US2552136A

US2552136A - Linear amplifier system

Info

Publication number: US2552136A
Application number: US678835A
Authority: US
Inventors: Charles A E Beurtheret
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1945-06-13
Filing date: 1946-06-24
Publication date: 1951-05-08
Anticipated expiration: 1968-05-08

Description

y 1951 c. A. E. BEURTHERET 2,552,136

LINEAR AMPLIFIER SYSTEM Filed June 24, 1946 2 Sheets-Sheet 1 Fig. I.

LOAD

Inventor-i Charles A.EZ. Beurthereb,

Hi5 Attorney.

y 3, 1951 c. A E. BEURTHERET 2,552,136

LINEAR AMPLIFIER SYSTEM Filed June 24, 1946 2 Sheets-Sheet 2 MOD.

VOLTAGE Inventor: Charles AB. Beurthereb,

His Att. orney.

Patented May 8, 1951 UNITED to General Electric Co New York STATES .PATENT OFFICE Paris, France, assignor mpany, a corporation of Application June 24, 1946, Serial No. 678,835 In France June 13, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires June 13, 1965 4 Claims.

My invention relates to electron tube amplifiers and more particularly to eliminating the distortion and tendency towards instability associated with the varying control electrode cur rent flow of such amplifiers.

It is an object of my invention to provide improved means to eliminate the undesirable effects of control electrode current flow in electron discharge devices.

Another object of my invention is to eliminate the undesirable consequences of control electrode current flow in electron discharge devices in a manner suitable for use in amplifier circuits.

A further object of my invention is to provide improved means capable of preventing the undesirable effects associated with control electrode current flow in an electron discharge device amplifier, which means inherently tend to cause the input impedance of the amplifier to appear constant.

Another object of my invention is to provide an improved means taking advantage of the characteristics of pentode and tetrode electron discharge devices to cause the input impedance of an amplifier to appear constant.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, both as to its organization and method of operation, may best be understood by reference to the following description taken in connection with the accompanying drawings in which Fig. 1 shows the schematic diagram of one embodiment thereof, Fig. 2 illustrates the operation of the circuit of Fig. 1, and Figs. 3 and 4 show other embodiments of the invention.

Generally stated, in accordance with this invention an auxiliary current path is provided in parallel with the cathode-control electrode current path of an electron discharge device. This current path is arranged to have characteristics similar to the conduction characteristics of the control electrode of the discharge device but so proportioned thereto as to cause the impedance of the combined system as seen by the preceding amplifier stage to be a constant value. That is, as the control electrode current of the discharge device decreases in proportion to the value associated with the presence of a constant impedance,

the auxiliary current path provides a compensating current increase so that the net effect as seen from the input circuit is similar to that of a constant impedance. In the particular embodiments of this invention described herein the current in the auxiliary path is caused to vary in accordance with the audio frequency voltage cycle of an audio frequency amplifier, thus compensating for inherent variations in the input impedance of that amplifier over the audio frequency voltage cycle.

In the circuit of Fig. 1, electron discharge device l is connected to act as an amplifier, as for example, a class B linear amplifier for amplifying an audio frequency wave. A similar elec-- tron discharge device and its associated circuits, shown generally as unit 2, are connected to operate in push-pull with device I, thus to reduce distortion in the amplified Wave and increase the power output of the system. Cathode-anode space path voltage for these devices is derived from unidirectional voltage source 5 through the primary winding of transformer 3. Audio frequency output voltage is applied to load 4 from the secondary winding of transformer 3.

Control electrode-cathode space path voltage for device i is provided from a driver stage shown generally at 6. In the case of a linear audio frequency amplifier, for example, this voltage is of audio frequency and has magnitude varying in accord with an intelligence. For purposes of iilustration driver stage 6 is shown as of conventional construction, using triode electron discharge device 8 having cathode-anode space path voltage derived from unidirectional voltage source 6! through the primary winding of transformer 9. Output voltage is applied to the junction of condensers i2 and is from the secondary winding of this transformer, thus to vary the control electrode-cathode space path potential of device i and the cathode-anode space path potential of device i in accord with the audio frequency voltage.

It is the purpose of electron discharge device l to improve the fidelity of reproduction of voltage across transformer 9 by causing the input impedance of the amplifier as seen by that transformer to be substantially constant. To this end, the cathode-anode space path of device 1 is connected through capacitor l3, resistance M, and capacitors i9 and 23 to ground so that the impedance seen by transformer 9 is influenced by the cathode-anode space path impedance of device l. Control electrode bias voltage for device is derived from unidirectional voltage source 2i through the circuits including potentiometer 22, resistance ll, and choke coil l6, thus providing adjustable means to establish the optimum unidirectional bias voltage at that device. Resistance 20 and capacitor l9 provide control electrode-cathode bias voltage for device i whereas choke coil I provides a current path for direct current flow from the common terminal of resistance i i and condenser l3 to ground while preventing flow of audio frequency current. Capacitor l8 by-passes audio frequency current from the cathode of device 1 to the movable terminal. of potentiometer 22.

It is the purpose of resistance H to cause the control electrode-cathode space path voltage of device i to vary in accordance with the value of control electrode current flow in device I. Thus, when current flow from control electrode of device I increases, the control electrode of device 1 becomes more negative relative to the cathode thereof and reduces the cathode-anode space current flow in device 7 a corresponding amount, thus causing less current flow to capacitor l3 and compensating for the additional cur- 0 rent flow through capacitor 52 associated with the increased control electrode current flow in device I. If the control electrode current iiow in device I increases in the opposite direction, the reverse efiect takes place and the cathode-anode current fiow of device 1 accordingly increases, thus compensating for the decreased current flow to capacitor l2.

While the current flow through device 1 is in one direction only (from anode to cathode) this current charges condenser l3 during the alternating voltage cycle in the same manner as control electrode current flow of device I charges (or discharges) condenser l2. This charge leaks oil through choke coil [5, the leakage current being substantially constant over the voltage cycle because of the inductance of choke H3. The value of the successive charges of condenser l3, and hence the alternating current flow through that unit, is determined by the control electrode-cathode Voltage at device 7 so that variations in the voltage drop of resistance I! alter the effective current flow through condenser 13 and hence the total efiective impedance seen by unit 6.

Fig. 2 illustrates the operation of the circuit of Fig. 1. In Fig. 2, curve POABCDE represents the control electrode current of device l as a function of the magnitude of the control electrode voltage. As the magnitude of this voltage is increased from zero, no control electrode current flows until point 0 i reached. At this point the bias voltage is overcome and the control electrode becomes positive with respect to the cathode, thus causing control electrodecathode space current to begin to flow. The control electrode-cathode space current then increases as the applied voltage is increased until point A is reached. At this time, secondary emission and other effects cause reverse current flow from the control electrode to such a degree that the normal increase in control electrode-cathode space current flow is masked and the net current flow decreases. Between point B and point D these secondary effects predominate to such a degree that the control electrode current flows in direction opposite the normal control electrode-cathode current flow (that is current flows to the control electrode rather than from it). Beyond point C, however, the increased normal control electrode-cathode current flow exceeds the changes in secondary emission and other effects so that increased control electrode current flow is associated with increased positive control electrode-cathode voltage.

The control electrode-cathode space current curve shown in Fig. 2 has two main undesirable effects on operation of the amplifier. In the region ABC, the space current decreases as the applied voltage increases in magnitude thus giving a characteristic analogous to a negative resistance and causing the amplifier to tend to produce low frequency parasitic oscillations in the control electrode circuits. While these oscillations can be eliminated or substantially reduced by using a resistance in the control electrode circuit, the power requirements at large levels of voltage from unit 8 are greatly increased and additional driving power from that unit is accordingly required. A second undesirable characteristic of the curve of Fig. 2 resides in the relatively great control electrode-cathode space current flow at large values of voltage in circuit 9. Since this current is very large in proportion to the magnitude of voltage at transformer 9, the impedance seen by that unit i decreased and distortion accordingly introduced into the system.

It is a well-known characteristic of electric circuits that any tendency for non-linearity in the impedances involved causes production or alternating voltages and currents having frequencies not contained in the exciting voltage. However, when linear impedances are provided, this effect does not take place and distortion is accordingly avoided. In accordance with the principles of this invention, such a linear impedance is substituted for the non-linear impedance representing the control electrode-cathode space path impedance of device I and the consequences of this non-linear impedance thereby avoided. To this end, current flow in addition to that of the actual control electrode current is produced and the value of this additional current varied in such fashion as to produce total current flow of magnitude substantially equal to the current flow existing in the presence of a constant cathodecontrol electrode impedance, thus causing driver 6 to operate as if a single constant impedance load is connected thereto.

In the simplified curves of- Fig. 2, the straight line PFIKG represents the current which would be absorbed by the tube 1 if its control grid were to remain at fixed potential. Curve FHIJKL represents the required current variation to compensate for the previously-mentioned variations in grid current of tube I.

The apparatus of Fig. 1 acts to cause the impedance seen by transformer 9 to appear substantially constant by causing the sum of the cathode-anode space current flow in device I and the control electrode current flow in device I to produce a total current in the secondary winding of that transformer which tends to be proportional to voltage. This is accomplished by suitable choice of characteristics of device 1 and resistances I l and ll, so that when the control electrode current of device I increases out of proportion to the voltage, current in device l decreases, and when the control electrode current of device decreases out of proportion to the voltage, current in device 1 increases.

Fig. 3 shows a modification of the circuit of Fig. 1 wherein the circuit automatically adjusts itself to maintain an apparently constant impedance across transformer 9. In this circuit, components representing similar units shown in Fig. 1 are identified with corresponding reference figures. Control electrode-cathode bias voltage for device l is provided by unidirectional voltage source 2| and potentiometer 22 through the circuit including resistance 24 and choke coil l6. Control electrode-cathode space path voltage for compensating electron discharge device 26 is de rived from the voltage drop across resistance 24 and a portion of potentiometer 22. Unidirectional control electrode-cathode space path voltage for that device is obtained from cathode resistance 28 and by-pass condenser 29. Cathodeanode space current flow in device 26 passes through the current path including resistance 39, resistance 28, potentiometer 22 and by-

pass capacitors

29 and 23. The moving terminal of potentiometer 22 is maintained at ground audio frequency potential by capacitors El and 23.

In the circuit of Fig. 3 resistance 24 is traversed by two currents, one being the cathode-anode space current of device 26 and the other being a current determined by the control electrode current of device I. Thus, when the control electrode of device I tends to draw increased current, the current flow through resistance 24 is likewise increased and the control electrode of device 25 becomes more negative with respect to the oathode, thereby reducin the current flow therethrough. This reduces the current through resistance 24 to the extent of the decreased current fiow through device 26, thus producing a tendency for increased current fiow in that tube. This feedback effect causes the system automatically to tend to maintain the total current flow in resistance 24 substantialy constant when any one voltage appears across transformer 9. The value of this constant current is substantially proportional to the voltage from transformer 9 so that the impedance seen by that unit is constant over the normal range of operating conditions.

The operation-of the circuit of Fig. 3 is analogous to that of a feedback amplifier or a closed cycle control system and the performance is improved as the amplification of device 26 is increased. In some instances it may be desirable further to improve the operation of the circuit by providing an auxiliary amplifier to increase the control electrode-cathode potential of device 26 above the voltage drop across resistance 24, thus in effect increasing the amplification in the feedback path.

Fig. 4 shows a modification of this invention for use in connection with a tetrode or pentode electron discharge device, and taking advantage of the high amplification factor of these devices. In the figure, driving power for device is obtained from amplifier 5 in the same manner as shown in Figs. 1 and 3. Likewise, unidirectional control electrode-cathode bias voltage is obtained from unidirectional voltage source 2|, potentiometer 22, and capacitor 23, in the same manner as in the case of the circuit of Fig. 1. Compensating current flow is achieved by reason of the cathodeanode current fiow through electron discharge device 3| which is connected to draw current depending on the current fiovv in the control electrode-cathode space path of device I. This current follows the path including resistance- 32, device 3|, and capacitor 23, to ground. The magnitude of this current is dependent on the voltage drop across resistance 35 since this voltage establishes the control electrode-cathode voltage of device 3|. The voltage drop of resistance 35 is in turn dependent upon the cathode-anode space current flow of electron discharge device 36, this current following the path that may be traced through resistance 31, source 38, potentiometer 22, and

resistances

39 and 34. The value of this current depends on the cathode-control electrode space path voltage applied to device 36 and consequently varies in accordance with the voltage drop across resistance 40 and the variations in current flow in electron discharge devices 4| and 42.

It is the purpose of electron discharge devices 4| and 42 to control the voltage drop across resistance 40 in accordance with two voltage components. One of these components is the actual voltage applied from unit 6 and the other component is a unidirectional voltage of value determined by the sum of the control electrode current of device I and the cathode-anode current flow in device 3|. The former component of voltage is applied to electron discharge device 42 from source 43 which may, for example, be one of the audio frequency amplifier stages prior to unit 6. The latter component of voltage is applied to the control electrode-cathode space path of device 4| from the voltage drop across resistance 39. Resistance as and condenser 45 provide control electrode-cathode bias voltage for devices 4| and 42. Cathode-anode space path current in these devices flows through the current path including resistance 46, devices 4| and 42, resistance 44, and potentiometer 22, the unidirectional voltage causing this current fiow being derived from unidirectional voltage source 2|.

The circuit of Fig. 4., at any one value of voltage of source &3, tends to maintain a constant value of current flow through resistance 39 by reason of the feedback effects due to electron discharge devices 4|, 35, and 3| and the associated circuits. If, for example, current flow in resistance 39 increases, the control electrode of device 4| is made more negative relative to the cathode thereof, thus decreasing the space current therethrough and reducing the voltage drop in resistance 45. The anode of device 4| accordingly becomes more positive and increased positive potential is applied to the control electrode of device 36 by reason of condenser ll. This action takes place because condenser 47 is made sufficiently large to prevent any significant charging thereof in a short period of of time. Device 3t accordingly conducts more current and the voltage drop across resistance .25 is increased, thus causing the control electrode of device 35 to become more negative and decreasing the space current therethrough. Due to the amplification of devices 4|, 36, and 35, this space current decrease is relatively far greater in magnitude than the space current change corresponding to the initial variation in voltage drop across resistance 39. Hence the system acts as a feedback or closed cycle control system to maintain constant the current flow in that resistance.

The current flow in resistance 39 is determined by the sum of the cathode-anode space current flow in electron discharge device 3| and the control electrode-cathode space current flow in electron discharge device I, this resistance acting in the same manner as resistance 24, Fig. 3, to measure the relative control electrode current flow of device I Inasmuch as the regulating system acts to maintain constant this current irrespective of changes such as variations in the characteristic of device I tending to change the value thereof, the system maintains constant the audio frequency current flow in the secondary of transformer 9 and hence controls the impedance seen by driver 6.

l'he tendency of the circuits of Fig. 4 (in the absence of device G2) to maintain a single constant value of current iiow in resistance 33 is independent or" the value of voltage across transformer circuit This results from the well known characteristic of pentode and tetrode electron discharge devices to draw a constant current flow over a great variation in cathode-anode space path voltage. Hence the value of current in resistance maintained constant by the system is the same irrespective of variations in the voltage across transformer a due to variations of the audio frequency voltage. Thus this circuit (in the absence of device 32) does not cause an apparently constant impedance load on unit 5 but on the contrary causes constant value of current fiow from that unit.

It is the function of the circuits including electron discharge device :2 to vary the value of the constant current flow in resistance in accordance with the value of voltage across transformer 9, thus to cause electron discharge device i and the associated circuits to appear as a constant impedance drawing current proportional to voltage. This action is accomplished by varying the space current flow in device l2 in accordance with the audio voltage derived from unit 43. If, for example, the audio voltage increases in the positive direction, current flow through device 42 is increased, device 35 accordingly draws less current, and device 3i is made more conductive so that for any one value or" current flow in resistance the corresponding current flow in device 3! is increased. Conversely, the value of current flow in resistance 39 maintained constant by the action oi

devices

42, 36, and 3! is varied in accordance with the value of current flow in device Since an inc eased positive voltage at device 32 requires greater voltage drop in resistance 39 to provide the constant current fiow maintain-ed, the effect of increased positive voltage from unit 63 is to increase the current flow in resistance 38 maintained by the system and hence cause the audio frequency current in condenser 22 to increase in a manner similar to the increase obtainable in the presence of a constant impedance load.

It will be understood that, While I have described herein a control circuit arranged to alter the current flow in only one half of a push-pull circuit, it is considered preferable to apply the system to provide similar control with respect to the other half of the circuit. 'Une method of accomplishing this result is to provide an identical circuit appropriately connected to alter the operation of the other portion of the push pull circuit (unit 2, Figs. 1, 3, and 4). In another method of accomplishing this result, the control circuits may be partially combined, as for example by using a common source 2 l.

While I have illustrated particular embodiments of my invention, it will be understood that I do not wish to be limited thereto since various modifications both in the circuit arrangements and the instrumentalities employed may be made, and I contemplate by the appended claims to cover any such modifications and alternative con- 8 structions as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A linear amplifier system comprising a source of signal potentials to be amplified, a main electron discharge amplifying device having a grid circuit coupled to said source and an output circuit, said grid circuit including a series impedance through which grid current flows when said device draws grid current, an auxiliary electron discharge device having an anode, a cathode and a control electrode, means connecting the anode-to-cathode path of said auxiliary device in parallel to said grid circuit for signal potentials, and means for varying the control electrode-tocathode voltage of said auxiliary device in response to variations in current through said impedance, said control electrode being driven negatively in response to an increase in said current and vice versa, whereby the anode current drawn by said auxiliary device varies inversely with the grid current drawn by said main device.

2. A linear amplifier system comprising a main amplifier having grid and output c"cuits, said grid circuit including a series impedance and a source of negative operating bias potential, a source of signals to be amplified coupled to said grid circuit, an auxiliary amplifier having an anode circuit connected in parallel to said grid circuit and having a control electrode, said anode circuit connected so as to pass no grid current means for impressing a negative operating bias potential on said control electrode, and means to vary the net potential or" said control electrode inversely with the current in said impedance, whereby the anode current drawn by sai' auxiliary amplifier varies inversely with grid current drawn bysaid main amplifier.

3. A linear amplifier system comprising a source of signal potentials to be amplified, a main electron discharge amplifying device having a grid circuit coupled to said source and an output circuit, said grid circuit including a series impedance through which grid current flows when said device draws grid current, an auxiliary electron discharge device having an anode circuit in shunt to said grid circuit and a control electrode circuit in shunt to said impedance, said impedance car- 1 rying both the grid current of said main device and the anode current of said auxiliary device, said auxiliary device being connected to draw decreased anode current in response to increased current in said impedance and vice versa, thereby tending to maintain the total current through said impedance substantially constant.

4. A linear amplifier system comprising a main amplifier having grid and output circuits, said grid circuit including a series impedance and a source of negative operating bias potential, a source of signals to be amplified coupled to said rid circuit, an auxiliary amplifier of the type including a screen grid, said auxiliary amplifier having an anode-cathode circuit connected in parallel to said grid circuit and having a control electrode, means impressing a negative operating bias potential on said control electrode, a third amplifier having an anode-cathode circuit coupled to said control electrode and having a control grid, means developing a first control voltage proportional to current in said impedance, means developing a second control voltage proportional to the signals from said source, and means to impress said control voltages concurrently on said control grid, said voltage each causing current changes in said impedance which are degenerative with respect to changes in grid current in said main amplifier.

CHARLES A. E. BEURTHERET.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Burton May 3, 1932 Number Number Number 10 396,143

Name Date Ballou Jan. 14, 1936 Fay Mar. 10, 1936 Numans May 16, 1939 Maxwell Dec. 19, 1944 Goodale May 18, 1948 FOREIGN PATENTS Country Date Great Britain Aug. 3, 1933