497,060

PATENT SPECIFICATION

Application Date: June 9, 1937. No. 15938/37

Complete Specification Left: March 28, 1938.

Complete Specification Accepted: Dec. 9, 1938.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Thermionic Amplifiers

I, ALAN DOWER BLUMLEIN, of 32 Audley Road, Ealing, London, W.5, a British subject, do hereby declare the nature of this invention to be as follows:

The present invention relates to thermionic amplifiers and more especially to so-called variable gain amplifiers.

According to the present invention a variable gain amplifier arrangement is provided comprising a thermionic valve circuit including a valve having negative feed back path, that is to say, including a valve which has a negative feed back impedance inserted effectively in its cathode circuit so that the potential of the cathode varies in correspondence with the potential of the grid, the negative feed back impedance in the cathode circuit of the valve comprising an impedance element of which the impedance varies with the amplitude of the signals applied to the valve, thus varying the gain of the valve as the signal amplitude varies. For example, the negative feed back impedance may be constituted by an impedance having a diode rectifier in shunt which becomes conductive at a particular signal level. Alternatively, the diode rectifier arrangement might be replaced by a thermionic valve having a control grid, the potential of which may be adjusted 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 amplification of picture signals. 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 grey shades, and it is therefore advantageous to amplify these signals in an arrangement which amplifies the signals corresponding to shades from grey to white to a greater extent than signals corresponding to shades from black to grey. The invention has thus found an important application in television systems, and will be described more fully with reference to the accompanying drawings, chiefly from the point of view of its use in such systems.

In the drawings:-

Fig. 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,

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

Fig. 3 is the diagram of a preferred embodiment of the invention.

Referring to Fig. 1, it will be seen that the arrangement therein shown comprising a thermionic valve 1 of the screen grid type which is arranged with a negative feed back 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 output derived from terminals II respectively.

In the shunt with the impedance 2 in the cathode-earth lead of the valve 1 is arranged a diode rectifier 5 in series with a battery 6 on the one hand and resistance 7 on the other hand, the anode 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 7 to the upper end of the impedance 2 and consequently to the cathode of the valve 1. This arrangement operates as follows:

Signals in the form of varying D.C. potentials are applied to the control grid of the valve 1 to cause the anode current of the valve to vary in well known manner, signals corresponding to white parts of a picture to be transmitted set up low or negative potentials on the grid of the valve 1 and signals corresponding to black parts of the picture to be transmitted set up 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 cathode-earth lead of the valve 1, increases, thus raising the cathode potential of the valve which therefore follows the potential of the control grid. The effective potential between the control grid and the cathode is thus reduced and the 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 falls when signals corresponding to the bright or white parts of the 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 intermediate black and white the diode 5 will become conductive; thus the impedance 2 will be shunted and its effective value reduced, thereby reducing the extent to which the variations in potential of the cathode follow those on the control grid, and consequently increasing the change of potential of the control grid with relation to the change of potential of the cathode of the valve 1 so that the gain of the valve 1 increases.

For example in the circuit shown in Fig. 1, for handling television signals, if the value of the resultant negative feed back impedance of the valve 1 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 feed back impedance to a fraction substantially

EQUAT. HERE

 

of the value without the said impedance.

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

Fig. 2 is a curve illustrating the operation of the circuit shown in Fig. 1. In this figure, 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 portions 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 grey for which signals may have an amplitude represented, for example, by Og, the slope of the curve C will be low as indicated, but when signals reach the level Og, then the diode 5 or equivalent device becomes conductive and reduces the value of the impedance 2 in the cathode lead of the valve 1 to a particular level whereby the gain of the circuit in respect of signals of the level Og 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 grey and grey and white respectively, are equal, the difference 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 grey and white respectively would have been equal to GW1 which is equal to BG. From Fig. 2 it will be obvious that the effect of the valve 5 has been to expand differentially the volume of the signals amplified by the arrangement of Fig. 1 from a volume represented by OW1 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.

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, but cease to become conductive when the potential drop across the loading impedance 2 becomes sufficiently high. Thus, for signals of high amplitude the effective impedance in the cathode lead of the valve 1 is small. 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 alternative preferred form of the invention is illustrated in Fig. 3. In this arrangement, the amplifier again comprises of valve 1 which may be similar to that shown and described in connection with Fig. 1. In the case of the arrangement of Fig. 3, however, the impedance 2 in the cathode-earth lead of the tube of the valve 1 is not shunted by a diode but is shunted by a triode valve 11 of which the anode is connected to the screen of the valve 1. High tension supplies for the anode of the valve 1 and for the screen of valve 1 and the anode of valve 11 are derived from a suitable source or sources not shown. The high tension supply should be reasonably constant and have a low or constant impedance over the whole frequency range including direct current. The bias potential for valve 11 may be derived from an adjustable tapping point on a potentiometer shunted across the anode and cathode of valve 1. With this arrangement by adjusting the bias on the control grid of valve 11 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 11, like the valve 1, includes impedance 2 in its cathode-earth lead, and is thus arranged to have negative feed back. Thus the impedance of valve 11, viewed from the point 14 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 11 a resistance 15, as shown, in order that the valve 11 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 D.C. 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 Fig. 2 for a particular brightness remains constant. Hence preferably the supply potentials to circuits should be derived from batteries, neon stabilisers 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.

Dated this 8th day of June, 1937

F. W. Cackett

Chartered Patent Agent

Complete Specification

Improvements in or relating to Thermionic Amplifiers.

I, ALAN DOWER BLUMLEIN, of 32 Audley Road, Ealing, London, W.5, a British subject, do hereby declare the nature of this invention and to what manner the same is to be performed to be particularly described and ascertained in and by the following statement:

The present invention relates to thermionic valve circuit arrangements and more especially to so-called variable gain amplifiers.

The object of the invention is to provide an arrangement which may be used for the amplification of picture signals in television systems and which is provided with means of varying the gain of the arrangement in accordance with the instantaneous level of the applied signal.

Television signals usually take the form of varying d.c. potentials, the instantaneous value of the potential corresponding to the value of the picture brightness which it is required to transmit. Thus, any arrangement by which the picture signals are repeated with variable gain should respond without lag to changes in the instantaneous level of the applied signals. Thus, an arrangement for amplifying or repeating picture signals with variable gain must differ in character from an arrangement used in repeating signals such as speech signals. In the latter case the signals are represented by an alternating current or voltage which varies in amplitude and frequency in accordance with the instantaneous level and frequency of the original tones which are required to be transmitted, and any arrangement by which speech signals are repeated with variable gain is required to have its gain varied substantially in accordance with the mean amplitude of any signal voltage or current variation occurring, but not in accordance with the instantaneous value of the signal voltage or current, in order that the arrangement may repeat the speech signals without distortion.

As indicated above, the invention has been primarily developed in connection with television systems for the amplification of picture signals. In this case, it may be desirable to increase the effect of signals corresponding to white shades or tones in the pictures in proportion to the effect of grey signals corresponding to grey shades, and it is therefore advantageous to amplify these signals in an arrangement which amplifies the signals corresponding to shades from grey to white to a greater extent than signals corresponding to shades from black to grey.

The object of the invention is to provide an arrangement which may be used for the amplification of picture signals of the kind mentioned and which is provided with means for varying the gain of the arrangement in accordance with the instantaneous level of the applied signal.

According to the invention, a thermionic valve circuit arrangement is provided including a thermionic valve, said valve having associated with it an impedance common to the input circuit and the output circuit of the valve, said impedance being arranged in such manner as to be traversed by the output current of the valve to develop feed-back potentials which are applied in a negative sense to said input circuit, and said impedance being of such a character that its value varies in dependence on the instantaneous value of the current therethrough in such manner that the effective gain of the valve is caused to vary in dependence on the instantaneous value of signals applied to the input. For example, said impedance may comprise a resistance having a diode rectifier being arranged to become conductive when the instantaneous amplitude of the applied signals attains or falls to a certain value, thereby reducing the effective value of the impedance and increasing the gain of the valve in respect of 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 grid, the potential of which may be adjusted as desired to determine the signal level at which the valve becomes conductive.

The invention and the method of carrying the same into effect will be described more fully with reference to the drawings accompanying the Provisional Specification.

Fig. 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,

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

Fig. 3 is the diagram of a preferred embodiment of the invention.

Referring to Fig. 1, it will be seen that the arrangement therein shown comprising a thermionic valve 1 of the screen grid type which is arranged with a negative feed back 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 output derived from terminals II respectively.

In the shunt with the impedance 2 in the cathode-earth lead of the valve 1 is arranged a diode rectifier 5 in series with a battery 6 on the one hand and resistance 7 on the other hand, the anode 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 7 to the upper end of the impedance 2 and consequently to the cathode of the valve 1. This arrangement operates as follows:

Signals in the form of varying D.C. potentials are applied to the control grid of the valve 1 to cause the anode current of the valve 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 1 and signals corresponding to black parts of the 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 cathode-earth lead of the valve 1, increases, thus raising the cathode potential of the valve which therefore follows the potential of the control grid. The effective potential between the control grid and the cathode is thus reduced and the 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 falls when signals corresponding to the bright or white parts of the 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 intermediate black and white the diode 5 will become conductive; thus the impedance 2 will be shunted and its effective value reduced, thereby reducing the extent to which the variations in potential of the cathode follow those on the control grid, and consequently increasing the change of potential of the control grid with relation to the change of potential of the cathode of the valve 1 so that the gain of the valve 1 increases.

For example in the circuit shown in Fig. 1, for handling television signals, if the value of the resultant negative feed back impedance of the valve 1 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 feed back impedance to a fraction substantially

EQUAT. HERE

 

of the value without the said impedance.

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

Fig. 2 is a curve illustrating the operation of the circuit shown in Fig. 1. In this figure, 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 portions 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 grey for which signals may have an amplitude represented, for example, by Og, the slope of the curve C will be low as indicated, but when signals reach the level Og, then the diode 5 or equivalent device becomes conductive and reduces the value of the impedance 2 in the cathode lead of the valve 1 to a particular level whereby the gain of the circuit in respect of signals of the level Og 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 grey and grey and white respectively, are equal, the difference 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 grey and white respectively would have been equal to GW1 which is equal to BG. From Fig. 2 it will be obvious that the effect of the valve 5 has been to expand differentially the volume of the signals amplified by the arrangement of Fig. 1 from a volume represented by OW1 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.

The invention is not only suitable for expansion of amplitude range, but by reversing the connections of the diode rectifier 5 for example, the rectifier might be made conductive for low input amplitudes when the grid of the valve 1 is more positive, but cease to become conductive when the potential drop across the loading impedance 2 becomes sufficiently high. Thus, for signals of high amplitude, i.e., when the potential of the grid of valve 1 has a low or negative value, the effective impedance in the cathode lead of the valve 1 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 range compression arrangement at the transmitter end of the line and a complementary range expanding arrangement at the receiver end of the line of substantially identical form except for a slight variation in the connections.

The arrangement in which the connection 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 1 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 Fig. 3. In this arrangement, the amplifier again comprises of valve 1 which may be similar to that shown and described in connection with Fig. 1. In the case of the arrangement of Fig. 3, however, the impedance 2 in the cathode-earth lead of the tube of the valve 1 is not shunted by a diode but is shunted by a triode valve 11 of which the anode is connected to a suitable source of high potential, for example the lead to the screen of the valve 1. High tension supplies for the anode of the valve 1 and for the screen of valve 1 and the anode of valve 11 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 be reasonably constant and have a low or constant impedance over the whole frequency range including direct current. The bias potential for valve 11 may be derived from an adjustable tapping point on a potentiometer shunted across the screen of valve 1 and earth. With this arrangement by adjusting the bias on the control grid of valve 11 it is possible to determine the signal level at which this valve becomes conductive. It is therefore possible to control the amount of expansion introduced by the amplifier and the region of input amplitude which is expanded. As will be seen, the valve 11, like the valve 1, includes impedance 2 in its cathode-earth lead, and is thus arranged to have negative feed back. Thus the impedance of valve 11, viewed from the point 14 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 11 a resistance 15, as shown, in order that the valve 11 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 D.C. 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 Fig. 2 for a particular brightness remains constant. Hence preferably the supply potentials to circuits should be derived from batteries, neon stabilisers 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 the output current might be used to change the gain of the amplifier.

I am aware that in British Patent Specification No. 479,485 which was not published at the date of the present application but which was lodged pursuant to an application dated prior thereto, there is claimed the provision in an audio amplifier or like apparatus including an amplifier valve and a load circuit, of a circuit connected to the said load circuit and comprising two or more elements having an impedance dependent in value upon the current flow therethrough, the value of the said impedance being only slowly variable with time, and degenerative feed-back connections for supplying the signal across one or more of said elements to the input circuit of the said amplifier whereby the amount of degeneration and hence the amplifier gain is dependent upon the current flow through the said impedance.

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. A thermionic valve circuit arrangement, said arrangement including a thermionic valve, said valve having associated with it an impedance common to the input circuit and the output circuit of the valve, said impedance being arranged in such manner as to be traversed by the output current of the valve to develop feed-back potentials which are applied in a negative sense to said input circuit, and said impedance being of such a character, that its value varies in dependence on the instantaneous value of the current therethrough in such manner that the effective gain of the valve is caused to vary in dependence on the instantaneous value of signals applied to the input.
  2. A thermionic valve circuit arrangement according to Claim 1 in which said impedance comprises a resistance having a diode rectifier connected in shunt therewith the rectifier being arranged to become conductive when the value of said output current falls to a certain level thereby reducing the effective value of the impedance and increasing the effective gain of said valve in respect of signals which give rise to an output current lower than said level.
  3. A thermionic valve circuit arrangement according to Claim 1 in which said impedance comprises a resistance having a diode rectifier connected in shunt therewith the rectifier being arranged to become conductive when the output current rises above a certain level thereby increasing the effective gain of said valve in respect of signals which set up an output current having a level higher than said level.
  4. A thermionic valve circuit arrangement according to Claim 1 in which the cathode of said thermionic valve is connected to the cathode of a further thermionic valve to a control electrode of which a bias potential is applied, and the anode of which is connected to earth through a source of high potential, whereby the potential variation of the cathode of the first mentioned valve arising from the variation of the current through said common impedance is applied to the cathode of said further valve, which valve is thereby caused to become conductive when said current falls to a predetermined level depending on the value of said bias potential thereby increasing the effective gain of the first mentioned valve in respect of signals which give rise to an output current lower than the said predetermined level.
  5. An arrangement according to the preceding claims for amplifying or repeating picture signals which take the form of varying direct current potentials, the arrangement being such that signals corresponding to shades in the picture to be transmitted from grey to white are amplified to a greater extent than signals corresponding to shades from black to grey.
  6. A thermionic valve circuit arrangement arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figures 1 and 3 of the drawing accompanying the Provisional Specification.

Dated this 26th day of March, 1938

F. W. Cackett

Chartered Patent Agent