CA1185006A - Image pickup apparatus with gain controlled output amplifier - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Solid-state image pickup apparatus, such as an MOS
imager, has a two-dimensional array of picture element units each formed of a photo sensitive element and a gating element. The picture unit elements discharge a signal charge onto vertical and horizontal transmitting lines in response to vertical and horizontal scanning pulses. Then, a resulting signal current is used to develop an output video signal. In order to give the output video signal a good S/N ratio, a gain-controlled current amplifier is employed. In several embodiments, the gain-controlled amplifier includes first through fourth transistors with the base-emitter junctions of the first and second transistors and of the third and fourth transistors connected in series, with a constant current source coupled to the first transistor, controlled current sources connected to the second and third transistors, and a load device coupled to the fourth transistor. In other embodiments, the gain controlled amplifier is formed of first, second, and third current mirror circuits connected in a balanced-current arrangement. Electrically variable resistances, e.g., MOS
transistors, are coupled to the output transistors of the first and second current mirror circuits to control the current gain.

Description

l~l ~ 06 BACKGROUND OF THE INVENTION
This invention relates to gain-controlled amplifiers, and is more particularly directed to a solid-state imaqe pickup device incorporating a gain-controlled current amplifier for converting a relativelv feeble video signal ~o a strong video output signal with a high signal-to-noise (S/N) ratio.
A solid-state image pickup element, such as an MOS
imager, generally provides a quite weak signal which must be amplified before ~eing applied to a successive stage. This weak video signal is particularly susceptible to n~ise due to stray currents, shot noise, and the likeO In an MOS
imager, the video signal is provided as a differentiated current. This signal must be integrated to obtain an average value, but integration lowers the overall level of the video signal. Thus, the integrated signal has a rather low S/N ratio.
~ n external amplifier is conventionally provided to amplify the output signal and thereby convert it to a level suitable for further processing. Unfortunately, this external amplifier amplifies the noise along with the video signal, and, because it is external, also introduces noise.`
Consequently, the resulting video output signal has a further deteriorated S~N ratio.
Furthermore, the average level of the output signal from the image pickup element varies qreatly with .

-2-, , 38/S01611 ~356~i s8 ~.Pl 3 2 changes in brightness or color temperature of a scene to be televised. If the gain of the circuit connected to the pickup element is too high, the peak level becomes clipped and the signal waveform becomes distorted when a bright scene is being shot. As a result, the picture becomes washed out, and lacks detail in its bright portions. On the other hand, if the gain of the circuit is too low, the average signal level is reduced and the S/N ratio can become unacceptably low when a dark scene is shot. Obviously, both of the foregoing situations are undesirable. Consequently, it would be advantageous to include a gain control element in the video signal processing circuitry so that the gain of the output circuitry associated with the pickup element changes in accordance with the brightness of the picture being picked up.
Unfortunately, conventional gain-controlled amplifiers generally are suitable only for am~lifying a continuous analog signal, and no such circuit has been previously proposed which can control the amplitude of a sampled, time-shared current signal such as the output of an MOS imager or other similar image pickup device.
OBJECTS AN~ SU~5~RY OF THE IN~7ENTION
It is an object of this invention to provide video image pickup apparatus generating a strong video signal representing a scene with a high signal-to-noise ratio, the level of which signal is controlled in accordance with brightness conditions of the scene.
It is another object of the invention to provide video image pickup apparatus incorporating a current ~5~6 S82P132 amplifier therein to avoid the aforementioned problems that occur when an external amplifier is used to amplify the video signal.
It is a more specific object to provide video image pickup apparatus employing a gain-controlled current amplifier of a type suitable for amplifying a sampled, time-shared current signal typical of the output of a solid-state imager.
According to an aspect of this invention, image pickup apparatus are provided in which there are a plurality picture element units, each formed of a photosensitive member generating a quantum of electrical charge depending on the amount of light falling thereon ~nd an electrically controlled gating circuit for transmitting the generated amount of charge in response to scanning pulses applied thereto, with the picture element units being disposed in a two~dimensional array of horizontal rows and vertical columns. Scanning signal generators sequentially provide -horizontal and vertical scanning pulses to the gating circuits of the respective picture element units, so that the electrical charges are presented, in row-by-row sequence, to an output circuit to produce a video signal.
In embodiments of this invention, the output circuit includes a gain-controlled current amplifier circuit formed of an input transistor to which the input signal current is applied and an output transistor at which an amplified output signal current appears. The gain of the circuit, i.e., the ratio of the output signal current to the input signal current, is controlled by a controllable element _g _ ~5~ 38 / sol 6 1 1 coupled to the input and output transistors so that the current gain can be selectively controlled, in accordance with illuminating conditions of a scene being shot by means of a control signal applied thereto.
In several embodiments the gain-controlled amplifier includes first, second, third, and fourth transistors each having a first and a second current-carrying electrode and a control electrode, with the second and third transistors having their first current-carrying electrodes coupled together to a point of voltage, having their control electrodes coupled together to the first current-carrying electrode of the first transistor, and having their second current-carrying electrodes coupled to the control electrodes of the first and fourth transistors, respectively. The first and fourth transistors have their second current-carrying electrodes coupled together to another point of voltage. A current source supplies a constant current to the first current-carrying electrode of the first transistor, and a load device is coupled to the first current-carrying electrode of the fourth transistor. An adjustable current source supplies adjustable constant currents to the second current-carrying electrodes of the second and third transistors. In these embodiments, the input signal current is applied to the first current-carrying electrode of the first transistor and the output signal current appears at the first current-carrying electrode of the fourth transistor.

~5~ 38/S01611 In other embodiments, the gain controlled current amplifier can include first, second, and third current mirror circuits, each having an input transistor device and an output transistor device. Current sources provide substantially constant currents to the input transistor devices of the first and second current mirror circuits;
while the output transistor devices thereof are respectively coupled to the output and input transistor devices of the third current mirror circuit. First and second resistive elements, which can include the source-to-drain resistances of MOS transistors, are coupled to the input transistor devices of the first and second current mirror circuits.
Third and fourth resistive elements, which can also include MOS transistors, are adjustable by means of a gain control voltage applied thereto, and are coupled to the output transistor devices of the first and second current mirror circuits. A load device is coupled to the output transistor device of the first current mirror circuit. Here, the input signal current is applied to the input transistor device of the first current mirror circuit and the output signal current appears at the output transisotr of the first current mirror circuit.
In many embodiments, the load device can include a load capacitor having one plate coupled to the second current-carrying electrode of the second transistor and another plate coupled to a reference po;nt, and a pre-charging transistor for charging the one plate of the load transistor during occurrences of a switching pulse.

a~ ;38~Sll6~l The above and m~ny other ~bjects, features, and advantages of this invention will become apparent from the ensuing description of several preferred embodiments thereof, which is ~o ~e considered in connection with the accompanying drawings.

BRIEF DESCRIPTION C)F TH~ DRAWINGS
__ _ _ Fi~. 1 is a schematic diagram of a conventional solid-state imaqe sensor.
Figs. 2A to 2G are waveform charts of scanning pulse si~nals used in con~unction with the image sensor of Fig. 1.
Fig. 3 is a diagram of a first embodiment of the improved image sensor according to the present invention.
Fig. 4 is a schematic diagram of a pertinent portion of the improved image sensor according to a second embodiment thereof.
Fig. S is a schematic diagram of a pertinent positon of the improved image sensor according to a third embodiment thereof.
Figs. 6A to 6F are waveform charts for explaining operation of the ~hird embodiment.
Figs. 7 and 8 are schematic diagrams illustrating fourth and fifth embodiments of this invention, respectively~
Fig. 9 is a schematic diagram illustrating a sixth embodiment of this invention.
Figs. lOA to lOF are waveform charts for explaining operation of the sixth embodiment of this invention.

4~~

3~/S01611 ~S~6 S82P132 Figs. ll to 13 are schematic diagrams of a pertinent portion of seventh, eighth, and ninth embodiments.
DETAI~ED DESCRIPTION OF SEVERA~ PREFERRED E~BODIMENTS
Initially, for purposes of background and to emphasize the advantages of this invention, a conventional solid-state image sensor will be described with reference to Fig. l.
In this conventional image sensor, an ~IOS
(Metal-Oxide-Semiconductor) imager 1 is constituted by a plurality of picture element units SOll to SEnm disposed in a two-dimensional, or X-Y array formed of m horizontal rows by n vertical columns.
A horizontal scanning c;rcuit 2 provides successive horizontal gating pulses ~Hl (Fig. 2A), ~H2 (Fig. 2B),. . . . to ~m (Fig. 2C) to the picture element units of each horizontal row in turn. At the same time, a vertical scanning circuit 3 provides interlaced odd and even vertical gating pulses ~VO1 ~Fig. 2D), ~V02 (Fig- 2E) ~VEl' VE2' ' ~V~n The interlacing of the vertical gating pulses corresponds to the interlacing of a television image, in which a field of odd lines and a field of even lines alternate.
In this image sensor, each of the m picture element units SOll, S0l2~ Olm odd line are disposed in line in the horizontal 3~/S~1611 S~6 S82pl32 direction, and the picture element units for the remaining lines are similarly disposed.
Each of the picture element units SOll, S012' ~ ' ' SOlm ~ SEnm is formed of a photo diode Dollr Dol2 ~ ' Olm ' Enm' a PN or PIN type, and an associated MOS gating 011' M012' ' M01m ~ MEnm- The photodiodes have their cathodes connected in common to a target voltage source VT and have their anodes coupled to the source electrode of the associated MOS
gating transistOrs Mo1~ Ol2~ MOlm' MEnm-drain electrodes of these transistors are connected to respective vertical transmitting lines T~vi~ ~2~
. . . . LVm, for each vertical column (i.e., for each successive horizontal position) while the gate electrodes of the M05 gating transistors of each particular horizontal row are connected in common to an output of the vertical scanning circuit 3 to receive a particular scanning pulse ~VOl' ~V02 . . . .
~VEn in accordance with the particular image line to be scanned. For example, the scanning signal ~VOl of Fig. 2D is furnished to the gate terminals of the transistors ~011 to MOlm of the first odd horizontal row during a first horizontal scanning period.
The remaining odd lines and all of the even lines of the picture element units SOll, SOl~. . . .
SOlm, . . . .SEnm are constituted in like manner.

S82P]32 Consequently, an odd field of video is generated in response to feeding of odd vertical scanning pulses ~VOl' ~V02 ~ . . . to the gate electrodes of the transistors Moll, Mol2, . . . .

Olm; M021' M022 ~ M02m in the sequential odd horizontal rows, and an even field of video is generated in response to feedipg of even vertical scanning pulses VEl' VE2 ' VEn g electrodes of the transistors MEll- MEl~ M
. . . . ~1Enm of the sequential even rows.
Each vertical transmitting line LVl, L
. . . . LVm has associated ~ith it a capacitance CV for storing the electric charge passed by the gating transistors Moll, Moll Molm -MEnm in response to each particular vertical scanning pulse ~VOl to ~V~n~ 50 that the capacitances Cv at any line interval hold the electrical charge generated in one particular horizontal row of picture element units S 11 to Solm~ SEnl Enm Each of the vertical transmitting lines L
to LVm also incluaes a respective MOS transistor Ml, M2 ~ ~ ~ 'Mm having a source electrode coupled to the associated capacitance Cv, and having a gate electrode coupled to a respective output of the horizontal scanning circuit 2 to receive a particular horizontal scanning pulse ~Hl' ~H~ ' ' ' ' E~m' 38/~t)1611 ~ 6 S82P132 electro~es of the MOS transi.stors Ml to Mm are connected in common to a horizontal output transmitting line LH. ~ capacitance C~ is associated with the horizontal transmission line LH, while a load resistor 4 of a value RL bridges between a DC voltage source VDc and the horizontal transmitting line LH.
~ ere, the capacitances CH and Cv are sho~n as lumped values, but in practice they are substantially distributed over the respective transmitting lines LH and LVl to LVn.
The horizontal output transmitting line LH
is also coupled to ~ gate electrode of an output .MOS
transistor 5 connected as a source follower and serving as an isolation stage, with its drain electrode connected to a drain potential Vd~ and its source electrode connected to voltage source 6 and also to a pre-amplifier 7 to amplify the video signal and provide the same to an output terminal 8.
In this MOS imager l, light incident on the diode Doll to DEnm of each picture element unit SOll to SEnm causes the aiodes to generate electron-hole pairs. Consequently, a signal charge QS is stored between the gate and source electrodes of each respective MOS yating transistor Moll to MEnm. The size of each signal charge QS depends on the strength of the light incident on the particular diode Doll to Enm 3~/S01611 ~2Pl32 ~3S~)~6 Occurance of the vertical scanning signals V~l' V02' ' VEn places at a high level all of ~he gate electrodes of the yating transistors Moll to Olm; 021 to M02m; ; MEml to ME for one partlcular horizontal xow. As a result, all of the signal charges QS for the picture element units S011 to SOlm; S021 to SO~m; SEnl to SEnm f that particular row are gated to the capacitances Cv for the respective vertical transmission lines LVl to Lvm .
As shown in Figs. 2A to 2C, the horiæontal scanning pulses ~Hl to ~Hm g from low to high levels at times t2, t3 . . . .to and return to their low levels so as not to overlap one another. However, each such scanning pulse ~Hl to ~m is Or a sufficient length to transfer the charge QS from each capacitance CV to the capacitance CH.
As shown in Figs. 2D and 2~, the vertical scanning pulse ~V01 is high from a time tl until a later time t4, at which time the next vertical scanning pulse ~V02 becomes high. In fact, all of the vertical scanning pulses.~vOl to VEn r a non-overlapping fashion.
If it is assumed that the pulse width of each horizontal scanning pulse ~Hl to ~Hm is T, the time t2 that the first scanning signal ~Hl is made .

~ 6 38/S016ll high occurs a predetermined time ater the time t1 or t4 following the onset of each vertical scanning pulse ~VOl to ~VEn.
As aforementioned, each transmitting line LH and LVl to LVn has a respective capacitance CH and Cv, the load resistor 4 has a resistance value RL.
The signal charge QS stored in each picture element unit SOll to SEnm generates a signal current is (Fig.
2F~ to flow whenever the associated horizontal and vertical scanning pulses are hiyh. For the charge Qso11 of the picture element SOll, these pulses are both high from the times t2 to t3 (Fig. 2A). Thus, the signal current is associated with this particular picture element unit SOll can be expressed i = QSoll ~ exp - ( t ~ . (1) S RL(CV+C~) ~RL(CV CH~

The resulting output voltage VOut at the input to the preamplifier 7 becomes V = QsOll . exp ~R (Cv+CH)~

Here, it is important that the time constant RLICv+CH) be sufficiently small that all of the charge QS is transferred during each on period (T= t2-tl~ of ~ S82P132 the horizontal scanning pulses ~1 to ~Hn. That is, or a value k defined by the equation ~ ( V H) k ' ' . . . . . . . . (3) be rather larger ~han unity, for example k=5.
The same requirement holds true for all the picture,element units S0ll to SEmn ~vr example, for ~he signal charge QS012 ~ the picture element 5012 the output voltage V~ut becomes 2 . exp~ ~ (C~+~H) ~

Thus~ as _ach unit S011 to ~Qnm and SE11 to SEnm is sequentially scanned, its respective sign~l charge QS~ll to QSEnm and QSE11 to QSEnm f its associat~d vertical signal transmitting line ~1 to ~ml and then through the horizontal transmitting line LH and the load resistor 4 as a signal current is, which is used to derive the video signal as ~he output voltage V0ut.
However~ it is clear from ~ig. 2G and from equations 12) and (4) above that the output voltage V0ut is a differential voltage, and cannot, with satisfactory results, be used directly as a stable input for a sample-hold or peak-hold circuitO

~.~

~35~

Therefore, it is generallv necessary to produce an average, or mean value of the output voltage VOUt by use of an integrator or low-pass filter. ~his results in an average output voltage [Vout]mean as expressed out]mean = ~ 1 ~ exp ~ t ~ dt QS . RL tCV H) CV+CH

X[exp _ t ] . 1 RL(CV H) = QS RL (1 e~k) t5) Here, k is set at k~5 as was true in equation (3) so that the factor e k can be disregarded. If the signal charge QS is thought of as being the product of the signal voltage Vs times the gate-to-source storage capacitance C0 of a particular picture element unit S, and the discharge time thereo is represented by the expression k~RL(Cv~C~) from equation (3), then the average output voltage [vout]mean ; -15-, 3~/S01611 ~ 8~ S82P132 [V ] - Co RL Vs k ~ (C -~C t co ~ Vs klCv+CH) .As k is large com~ared to unity, and the capacitance CO is small compared to the capacitance~
CV and CH, it becomes readily apparent from equation (6) that tne mean value [VOut~mean is extremely low.
Therefore, the preamplifier 7 is required to have both a high gain and a high SIN ratio, and, con~sequently, is quite expensive.
Further, in a usual MOS imager, the circuit elements from the light-receiving portion 1 to the source-follower transistor 5 are integrated onto a single IC chip, and the preamplifier 7 is an external device connected to the transistor 5. Outside noise, such as clock noise, is likely to be introduced at the input of the preamplifier 7. Also, because the average value signal [VOut~mean is so small, it is easily affected by outside noise, and the S/N ratio suffers greatly.

-16~

3~/S01611 ~5~ s~ 2Pl32 One possible approach to improving the signal strength of the output signal [VoUtlmean has been to make the value k small (k-l), i.e., correspondingly to increase the value RL of the load resistor 4. However, if the resistor 4 is too large, the complete signal charge QS for each picture element unit S will not be entirely transmitted during a sing~e horizontal scanning pulse period ~, and there is noticeable deterioration in both horizontal and vertical resolution.
A first embodiment of this invention is shown in Fig. 3, wherein elements common to the circuit of Fig. 1 are identified with the same reference characters, and a detailed description thereof is omitted.
Here, instead of the load resistor 4, the source follower 5, and the preamplifier 7, the horizontal signal transmitting line LH ends in a gain-controlled current amplifying circuit 10.

~ 6 Sg2P132 This gain-controlled amplifying circuit 10 includes first, second, third, and fourth junction transistors ll, 12, 13, and 14, each of which in this embodiment is an NPN transistor. The second and third transistors 12, 13 have their bases joined together to the collector of the first transistor ll and have their collectors coupled to a source of collector voltage Vcc.
The first and fourth transistors 11, 14 have their emitters connected to another ~oint of voltage VDcl and have their bases connected to the emitters of the second and third transistors 12, 13, respectively. A load resistor 4' of value RL bridges between the source of collector voltage Vcc and the collector of the fourth transistor 14. An isolation stage formed of an emitter follower transistor 15 and an emitter resistor 16 couples the collector of the fourth transistor 14 to an output terminal lS.
A current source 21 connected between the source of collector voltage Vcc and the collector of the Eirst transistor 11 furnishes a constant current Io thereto. A
pair of controllable current sources 22 and 23 coupled between ground and the emitter of the second ana third transistors, respectively, provide controllable constant emitter currents IC to the latter transistors. Preferably, the constant emitter currents I~ have equal values, and both sources 22 and 23 are ganged together so that, when adjusted, the respective emitter currents IC remain equal.
Here, the input signal current iS is applied to the base of the first transistor 11 and an amplified output ~8~0~ 38/SOl611 signal current iSo~t appears at the collector of the fourth transistor 14.
The operation of the circuit lO as a gain-controlled current amplifier can be explained as follows.
The base-emitter junctions of the transistors 11 and 12 are connected in series and those of the transistors 13 and 14 are likewise connected in series, with the bases of the transistors 12 and 13 joined and the emitters of the transistors 11 and 14 joined. Consequently, the base_emitter forward voltages VBEl' vsE2~ VBE3' sE4 the first through fourth transistors ll, 12, 13, 14, respectively will have the relationship BEl vsE2 VsE3+VsE4 ~ (7) In general, the base-emitter voltage VB~ of a junction transistor is logarithmically related to the emitter current IE flowing these through and emitter area A
thereof:
VBE ln(IE/A) . . . . . . (8) Using the above relationships (7) and (8), if the transistors ll to 14, respectively, have emitter currents I1 to I4 and emitter areas Al to A4, the following relationship results: ~

A A A A ' ~ ' ( ) In the embodiment of Fig. 3, the emitter current I1 is substantially the same as the constant collector current lo provided from the current source 21. Also, the emitter current I2 is equal to the difference between the constant current IC and the input signal current iS (I3 =
IC-is) while the emitter current I3 is equal to the constant current Ic. Since the emitter current I4 for the fourth transistor 14 is substantially equal to its collector current IoUt, the relationship t9) above can be rewritten Io~Ic-is) = Ic Iout Al 2 3 4 . . . . (10) The output current IoUt is then I A~-A4 lo ~ is . . tll) Al-A2 Ic The output voltage VOut at the junction of the collector of the transistor 14 with the load resistor 4' becomes out Vcc Rl IOut-vBE . . (12) This output voltage VOut has an AC signal component VOUt Vout = L A1'A2 ~ IC} ~ ~ (13) obtained by substituting the above relationship (11) into the relationship (12), and considering only terms including the input signal current iS. Furthermore, by using an integrator or low pass filter, an average signal voltage ~V ] can be obtained having a value out means [ out]mean = A3'A4 Io RL
Al.A2 IC

re2~Cv+CH) exp [ r ~ )]dt . I . . (14) In equation (14), re2 is the resistance value at ~5Q~ 3~/S0l6ll the emitter of the transistor 12, and can be determined by the relationship r 2 = kT ~ 26mV
qIc Ic where ~s is the signal charge stored on any given picture element unit SOll to SEmn, k is Boltzmann's constant, and T
is the absolute Kelvin temperature of the base emitter junction of the transistor 12.
If a scanning period for the picture element units SOll to SEmn is selected to satisfy the relationship Tre2[Cv+CH) . ~ . . . . . . . . . . . . . ~15) equation (14) above becomes [ out~mean = ~3'A4 Io R1 ~S

Al-A2 IC T ..................... (16) As is apparent from the relationship (15) above, the rate of transfer of the signal charge QS from the vertical and horizontal signal transmitting lines LVl to LVm and LH is essentially determined by the value of the emitter resistance re2 of the transistor 12. The rate of charge transfer is independent of the resis-tance value RL of the laod resistor 4'. Consequently, even when the value RL f the resistor 4' is large and the gain of the circuit 10 is high, the deleterious effects of the prior art, such as loss of resolution, do not occur. Also, because the signal current iS can be amplified directly to obtain a large amplitude voltaqe signal vS, the S/~ ratio for the entire image pickup apparatus is greatly improved. Furthermore, in accordance with the relationship (16) abover if the ~3S~6 S8 2P1 32 transistors 11 to 14 have their emitter areas A1 to A4 selected to satisfy the relation A3.A4 ~ Al A2 and the constant currents Io and IC are selected Io ~ Ic, the average output voltage [VOut]mean can be amplified and the gain of the circuit 10 changed simply ~y controllably shanging the value of the constant current Ic.
~ second embodiment of this invention is illustrated in Fig. 4. In this embodiment, circuit elements that are also shown in Fig. 3 are ident~fied with the sarne characters, and a detailed description thereof is omitted.
Here, an additional current source 24 is connected between the source of collector voltage Vcc and the collector of the transistor 14. A load resistor 27 having a value RL is connected between the collector of the transistor 14 and a point of reference potential VDc2. The additional current source 24 provides a constant current I~C to the transistor 14 so that an output signal current iout having only AC
signal components flows through the load resistor 27.
In other words, the emitter current I4c~ which is substantially equal to the collector current thereof, is equal to the output current IoUt in the relation (ll) above J
and can be expressed as follows:
A3~A4 _ . iS) However, if the constant current source 24 supplies to the collector of the transistor 14 a constant DC current corresponding to the DC component of the current iout of the ~ S82P132 relationship (17~ above, the current flowing through the load resistor 27 becomes A3.A4 out A A 4 A3.A4 Io AI~A2 IC . ' ' (18) As is apparent, this output current iout has only AC signal eomponents. Consequently, the output voltage vOut in equation ~13) above, which is the AC component of the output signal VOut ean be obtained directly at the eollector of the transistor 14, and its average value ean be obtained aeeording to equation ~16) above.
In this case, the eonstant eurrent supplied fro~
the eurrent souree 24, will have the value A3.A4 I4 = O
~ Al A2 A third embodiment of this invention is illustrated in Fig. 5, in which elements shared with the embodiment of Fig. 4 are identified with the same reference characters. In this embodiment, in place of a load resistor, the collector of the transistor 1~ is connected to one plate of load capacitor 36 having a capacitance of Cp, another plate of which is eonnected to ground. A PNP
eharging transistor 35 has its emitter eonneeted to the one plate of the eapaeitor 36, its eolleetor connected to ground, and its base connected to receive precharge pulses ~p. The one plate of the capacitor 36 is also eoupled through an output stage formed of a Darlington arrangement of NPN transistors 37 and 38 to the output terminal 18.

3~/S01611 ~ S82P132 Here, at the onset of each of the horizontal scanning signals ~H1~ ~H2~ ~H3 (Fig- 6A), the signal charge Qs stored in each picture element unit SOll to SEmn is transfered through the transmittlng line L~ and LH and appears a differentiated signal current iS (Fig. 6B). In this embodiment, both the inRut signal iS and the output signal current iSout (Fig. 6C) are both positive, as the latter represents a charging of the load capacitor 36.
Thus, it is required to ~reset the load capacitor 36 to a constant initial low value, such as ground potential. For this reason, the clock pulse signal ~p (Fig. 6D) is supplied to the base electrode of the transistor 35 to drive the same on for a brief period immediately before the output signal current iSout flows. Consequently, the capaci-tor 36 is discharged before being partially charged up by the output signal current iSout. Thus, the load capacitor 304 has an output waveform v5Out as shown in Fig. 6E. ~ere, if the discharging ON time of the transistor 35, as governed by the pre-charge signal ~p is TA, the final output voltage vSOut appearing at the one plate of the capacitor 36 becomes:

Vsout 1 ol sout Cp Cp ~ 2 ~c X IA QS . exp _ t dt O re2 ~Cv+CH) re2 (Cv~CH) If the ON time TA is set to a value sufficiently greater than the time constant re2 (CD~CH), then the integral te~m ~ S82P13~

in the equation (19) above is nearly equal to QS' and the output voltage vsOut results as follows:

V 3 4 . ~ . Q
sout = S
Cp A1 A2 IC . . . . ~20) As is apparent from the above equation t20), if the capacitance Cp of the load capacitor 36 is small, the emitter area ratio 3 A4 A A
is large, and the constant current Io is also sufficiently large, compared with the gain-control constant current Ic, the circuit 10 can attain a high gain, and this gain can be easily controlled ~y selecting appropriate values of the constant current Ic.
Furthermore, because the output waveform vOut, as shown in Fig. 6E, is generally stable at a point just pri3r to the leading edge of the pre-charge pulse signal ~p, a samDle-hold operation can be carried out at the times indicated by vertical arrows in Fig. 6E, to produce a sampled and held voltage SH as shown in Fig. ~F.
Fig. 7 illustrates a fourth embodiment of this invention, wherein each of the current sources 21, 22, 23, and 24 are constructed of respective junction transistors 31, 32, 33, and 34. In particular, PNP transistors 31 and 34 are employed as the constant current sources for supplying the DC current lo to the collectors of the transistors 11 and 14, while NPN transistors 32 and 33 supply an adjustable constant current IC to the emitters of the transistors 12 and 13, respectively.

~ ~SV ~ S~2P132 The transistors 32 and 33 have their collectors coupled to the emi~ters of the respective transistors 12 and 13, and have their emitters coupled together to groundO
These transistors 32 and 33 have their bases connected to the base of a drive transistor 40, which has its collector joined to its base and to a controllable current source 41 supplying a control current ICl thereto. The emitter of this drive transistor 40 is connected to ground.
The transistors 31 and 34 have their collectors connected respectively to the collectors of the transistors 11 and 14, and have their emitters connected together to the point of collector voltage Vcc. These transistors 31 and 34 have ~heir bases joined together to the base of a drive transistor 42 whose emitter is connected to the voltage Vcc and whose collector is joined to its base, and is also coupled through a collector resistor 43 of value Ro to ground.
The transistors 42, 31, and 34 form a first current mirror circuit for supplying current to the transistors 11 and 14~ while the transistors 32, 33, and 40 form a second current mirror circuit for supplying current to the transistors 12 and 13.
In the above-described embodiment, if the transistors 31, 32, 33, 34, 42, and 40 have respective emitter areas of A31~ A32~ A33~ A34~ A42~ 40 output signal current iSout can be obtained substantially free of any DC current co~ponent, if following relationship are established:

~3 ~4 32 A33 A40 . ' . . . . ' (21) 5~ ~ S82P132 In addition, if the transistor 42 has a base-emitter voltage of VBE, and the resistor 43 has a resistance value of Ro~
the DC constant current Io flowing to the collector of the transistor 11, will have a value established as follows:
Vcc ~ VBE A3l Ro A42 . (22) Fig. 8 illustrates a fifth embodiment of this invention which is generally similar to the third embodiment, but wherein the bipolar transistor 35 is replaced with an MOS field effect transistor 35F. The transistor 35F, operated in a non-saturation mode, has its drain electrode connected to a source of drain ~otential Vdd, its source electrode coupled to the one plate of the load capacitor 36, and its gate electrode coupled to receive the pre-charge pulses ~p.
Fig. 9 shows a sixth embodiment of this invention, in which a gain-controlled amplifier 100 is supplied with the input signal current is from the MOS imager 1. In this gain-controlled amplifier 100, three current mirror circuits are employed to form a double-balance arrangement wherein any DC component of the output signal current isout is eliminated.
In this embodiment, first and second diode-arranged NPN transistors 101 and 102 have their bases joined to the bases of respective NPN transistors 103 and 104~ The transistor 104 has its collector joined to the collector of a diode-arranged PNP transistor 106, while the ~ S~ ~ ~ 38/S01611 transistor 104 has i-ts collector coupled to the collector of another transistor 107, whose base is joined to the base and collector of the transistor 106. These PNP transistors have their emitters joined to a source of potential Vcc, while the emitters of the transistors 101 to 104 are coupled to another reference voltage VDcl by means of source-drain paths of respective MOS transistors 111, 112, 113, and 114.
The MOS transistor 111 and 112 have their gate electrodes connected to a common DC bias voltage source 115 supplying a DC voltage VD thereto to maintain these transistors 111 and 112 in a non-saturation state. The MOS transistors 113 and 114 have their gate electrodes connected in common to a series combination of a DC bias voltage source 116, which supplies the DC voltage VD, and a gain-control voltage souree 117, which supplies a eontrolled variable voltage vc.
It should be appreeiated that the transistors 101 and 103 form a first eurrent mirror eircuit 121, the transistors 102 and 104 form a second current mirror circuit 12~, and the transistors 106 and 107 form a third current mirror eireuit 123. In these eurrent mirror eireuits, the transistors 101, 102, and 106 aet as input transistors, while the other transistors 103, 104, and 107 act as output transistors.
~ ere, constant current sources 131 and 132 each supply a constant current Io to the eollectors of the input transistors 101 and 102, respectively.
In this embodiment, the collector of the input transistor 101 of the first current mirror circuit 121 is eoupled to the horizontal signal transmitting line LH to ~ 6 S~2Pl32 receive the input signal current is. Accordingly, the output transistor 103 of the first current mirror circuit 121 is the point at which the output signal current 5out appears.
Similarly to the foregoing third, fourth, and ~ifth embodiments, a pre-charging transistor 135 and a load capacitor 136 are coupled to the collector of the transistor 103, and are followed by an isolation stage formed of an emitter-follower NP~ transistor 137 connected in series with a current source 138. The emitter of the transistor 137 then supplies an isolated output signal to an output terminal 139.
The operation of this circuit can be explained with reference to Figs. 10~ to 10F. To simplify the explanation, it will be assumed that the transistors 101 to 104l 10~, and 107 have identical characteristicsl and that the MOS transistors 111 to 114 also have identical characteristics. However, this condition is not necessary to practice this invention.
Each of the transistors 101 to 104 has an emitter current Il to I4l respectivelyl flowing therethrough.
Initiallyl if each of the MOS transistors 111 to 114 has a respective source-to-drain resistance of R11, R12, R13l and R14l the collector current I3 of the transistor 103 can be established as I = (I +i ) ~

The source-to-drain resistances R11 to R14 can be established according to the following relationships R = R = 1 (V -~V

R13 = Rl4 = l~(VD Vc th . . . (25) in which ~ is the gain characteristic of the MOS
transistors, and can be expressed as follows ~N EOX
(W) 2tox L . . . . (26) wherein ~N is the channel carrier mobility, EOX and toX are the dielectric constant and layer thickness of the oxide insulation layer on the gate region of the MOS transistor, respectively, and W and L are the gate width and gate length thereof, respectivelv. In the above relationshi~ 24 and 25, Vth is the threshold voltage for each OL the I~OS
transistors.
The collector current I7 flowing through the transistor 107 can be expressed as follows:

7 0 R14 . O . . . (27) From a consideration of both the foregoing equations (273 and (23), the output signal current iSout flowing into the load capacitor 136, is just the diEference between the currents I3 and I7, and can be expressed as follows:

sout = I3 I7 =(I +i ) ~ - I R12 i D c th D th l+ v s VD-Vth . . . . . . (28) ~5~ S82P132 Thus, the output signal current iSout will be provided substantially free of any DC components, and the gain o~ the circuit 100 will be substantially proportional to the level of the gain control voltage vc.
Similarly to the third embodiment (Fig. 5), in response to the leading edges of the horizontal scanning pulses ~hl~ ~h2~ ~h3 (Fig. lOA~, a differential input signal current iS (Fig. lOB) flows into the collector of the input transistor 101 of the first current mirror circuit, and an amplified output signal current iSout (Fig. lOC) flows from the collector of the output transistor 103. If, as shown in Fig. 10~, the pre-charging pulse ~p has a period TA representing the separation between successive ON times of the transistor 135, an output signal voltage vSOut will appear on the one plate of the load capacitor 136, and the same can be sampled at the times indicated by vertical arrows to provide a sampled and held signal SH as shown in Fig. lOF.
Similarly to the third embodiment discussed hereinabove, it should be apparent that the output voltage vSOut appearing at the output terminal 139 will have a value which can be calculated as follows:

1 ~A
p = 1 (1+ VC
Cp ~d-V h Q
X olAR ~cs~c )~ exp ~ ~ (C ~C ~ dt ~S~

vCC
cp(l V~--Vth) QS . . . . . . (29) In the above embodiment, it has been assumed that the MOS transistors lll to 114 have identical characteristics. ~owever, assuming that the ratio of gate widths to gate lengths of the MOS transistors 111 and 112 are equal to a value (W/L~IN~ while the ratio of the gate widths to the gate lengths of the MOS transistors 113 and 114 are equal to a value (W/L)oUT~ the foregoing equation (29) becomes sout C V -V ( / O~T Qs From this equation (30), it should be apparent that by setting the ratio (W/L)oUT to the value (W/L)IN to be large, a large-amplitude signal output can be readily obtained.
Fig. 11 illustrates a seventh embodiment of this invention, in which the gain-control circuit 100 is generally similar to that of the embodiment of Fig. 9, except that MOS transistors 141, 142, 143, 144, 145, 146, 147, and 157 are used in place of the junction transistors 101, 102, 103, 104, 135, 106, 107, and 137 of the sixth embodiment. In this embodiment, the MOS transistors 141 to 145 are N-channel type transistors, while the MOS
transistors 146 and 147 are P-channel type transistors.
In this seventh embodiment, the output voltage v50ut appearing at the one plate of the load capacitor 136 can be expressed as follows~ where the MOS transistors 141 and 143 have transconductances of gm1 and gm3, respectively, 5(~ 3~/S01611 and the MOS transistors 111 and 113 have source-to-drain resistances o~ Rl1 and R13, respectively:

_ +

Vsout = C ~
g m3 Rl 3 The operation and advanta~es of this embodiment are similar to those of the aforementioned sixth embodiment.
However, because the gain-control amplifier 100 is constituted entirely of MOS transistors, the entire image pickup apparatus can be integrated on a single semiconductor chip, ana the same MOS construction techniques used to prepare the MOS imager 1 can also be used to prepare the MOS
gain-controlled current amplifier 100.
Fig. 12 illustrates an eighth embodiment of this invention, which is generally similar to the Fig. 7 embodiment, but wherein depletion-type field ef~ect transistors 15l and 152 respectively ~orm constant current sources providing the constant current Io to the input transistors of the first and second current mirror circuits, 121 and 122, respectively. Also, in this embodiment, the current mirror circuits 121 and 122 are feed-forward current mirror circuits. That is, the first current mirror circuit 121, has an input MOS transistor 211 and a pair of output MOS transistors 212 and 213 connected in series. The second current mirror circuit 122 also has an input MOS transistor 221 and a pair of output MOS transistors 222 and 223 connected in series. The transistors 211 and 212 have ga-te electrodes coupled to each other and also to the drain electrode of the transistor 212, while the gate electrode of the transistor 213 is connected to the drain electrode of the transistor 211. Similarly, in the second current mirror circuit, the gate electrodes of the MOS transistors 221 and 222 are coupled together to the drain electrode of the MOS
transistor 222 and the source electrode of the transistor 223, while the gate electrode o the transistor 223 is coupled to the drain electrode of the MOS transistor 221.
This embodiment has the advantaye of a higher speed of operation than the foregoing embodiments, because of its feed-forward construction in the first and second current mirror circuits 121 and 122.
Fig. 13, which is on the same sheet as Fig. 4, shows a ninth embodiment of this invention. In this embodiment, the elements shown are generally the same as in the seventh embodiment, except that a load resistor 167 extends between a source of DC voltage VDc2 and the drain electrode of the output MOS transistor 143 of the first current mirror circuit 121. Here, the load capacitor 136 and the pre-charging transistor 145 are omitted. It should be appreciated that in this embodiment, only the AC output signal isout flows through the resistor 167, and, therefore, the source of the DC voltage VDc2 need not be closely controlled.
While several embodiments of this invention have been described in detail hereinabove, it is urged that the embodiments are representative only, and that many modifications and variations thereof ~ill be apparent to ~S~6 S82P132 those skilled in the art without departing from the scope and spirit of this invention, as defined in the appended claims.

Claims (23)

WHAT IS CLAIMED IS:

1. Image pickup apparatus comprising a plurality of picture element units each formed of a photosensitive member generating a quantum of electrical charge depending on the amount of light falling thereon, and an electrically controlled gating circuit for transmitting the generated amount of charge in response to scanning pulses applied thereto, said picture element units being disposed in a two-dimensional array of horizontal rows and vertical columns; scanning circuit means for sequentially providing said scanning pulses to the gating circuits of the respective picture element units; and output circuit means coupled to receive the electrical charges in sequence from said picture element units to produce a video signal, wherein said output circuit means includes a gain-controlled current amplifier having a signal input receiving said electrical charges as an input signal current, an output providing an amplified output signal current whose strength is related to that of the input signal current by a current gain of the gain-controlled current amplifier, and a controllable element for selectively adjusting said gain in accordance with a control signal applied thereto.

2. Image pickup apparatus according to claim 1, wherein said gain-controlled amplifier includes first, second, third, and fourth transistors, each having a first current-carrying electrode, a second current-carrying electrode, and a control electrode, with said second and third transistors having their first current-carrying electrodes coupled together to one point of voltage, having their control electrodes coupled together to the first current-carrying electrode of said first transistor, and having their second current-carrying electrodes coupled to the control electrodes of said first and fourth transistors, respectively, the first and fourth transistors having their second current-carrying electrodes coupled together to another point of voltage; a current source coupled between said one point of voltage and the first current-carrying electrode of said first transistor to provide a constant current thereto; load means coupled between said one point of voltage and the first current-carrying electrode of said fourth transistor; and adjustable current source means providing adjustable constant currents to said second current-carrying electrodes of said second and third transistors, respectively; with said input signal current being supplied to said control electrode of said first transistor and said output signal current appearing at said first current- carrying electrode of said fourth transistor.

3. Image pickup apparatus according to claim 2, wherein said picture element units and said first through fourth transistors are all formed as an integrated circuit on one semiconductor chip.

4. Image pickup apparatus according to claim 2, wherein said load means includes a load resistor.

5. Image pickup apparatus according to claim 2, wherein said load means includes a further current source coupled to the first current-carrying electrode of said fourth transistor providing a constant current thereto, and another load element coupled to such first current-carrying electrode.

6. Image pickup apparatus according to claim 5, wherein said other load element includes a load capacitor having one plate coupled to the first current-carrying electrode of said fourth transistor and another plate coupled to a reference point, and a pre-charging transistor having a first current-carrying electrode connected to a voltage reference point, a second current-carrying electrode coupled to said one plate of said load capacitor, and a control electrode connected to receive a switching pulse.

7. Image pickup apparatus according to claim 6, further comprising an output isolation stage having an input coupled to said one plate of the load capacitor and an output.

8. Image pickup apparatus according to claim 7, wherein said output isolation stage includes a darlington pair of transistors having a control electrode coupled to said one plate of the load capacitor and an output electrode connected to said output of the isolation stage.

9. Image pickup apparatus according to claim 6, wherein said pre-charging transistor includes an MOS
transistor having source and drain electrodes as its current-carrying electrodes and a gate electrode as its control electrode.

10. Image pickup apparatus according to claim 1, wherein said gain-controlled amplifier includes first, second, third, and fourth junction transistors each having a collector, an emitter, and a base, with the second and third transistors having their collectors coupled together to one point of voltage, having their bases coupled together to the collector of said first transistor, and having their emitters coupled to the bases of the first and fourth transistors, respectively, the first and fourth transistors having their emitters coupled together to another point of voltage; one current source coupled between said one point of voltage and the collector of the first transistor to provide a constant current thereto; load means coupled to the collector of the fourth transistor; and adjustable current source means providing adjustable constant currents to the emitters of the second and third transistors, respectively; with said input signal current being applied to the base of the first transistor and the output signal current appearing at the collector of the fourth transistor.

11. Image pickup apparatus according to claim 10, wherein said load means includes a further current source coupled to the collector of said fourth transistor providing a constant current thereto, and another load element coupled to such collector.

12. Image pickup apparatus according to claim 11, wherein said first, second, third, and fourth transistors are formed with respective emitter areas A1, A2, A3, and A4, said one current source provides the constant current IO to the collector of the first transistor, and the further current source provides the constant current to the collector of the fourth transistor at a level I4C, where

13. Image pickup apparatus according to claim 11, wherein said one and said further current sources are formed of respective one and further transistors each having an emitter connected to said one point of voltage and a collector joined to the collector of a respective one of said first and fourth transistors, and having their bases joined together; and further comprising a diode-connected transistor having an emitter joined to said one point of voltage, a base connected to a collector thereof and to the bases of the one and further transistors, and a resistor connected between the collector of said diode-connected trasnsitor and another point of voltage.

14. Image pickup apparatus according to claim 10, wherein said adjustable current source means includes a current mirror circuit formed of fifth, sixth, and seventh transistors, each having a collector, an emitter, and a base, with their emitters joined together to a reference point, their bases joined together, the collectors of the fifth and sixth transistors being joined to the emitters of the second and third transistors, respectively, and the collector of the seventh transistor being joined to the base thereof; and a controllable current source coupled to the joined-together bases of the fifth, sixth, and seventh transistors.

15. Image pickup apparatus according to claim 1, wherein said plurality of picture element units provides said input signal current as a time-shared sampled signal.

16. Image pickup apparatus according to claim 1, wherein said gain-controlled current amplifier includes first, second, and third current mirror circuits each formed of an input transistor device and an output transistor device, each transistor device having first and second current-carrying electrodes and a control electrode, with the control electrodes of both said transistor devices of each said current mirror circuit being joined together, the current mirror circuits being arranged so that current flowing into the first current-carrying electrode of the output transistor device thereof changes in accordance with a current flowing into said first current-carrying electrode of the input transistor device thereof; current source means providing substantially equal constant currents to the first current-carrying electrodes of the input transistor devices of said first and second current mirror circuits, respectively; the first current-carrying electrodes of the output and input transistor devices of the third current mirror circuit being joined to the first current-carrying electrodes of the output transistor devices of said first and second current mirror circuits, respectively, the second current-carrying electrodes of the transistor devices of the third current mirror circuit being coupled to a point of reference potential; first and second resistive means coupled to establish substantially equal fixed resistances between a reference point and the second current-carrying electrodes of the input transistor devices of said first and second current mirror circuits, respectively; and third and fourth resistive means coupled to establish substantially equal variable resistances between the reference point and the second current-carrying electrodes of the output transistor devices of said first and second current mirror circuits, respectively; with said input signal current being applied at the first current-carrying electrode of the input transistor device of said first current mirror circuit and with said output signal current appearing at the first current-carrying electrode of the output transistor device of said first current mirror circuit.

17. Image pickup apparatus according to claim 16, wherein said first, second, third, and fourth resistive means each include a respective first, second, third, and fourth MOS transistor having source and drain electrodes coupled in series between said reference point and the second current-carrying electrodes of the input transistor device of said first current mirror circuit, the input transistor device of said second current mirror circuit, the output transistor device of said first current mirror circuit, and the output transistor device of said second current mirror circuit, respectively, and each said MOS
transistor also having a gate electrode; and further comprising a fixed bias source coupled to the gate electrodes of said first and second MOS transistors and a selectively variable bias source coupled to the gate electrodes of said third and fourth MOS transistors.

18. Image pickup apparatus according to claim 16, wherein the input and output transistor devices for said first, second, and third current mirror circuits each include a junction transistor having a collector/ an emitter, and a base as its first current-carrying electrode, second current-carrying electrode, and control electrode, respectively, with the collector and base of each input transistor being coupled together.

19. Image pickup apparatus according to claim 18, wherein the junction transistors of said first and second current mirror circuits are all of one type and the junction transistors of said third current mirror circuit are both of another, complementary type.

20. Image pickup apparatus according to claim 16, wherein the input and output transistor devices for said first, second, and third current mirror circuits each include an MOS transistor having a drain electrode, a source electrode, and a gate electrode serving as its first current-carrying electrode, its second current-carrying electrode, and its control electrode, respectively.

21. Image pickup apparatus according to claim 20, wherein said MOS transistors for said first and second current mirror circuits are of one of an N-channel and a P-channel type; while the MOS transistors for said third current-mirror circuit are of the other of said N-channel and said P-channel type.

22. Image pickup apparatus according to claim 20, wherein the output transistor device for each said current mirror circuit includes a first and an additional MOS
transistor having source, drain, and gate electrodes, with the source and drain electrodes of the additional MOS
transistor being coupled in series with the drain electrode of the first MOS transistor, and with the gate electrode of the additional MOS transistor being connected to the drain electrode of the input MOS transistor, such that each said current mirror circuit operates as a feed-forward circuit.

23. Image pickup apparatus according to claim 16, further comprising a load capacitor having one plate coupled to the first current-carrying electrode of the output transistor device of said first current mirror circuit, and another plate coupled to a reference point; and a pre-charging transistor having a first current-carrying electrode coupled to a source of charging voltage, a second current-carrying electrode coupled to said one plate of the load capacitor, and a control electrode coupled to receive a switching pulse.