US2222933A - Thermionic amplifier - Google Patents

Description

Nov. 26, 1940. A. p. BLUMLEIN 2,222,933

THERMIONIG AMPLIFIER Filed May 24, 1938 W :5 I a i I 7 I w EE 5 I 4 ;g 2-- i a: 6 "EKD 2 6 l JV g-. i b 9 w INVENTOR ALAN D. BLUMLE/N ATTORNEY Patented Nov. 26, 1940 UNITED STATES PATENT OFFICE 2,222,933 THERMIONIC AMPLIFIER Great Britain Application May 24, 1988, Serial No. 209,651 In Great Britain June 9, 1937 2 Claims. (01. 1'l9171) The present invention relates to thermionic valve circuit arrangements and more especially to so-called variable gain amplifiers.

According to the present invention a variable gain thermionic valve circuit arrangement is provided comprising a thermionic valve having a negative feedback path including an impedance across which feedback potentials are developed and which is adapted to be traversed by the outit) put current of the valve, said impedance comprising an element of which the impedance varies with the amplitude of the signals applied to the valve, whereby the gain of the valve is caused to vary as the signal amplitude varies. Preferably, in a thermionic valve circuit arrangement according to the invention the impedance across which feedback potentials are developed is included between the cathode of the valve and earth. For example, said impedance may comprise a resistance having a diode rectifier connected in shunt therewith the rectifier being arranged to become conductive when the amplitude of the applied signals attains or falls to a certain value, thereby reducing the efiective value of 2 the impedance and increasing the gain of the valve in respect to signals of higher or lower amplitude as the case may be. Alternatively, the diode rectifier arrangement might be replaced by a thermionic valve having a control 80 grid, the potential of which may beadiusted as desired to determine the signal level at which the valve becomes conductive.

The invention has been primarily developed in connection with television systems for the SIS amplification of picture signals which may take the form of varying direct current potentials. In this case, it may be desirable to increase the effect of signals corresponding to white shades or tones in the picture in'proportion to the effect of signals corresponding to gray shades, and it is therefore advantageous to amplify these signals in an arrangement which amplifies the signals corresponding to shades from gray to white to a greater extent than signals corresponding to shades from ;black to gray. The invention has thus been found an important application in this connection, and will be described more fully with reference to the accompanying drawing, chiefly from the point of view of its use in television systems.

In the drawing:

Figure 1 is a diagram showing a thermionic valve arrangement according to the invention such as might be used for repeating picture signals in a television system,

Figure 2 is a curve illustrating the mode of operation of the arrangement according to the invention, and

Figure 3 is the diagram of a preferred embodiment of the invention. '6 Referring to Figure 1, it will be seen that the arrangement therein shown comprises a thermionic valve I of the screen grid type which is arranged with a negative feedback impedance 2 in the cathode-earth lead.

The anode of the valve is connected to a suitable high tension supply through an impedance 3 and the valve is provided with a grid leak resistance 4, the input signals being applied between terminals I and the ouput derived from 15 terminals II respectively.

In shunt with the impedance 2 in the cathodeearth lead of the valve I is arranged a diode rectifier 5 in series with a battery 6 on the one hand and resistance 1 on the other hand, the anode 20 side of the rectifier being connected to the positive terminal of battery 6 and thence through battery 6 to earth, and the cathode side of the rectifier being connected through the resistance 1 to the upper end of the impedance 2 and conse- 25 quently to the cathode of the valve I. This arrangement operates as follows:

Signals in the form of varying direct current potentials are applied to the control grid of the valve I to cause the anode current of the valve 80 to vary in well known manner, signals of high amplitude corresponding to white parts of a picture to be transmitted causing the setting up of low or negative potentials on the grid of the valve I and signals corresponding to black parts of the 35 picture to be transmitted causing the setting up of high or positive potentials on the grid of the valve. As the control grid becomes more positive the anode current of the valve rises so that the potential developed across impedance 2 in the 4 cathode-earth lead of the valve I,' increases, thus raising the cathode potential of the valve which therefore follows the potential of the control grid. The eifective potential between the control grid and the cathode is thus reduced and the 45 valve operates with a gain which is lower than the gain at which it would operate without the impedance 2.

As the potential of the grid fails when signals corresponding to the bright or white parts of the 50 picture appear on the grid the potential at the upper end of the impedance 2, and hence the cathode of the diode 5, becomes more negative with relation to the anode and, if the battery 6 has a suitable value, for some signal level interand consequently increasing the change of potential of the control grid with relation to the change of potential of the cathode of the valve I so that the gain of the valve I increases.

For example, in the circuit shown in Figure 1, for handling television signals, if the value of the resultant negative feedback impedance of the valve I is S, and the total mutual conductance (i. e. anode and screen) of the valve g, it may be shown that the gain of the stage is reduced due to the provision of the negative feedback impedance to a fraction substantially 1 1+gS of the value without the said impedance.

The effect of varying the value 8 of the impedance 2 will thus be evident.

Figure 2 is a curve illustrating the operation of the circuit shown in Figure 1. In thisflgure, input signal amplitudes are represented horizontally and output signal amplitudes are represented vertically, the curve C indicating the relation between input signal amplitudes and output signal amplitudes. It will be seen that the curve C consists substantially of two straight line pertions meeting in the region D. The slope of each of these two straight line portions gives the gain at which the amplifier operates in respect of input signals of amplitude within the range corresponding to the region in which the lines lie. For example, assuming that the amplitudes Ob and Ow represent the extremes of black and white respectively, then for input signals up to a shade of gray for which signals may have an amplitude represented, for example, by 09, the slope of the curve C will be low as indicated, but when signals reach the level 09. then the diode 5 or equivalent device becomes conductive and reduces the value of the impedance 2 in the cathode lead of the valve I to a.- particular level whereby the gain of the circuit in respect of signals of the level 09 and of higher levels increases as indicated by the increased steepness of the portion of the curve C above the region D. Thus, assuming by way of example, that the differences in amplitude between signals for black and gray and gray and white respectively, are equal, the difierence between the amplitudes of the corresponding output signals will not be equal as will be seen by comparing the vertical distances BG and GW. If the gain of the amplifier had remained unchanged throughout its range of operation the amplitude of the output signals corresponding to gray and white respectively would have been equal to GW' which is equal to BG. From Figure 2 it will be obvious that the effect of the valve 5 has been to expand differentially the volume of the signals amplifier by the arrangement of Figure 1 from a volume represented by OW to a volume represented by OW. The point at which differential expansion occurs can be adjusted by arranging for the diode 5 to become conductive at different signal levels. It will be appreciated that curves corresponding to that shown in Figure 2' obtained in practice may show less abrupt changes of slope than that indicated at D.

The invention is not only suitable for expansion of volume, but by reversing the connections of the diode rectifier 5 for example, the rectifier might be made conductive for low input amplitudes when the gridnof the valve I is more positive, but cease tobecome conductive when the pgtil tialdropacross the loading impedance 2 becomes sufficiently high. Thus, for signals of highamplitude i. e. when the potential on the grid of valve I has a low or negative value, the effective impedance in the cathode-lead of the valve I is large. With such an arrangement the gain of'the valve is high for signals of low amplitude and low for signals of high amplitude. It is thus possible, according to the invention, to provide in a'signal transmission channel in which the signal volume which the channel can transmit satisfactorily is restricted, a volume compression arrangement at the transmitter end of the line and a complementary volume expanding arrangement at the receiver end of the line of substantially identical form except for a slight variation in the connections.

An arrangement in which the connections of the diode rectifier 5 are reversed, might also be employed in amplifying television signals before transmission in the case where signals corresponding to white cause the setting up of high or positive potentials on the grid of valve I and. signals corresponding to black cause the setting up of low or negative potentials on said grid.

An alternative preferred form of the invention is illustrated in Figure 3. In this arrangement, the amplifier again comprises a valve I which may be similar to that shown and described in connection with Figure 1. In the case of the arrangement of Figure 3, however, the impedance 2 in the cathode-earth lead of the tube of the valve I is not shunted by a diode but is shunted by a triode valve II of which the anode is connected to a suitable source of high potential, for example the lead to the screen of the valve I. High tension supplies for the anode of the valve I and for the screen of valvel -I and the anode of valve II are derived from a suitable source or sources not shown and of which the negative terminals will be connected to earth. The high tension supply should bereasonably constant and have a low or constant impedance over the whole frequency range including direct current. The bias potential for valve II may be derived from an adjustable tapping point on a potentiometer I 6 shunted across the screen of valve I and earth. With this arrangement by adjusting the bias on the control grid of valve II it is possible to determine the signal level at which this valve becomes conductive. It is therefore possible to control the amount of volume expansion introduced by the amplifier and the region of input amplitude which is expanded. As will be seen, the valve II, like the valve I, includes impedance 2 in its cathode-earth lead, and is thus arranged to have negative feedback. Thus the impedance of valve II, viewed from the point I4 will therefore be substantially the reciprocal of its mutual conductance and of .the order of a few hundred ohms. It may therefore be desirable to include in series with the cathode of valve II a resistance I5, as shown, in order that the valve II may not exert too great a shunting action when it becomes conductive.

It will be appreciated that when the invention is applied to television systems, the direct current component (i. e., average picture brightness) should preferably be present at the input of the circuits in accordance with the invention so that an absolute potential of the grid.always represents a certain picture brightness irrespective of average picture brightness whereby a position on the curve of Figure 2 for a particular brightness remains constant. Hence preferably the supply potentials to circuits should be derived from batteries, neon stabilizers or like devices.

While the invention has been described with reference to arrangements involving a thermionic valve circuit having a thermionic valve impedance for shunting the cathode loading impedance, it may be possible that other impedances variable with applied voltage in shunt with the cathode impedance might be used to change the gain of the amplifier.

I claim:

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. An amplifier circuit comprising a first electron discharge device having at least a. cathode, a control grid, a screen grid and an anode, a second electron discharge device having at least a cathode, a control electrode and an anode, means for connecting the anode of said second discharge device to the screen grid of said first discharge device, means including a resistance for connecting the cathode of said first discharge device to the cathode of said second discharge device, means for maintaining said anodes positive with respect to said cathodes, said last named means including a resistance connected between the cathode of said firstdischarge' device and the negative terminal of a source of potential, means for impressing a signal between the control electrode of said first discharge device and the negative terminal, and an output circuit connected to the anode of said first discharge device.

2. An amplifier circuit comprising a first electron discharge device having a cathode, a control electrode and an anode, means for maintaining the anode positive with respect to the cathode including a resistance connected between the source of current and the cathode, a second electron discharge device having a cathode, a control electrode and an anode, resistance means for connecting the cathode of said first electron discharge device to the cathode of said second electron discharge device, means for maintaining the anode of said second electron discharge device positive with respect to its cathode, means for maintaining the control electrode of said second electron discharge device at a predetermined relatively fixed potential, and means for impressing signal variations between the control electrode and the cathode of said first electron discharge device.

ALAN DOWER BLUMLEIN.

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