GB2024552A

GB2024552A - Amplifier for TV camera

Info

Publication number: GB2024552A
Application number: GB7922336A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1978-06-30
Filing date: 1979-06-27
Publication date: 1980-01-09
Also published as: NL7807071A; DE2926196A1; IT7923917A0; IT1127604B; FR2430147A1; JPS558200A

Abstract

An amplifier arrangement for a television camera, in which a first transistor 4, which may be located near a television pick-up device 1, is connected to the output 3 of the device 1. The first transistor 4 is connected to the input 10 of a preamplifier 10-28 by means of a cable 7, which input 10 is connected to a control electrode of a second transistor 13 and, through a series arrangement of a resistor 15 and an inductor 14 to a voltage-carrying terminal. The inclusion of the inductor 14 provides the advantage that a low supply voltage can be used for the preamplifier resulting in an improved signal-to-noise ratio. <IMAGE>

Description

SPECIFICATION Amplifier Arrangement for a Television Camera The invention relates to an amplifier arrangement for a television camera comprising a first transistor a control electrode of which is adapted to be coupled to an output of a television pick-up device, and a preamplifier including a second transistor, an output electrode of said first transistor being coupled to an input of said preamplifier which input is connected to a control electrode of said second transistor and to a voltage-carrying terminal through a resistor.

Such an amplifier arrangement is described in an article in "Journal of the SMPTE" July 1973, pages 547 to 551 inclusive, pages 548 (Figure 4) and 549 in particular where is described that the first transistor is of the type having an insulated gate electrode. This transistor is provided near a pick-up device, which produces the picture signal, and the gate electrode is connected as control electrode to the output of the pick-up device. By means of a cable the drain or output electrode of the transistor is connected to the input of a remote preamplifier. Herein the base electrode of the second transistor, which is of the npn-type is connected as control electrode to the preamplifier input which is connected to a terminal which carries a voltage of +50V, through the supply resistor.The base and the collector electrode of the second transistor are further interconnected through a feedback resistor and the collector is connected to the terminal which carries a voltage of +50V, through a further resistor. In addition, the preamplifier comprises transistors and resistors which, however, are of no importance for the present patent application.

In the described preamplifier the high supply voltage of +50V is used to enable the processing of large signal peaks occurring in the input signal. These signal peaks occur in a pick-up device which comprises a pick-up tube provided with a anti-comet tail gun. The high supply voltage has the further advantage that a supply resistor having a high resistance value of, for example, 10 kQ can be used, the advantage being a good signal-to-noise ratio in the amplified picture signal, as a higher resistance value results in a smaller noise current at the supply resistor which functions as a noise current source.

The mean square of the noise current fluctuations in a (short-circuited) resistor is inversely proportional to the resistance value.

A reduction of the supply voltage must be accompanied by a decrease in the value of the supply resistor. The voltage drop across the resistor should, namely, be so large that a sufficient voltage is still available at the drain of the transistor which is provided at the pick-up device. However, the lower value of the supply resistor causes the signal-to-noise ratio to decrease.

It is an object of the invention to provide a preamplifier which can operate with a lower supply voltage, but which is capable of providing an improvement in the signal-to-noise ratio.

The invention provides an amplifier arrangement for a television camera comprising a first transistor a control electrode of which is adapted to be coupled to an output of a television pick-up device, and a preamplifier including a second transistor, an output electrode of said first transistor being coupled to an input of said preamplifier which input is connected to a control electrode of said second transistor, an inductor and a resistor being provided in series between the input and a voltagecarrying terminal.

The invention also provides a television camera incorporating a pick-up device and a preamplifier as described above.

A practical embodiment of a preamplifier wherein an improved signal-to-noise ratio of 2 dB occurred had the resistor of resistance approximately four hundred ohms and the inductor had an inductance of approximately five hundreds of a millionth of a henry.

The invention will be explained in greater detail with reference to the accompanying drawing wherein: Figure 1 shows a first embodiment of a television camera provided with a preamplifer according to the invention, and Figure 2 shows a second embodiment.

In Figure 1 reference numeral 1 denotes a television pick-up device which, for example, comprises a pick-up tube, not shown. A lead 2, which is connected to a terminal which carries a voltage of -45V is shown at the device 1. The voltage-carrying terminal and other terminals which are still further to be described, are part of voltage sources, another terminal of which is assumed to be connected to ground. Furthermore the device 1 comprises in the Figure an output 3 at which a picture signal occurs which is generated bathe pick-up device 1.When the device 1 is provided with a television pick-up tube, the lead 2 is connected to the cathode of an electron gun, whereas the output 3 is connected to a signal electrode at which a potential image is formed which corresponds to a scene to be televised, the potential image being converted line-by-line and field-by-field into the picture signal by means of electron beam scanning. As the construction of the pick-up tube 1 is not relevant for an understanding of the invention it is not further described.

The output 3 which carries the picture signal is connected to the gate electrode as control electrode of a transistor 4 of the type having an insulated gate electrode, for example a field effect transistor, and is connected to one lead of a resistor 5. The source s of the transistor 4 is connected to the ground and the drain d is connected to terminal 6 of a coaxial cable 7. The other lead of the resistor 5 is connected to a terminal 8 of a coaxial cable 9. The sheaths of the coaxial cables 7 and 9, respectively, are connected to ground and the inner cores are connected to an input 10 and an output 11, respectively, of a preamplifier, still further to be described, which further comprises an output 12, which is connected to the output 11, at which output 12 the amplified picture signal is available for supply to further processing circuits in a television camera.

The preamplifier input 10 is connected to the emitter electrode as control electrode of a pnp transistor 1 3 and is further connected to a terminal carrying a voltage of +5V, through the series arrangement of an inductor 14 and a resistor 1 5. The base electrode of the transistor 13 is connected to ground through a resistor 1 6 and to the terminal carrying the voltage of +5V through the parallel arrangement of a resistor 1 7 and a capacitor 1 8.The collector electrode of the transistor 13 is connected to the base electrode of npn transistor 19, and through a resistor 20, to a terminal carrying a voltage of -5V. The collector electrode of the transistor 1 9 is connected to ground and the emitter electrode is connected to the emitter electrode of an npn transistor 21, these two emitter electrodes being connected through a resistor 22 to the terminal which carries the voltage of -5V. The base electrode of the transistor 21 is connected through a resistor 23 to the terminal which carries the voltage of +5V and through the parallel arrangement of a resistor 24 and a capacitor 25 to the terminal which carries the voltage of -5V. The collector electrode of the transistor 21 is connected to the base electrode of a npn transistor 26 and, through a resistor 27, to the terminal which carries the +5V voltage. The collector electrode of the transistor 26 is connected to the terminal which carries the voltage of +5V and the emitter electrode is connected to the outputs 12 and 11 and, through a resistor 28, to the terminal which carries the -5V voltage. The parallel circuits (17, 18) and (24,25) have been provided for a high-frequency decoupling with respect to the terminals having the supply voltage of + and -5V.

The preamplifier shown in Figure 1 having, for example, a gain factor equal to 200, the value of the resistor 5 being equal to 1 MS2, operates as follows. It is assumed that at the maximum value of the picture signal, that is to say the peak white value, a current of 200 nA flows through the resistor 5.

Neglecting the resistance of the cable 9 this produces a voltage at the outputs 11 and 12 which is 200 mV higher than the voltage at the gate electrode of the transistor 4. It is now assumed that the pick-up device 1 starts processing a not so bright scene portion. The current through the resistor 5 then decreases, which results in an increase in the voltage at the gate electrode of the transistor 4. This increase in voltage makes the tran;jistor 4 more conductive resulting in a larger current through the series arrangement of the resistor 1 5 and the indicator 14. The consequent decrease in voltage at the emitter electrode of the transistor 13 gives less current through the resistor 20 and a decrease in the voltage at the base electrode of the transistor 1 9 which, consequently, starts carrying less current.In response thereto the transistor 21 will start carrying more current as a result of which the transistor 26 is rendered less conductive owing to the voltage drop at the collector and base electrode, respectively, of the transistor 21 and 26, respectively, so that the voltage at the outputs 12 and 11 decreases. A drop in voltage at the gate electrode of the transistor 4 has the reverse effect. It is apparent that a negative feedback effect is obtained through the resistor 5. Assuming that for the darkest scene portions a negligible current flows through the resistor 5 it follows from the specified values that a picture signal having a 200 mV peak-peak value is produced at the outputs 12 and 11, in case of a 1 mV variation at the gate electrode of the transistor 4, which carries a voltage of, for example, -1 V.

From the preceding it follows that to achieve optimum operation sufficient current must be supplied to the transistors 4 and 1 3 through the series arrangement of the resistor 1 5 and the indicator 14. In the absence of the indicator 14 and solely in the presence of a resistor 1 5 and the value of the resistor 1 5 must not be so high that the voltage which becomes available for the drain d of the transistor 4 and the emitter electrode of the transistor 13 is insufficient for optimum operation.

However, the value of the sole resistor 15 must not be low, as, when the value is low, the influence of a noise current source 29 which, in Figure 1, acts in parallel with the resistor 1 5 deteriorates the signalto-noise ratio in the picture signal at the outputs 12 and 11 to an impermissible extent. The essential influence of the coil 14 can be explained as follows.

For noise produced by a resistor Nyquist calculated that 4kT I n2=-Af(A2) (1) R where In2 gives the mean square of current fluctuations In in a short-circuited resistor of R ohm, within the frequency range Af of Hz, for an absolute temperature of T k, k being the Boltzmann constant.

It follows that the mean square of the current fluctuatidns In is inversely proportional to the resistance value R, so that it is advantageous to choose the highest 'possible resistance value for R in order to obtain an optimum signal-to-noise ratio.

The use of the inductor 14 splits the noise current In into two currents, namely a current Ir through the resistor 1 5 and a current II through the inductor 14. Leaving the emitter-base resistance of the transistor 13 and the impedance of the parallel arrangement of the resistor 1 7 and the capacitor 1 8 out of consideration it can be derived that: R il= .In (2) R+jwL where w=27rf and L is the inductor inductance in henries. For the magnitude of the current it follows

From the expressions (1) and (3) it follows that

wherein the frequency range has been taken from 0 to fm as the maximum frequency.

From the expression (4) it can be derived that:

For L,=O, that is to say the inductor 14 is absent and the sole resistor 15 is present, the limit: 4kTfm Fill 2= (8) R folldws from expression (7).

From the expressions (7) and (8) a noise reduction factor N follows where: iTil 2(7) arctg 2xfmL/R N= IllI 2(8) 2nfmllR The noise reduction factor N can be utilized to calculate the value of the sole resistor which would furnish the same noise contribution as the series arrangement of the resistor 1 5 and the inductor 14.

Thus it follows that, when using a resistor 15 with R=390Q and an inductor 14 with L=470 H, at a maximum frequency fm=5 MHz, the factor arctg 12# artg37.68 1.54 N= = = =0.04.

12n 37.68 37.68 From this it,follows that the series arrangement of the resistor 1 5 with R=390Q and the inductor 14 with L=470 yH provides a noise contribution which is equally as large as the noise contribution of a sole resistor of 25 times 390=9.75 kQ. In practice this results in an improvement of 2 dB in the signalto-noise ratio compared with the situation where a sole resistor 15 having a value of 390Q is used.

In the circuit shown in Figure 2 the following items have been given the same reference numerals as in Figure 1: the pick-up device 1 with the lead 2 and the output 3, the feedback resistor 5, the cables 7 and 9 with terminals 6 and 8, which cables are connected to the input 10 and output 11, respectively, of the preamplifier of which the output 1 2 is also indicated. Reference numeral 4' denotes a npn transistor the base electrode of which is connected as control electrode through a capacitor 30 to the output 3 which is further connected through a resistor 31 to a terminal which carries a voltage of +45V. The lead 2 is connected to ground. The emitter electrode of thetransistor 4' is connected to ground and the collector electrode is connected to the terminal 6 of the cable 7.The preamplifier input 10 is connected in series with an inductor 14' through a resistor 15' to a terminal which carries a voltage of + 1 2V. A noise current source 29' is shown in parallel with the resistor 15'.

The input 10 is further connected to a control electrode, which is the base electrode, of a npn transistor 32 and to a lead of a feedback resistor 33 whose other lead is connected to the collector electrode of the transistor 32. Through a resistor 34 the collector of the transistor 32 is connected to the terminal which carries the + 1 2V voltage. The emitter electrode of the transistor 32 is connected to the emitter electrode of a pnp transistor 35 the base electrode of which is connected to ground through a resistor 36 and through the parallel arrangement of a resistor 37 and a capacitor 38 to the terminal which carries the +12V voltage. The parallel arrangement (37, 38) provides high frequency decoupling.

The collector electrode of the transistor 35 is connected to the base electrode of a npn transistor 39 and, through a resistor 40, to a terminal which carries a voltage of-i 2V. The collector electrode of the transistor 39 is connected to the terminal which carries the + 1 2V voltage and the emitter electrode is connected through a resistor 41 to the terminal which carries the -1 2V voltage and directly to the outputs 12 and 11.

The circuit shown in Figure 2 operates so that an increase in voltage at the base electrode of the transistor 4' renders this transistor more conductive, so that the voltage at the base electrode of the transistor 32 decreases, which transistor then conducts less current, the same as the transistor 35. The resulting decrease in voltage at the base electrode of the transistor 39 causes this transistor to become conductive to a lesser extent so that a voltage drop occurs at the outputs 12 and 11. When the voltage at the base electrode of the transistor 4' decreases the reverse effect is obtained.

The influence of the inductor 14' is similar to the influence of the coil 14 in the description of the preamplifier of Figure 1 as regards the noise reduction in the noise contribution caused by the resistor 15 (15'). The noise contribution of the resistors 33 and 34 is negligible compared to the contribution by the resistor 15' as the resistor 33 is of a relatively high value and, at the higher signal level at the collector electrode of the transistor 32, the noise contribution of the resistor 34 as compared to the contribution of the resistor 1 5' at the low signal level present there, may be neglected.

The foilowing values are given by way of illustration of a preamplifier as shown in Figure 2: resistor 33 :1 OkQ resistor 34 :1.6kQ resistor 15' :500Q inductor 14t:200yH In practice the signal-to-noise ratio is improved by 1 dB compared to the situation in which the inductor 14' is omitted.

Claims

1. An amplifier arrangement for a television camera comprising a first transistor a control electrode of which is adapted to be coupled to an output of a television pick-up device, and a preamplifier including a second transistor, an output electrode of said first transistor being coupled to an input of said preamplifier which input is connected to a control electrode of said second transistor, an inductor and a resistor being provided in series between the input and a voltage-carrying terminal.

2. An amplifier arrangement as claimed in Claim 1, in which when the resistor has a resistance value of some hundreds of ohms whilst the inductor has an inductance value of the order of some hundred of a millionth of a henry.

3. An amplifier arrangement as claimed in Claim 1, in which when the resistor has a resistance of approximately four hundred ohms the inductor has an inductance of approximately a five hundred millionth of a henry.

4. An amplifier arrangement for a television camera substantially as herein described with reference to the accompanying drawings.

5. A television camera incorporating a pick-up device and an amplifier arrangement as claimed in any of the preceding Claims.

6. A television camera substantially as herein described with reference to the accompanying drawings.