US2361198A - Feedback amplifier - Google Patents

Description

0st. 24, 1944. R. N. HARMON m1 2,361,198

FEEDBACK AMPLIFIER Filed June 12, 1942 WITNESSES: INVENTORS.

ATTORNEY Patented Oct. 24, 1944 FEEDBACK AMPLIFIER Ralph N. Harmon and Fred W. Fischer, Baltimore, Md., assignors to Westinghouse Electric & Manufacturing Company, East'llttsburgh,

Pa... a corporation of Pennsylvania Application June 12, 1942, Serial No. 446,748

3 Claims.

This invention relates to wave translating systems, and particularly to amplifiers utilizing inverse feedback for suppressing the tendency of distortion.

The invention refer particularly to audio frequency amplifiers having a large power output used for modulating radio transmitters at high level modulation.

It is a particular object of the invention to pointed out in particularity by the appended claims and taken in connection with the accompanying drawing, in which the single figure shows a schematic circuit of apush-pull amplifier having component elements arranged in accordance with this invention.

Amplifiers of the push-pull type are known in the art and extensively used because of various advantages inherent in a push-pull'circuit. It is generally acknowledged that in a transformer coupled push-pull amplifier, there is no current at signal frequency flowing through the source of plate power, which means that a push-pull amplifier produces no regeneration even when there is a plate impedance common to the power stage and other stages. It is also known that the cancelling effect of the load impedances in a push-pull amplifier provides a reduction of hum, and particularly a cancellation of even harmonics and even-order of harmonic frequencies. The teachings of the prior art are found to be true, in general, for a wide variety of applications to which amplifiers of the push-pull type are parvticularly suited. It is true also for various types of operation of vacuum tubes, and particularly manifest in the operation referred to as class A wherein there is no grid current because the signal voltage isnot allowed to exceed the bias required to maintain operation of the tubes on the straight-line portion of their characteristic.

Aside from the advantages of a push-pull amplifier with respect to cancellation of even-harmonics, a further reduction of distortion due to odd-order of harmonics may be obtained by an inverse feedback circuit in a variety of forms, as described by H. S. Black, in Electrical Engineering, January, 1934. In these circuits, the signalling waves including the frequency range of the fundamental or applied waves are so fed from the output to the input of the amplifier as to reduce their output amplitude and render modulation or. distortion generated in the amplifier less than without feedback. Combining the inherent advantages of a push-pull circuit with the advantages of a feedback circuit in inverse phase, the fidelity of an amplifier can be greatly enhanced. To a certain extent, these circuits in their form known at present will fulfill their function within a comparatively wide output power obtained from an amplifier.

. In certain applications, particularly for modulation of broadcast transmitters, the amplifiers are called upon to deliver very large amounts of power. By large amount is meant several kilowatts of audio frequency energy. In designing amplifiers for such large power output, it was found that the fundamental characteristics of the push-pull circuit do not hold above certain power levers. In order to handle large power, large circuit components are necessary; large iron-core inductances, particularly large output transformers, which have, due to their size, certain inherent disadvantages. The advantages of the push-pull circuit above described depend directly on the electrical characteristics of the com ponents, namely, the transformers utilized for coupling the amplifier stages. In these transformers, it is important that a perfect balance shall be maintained between the windings which divide the two sides of the amplifier, and it is important that the, leakage reactance should be reduced to the minimum. These components are the essential elements which cooperate in the push-pull circuit for the elimination of evenharmonics and for the suppression of regeneration. It was found that for amplifiers at low power level, this can easily be accomplished by the careful design of the coupling impedances. For amplifiers operating at average output levels, the transformers do not create a special problem. On the other hand, for amplifiers operating at several thousands of watts output, which must have very large output transformers weighing several tons instead of several pounds as for low level amplifiers, the problem of reducing leakage reactance faces the designer. In transformers of such size, as a matter of fact, even-harmonics will not balance out emciently at the higher audio frequencies because of the physical diiflculty of reducing leakage reactance between windings caused by high voltage insulation and large powers handled.

The large power handling in an amplifier of the push-pull type is generally restricted to the output stage, which must be driven satisfactorily by the push-pull driver stage. The transformer used in the driver stage need not handle higher power than is needed for full excitation of the and 1' respectively return through suitable bias output stage. Consequently, the coupling transformer of the driver stage may be of such size that the problem of reducing leakage reactance can be satisfactorily solved by proper design. In other words, the driver transformer will inherently cancel the even-order of harmonics. We have now the situation whereby in the output transformer, even harmonic voltages are present, and

in the driver stage, the output circuit will cancel all even harmonics. If we desire to employ inverse feedback to improve the output wave form of the amplifier and by this means reduce the distortion due to all harmonics, even or odd, we are faced with the problem of feeding the inverse voltage from the output stage to the input stage in such manner that it shall also be effective for the even harmonics present in the output circuit of the amplifier. Assuming that the feedback circuit. is capable of delivering to the driver stage at a point suitable therein, voltages in inverse phase at all order of harmonics, that is, also at the even harmonics, the correctiveeflect of this feedback circuit will be nullifled in the output circuit of the driver stage as long as the latter inherently cancels out even harmonics. In other words, the driver stage will not feed to the output stage voltages derived from the output stage at even harmonics.

Under such conditions the advantages of inverse feedback are lost and even-harmonic distortion cannot be converted unless means are provided to permit the connective feed-back voltage to be impressed on the grids of the output tubes. By the circuit arrangement in accordance with this invention, as will be seen, it is possible to apply reverse feedback at evenharmonics as well as at odd-harmonics.

A better understanding can be had of the problems which this invention. is set out to solve by first considering the circuit shown in the figure. While it is a conventional push-pull circuit, it has the essential modification in the choice of coupling impedances which enables the utilization of the inverse feedback circuit to the full extent in reducing distortion due to the generation of even-harmonics in the output circuit. For simplicity of illustration, only the output stage and the driver stage are shown. Preceding stages operating at lower levels may be arranged in the conventional form and need not be considered here. The input circuit to the amplifier comprises the push-pull input transformer I having a primary winding 2 and balanced secondary windings 3 and I. These windings are on a common magnetic core 5. The high potential side of the winding 3 connects to the control electrode 6 of the driver tube 1, whereas the high potential side of the winding 4 connects similarly to the control electrode 6' of the driver tube I. The secondary winding 3 is also shunted by resistor 8 and the secondary winding 4 by resistor 9. The grid return circuitthrough the windings 3 and l terminates at ground potential to which the cathodes II and H of the tubes 1 resistors l3 and I3. In the output circuit of the tubes 1 and 'I' are individual reactors II and It. The reactor it comprises the anode load for the tube 1 and is connected between the anode i8 and the positive side of the anode potential source shown here by battery i9. Similarly, the reactor I8 formsthe anode load for the tube I and is connected between the anode i8 and the positive terminal of the battery E9. The negative terminal of the latter is connected to ground.

In order to make the understanding of the circuit clearer and easier to follow, batteries are shown for the various sources of potential required for the energization of the tubes. It is to be understood that all types of rectifier power supplies or sources of direct-current potential of the proper polarity may be employed, depending upon the types of tubes used. Furthermore, the cathode heater circuit has been omitted since this is not essential in the understanding of the operation of the circuit. It is common practice to provide alternating current for the energization of filaments in various tubes, particularly in push-pull amplifiers where hum due to alternating-current operation of the filaments is inherently reduced.

Continuing the description of the circuit, it should be noted in particular that the output circuit of the driver stage differentiates from its input circuit in that the output lmpedances have no common core. each being a separate reactor. As will be seen later, this is an important point and an essential structure for the intended operation of the circuit in accordance with this invention. The signal output from the driver stage is fed to the respective control electrodes 20 and 33' of the output tubes 3| and 3| through the coupling condensers 23 and 34. The former connects from the anode l3 to the control electrode 20 and the latter from the anode II to the control electrode 20'. The input circuit between control electrode 23 and cathode ll of the tube 2! comprises the grid resistor 25 and the bias potential source shown here by the battery 33. Similarly, the input circuit between grid 20' and cathode i1 includes the grid resistor 21 and the battery 2.. In some cases where the output tube draws grid current resistors 25 and 31 may be replaced by suitable chokes so bias will not change due to grid current. The output circuit of the amplifier comprises the output transformer 39 having primary windings 30 and 3| and a secondary winding 32. The circuit including the primary winding 30 is completed between the anode 33 of the tube 2| and the cathode I'I through the anode supply source represented by the battery 34. Similarly, the anode 33' of the tube 2| returns to the cathode through the primary winding 3| and the battery 3|. The inverse feedback circuit for each side of the amplifler comprises a current conductive path formed .by a resistor and a condenser between the an- Similarly, for the tubes 1' and from the output circuit to a point in the input circuit where the signal potentials are of opposite phase. In its operation, it will function to provide high output power at a minimum distortion, particularly of harmonic distortion. The operation and particular function of the feedback circuit need not be elaborated on in detail, in that it operates in the same manner as similar circuits in conventional amplifiers. The important point is that in the arrangement herein shown, its function is rather extended to minimize distortion due not only to odd-harmonic voltages, but also due to even-harmonic voltages present in the output circuit. which deliver comparat vely small power, let us say, not over 'a kilowatt, the problems of frequency distortion are not as severe as in amplifiers delivering several hundred kilowatts. The

' circuit shown here should be considered with the understanding that the amplifier is of the type delivering a very large amount of power and that the output transformer 29 is of very large physical proportions, weighing a ton Or more. As such, it has an appreciable leakage reactance which is unavoidable due to its physical size. The leakage reactance of the transformer 29 will cause appreciable even harmonic voltage output which is strongest in second harmonics and contain ng also appreciable higher order even harmonics. To make the inverse feedback circuit effective to reduce the even harmonic voltages, obviously it is necessary that when these voltages are applied to back channel in its cathode circuit was cancelled in the anode circuit between the center tapped primary winding of the input transformer. When this transformer was eliminated and the out- As stated previously in amplifiers the driver stage in inverse relation, they should also be fed to the grids of the output tubes in inverse phase. If the output circuit of the driver stage is of the conventional type comprising an output transformer or an output choke on a common core, it is evident that cancellation of even harmonics will be effected. Cancellation in this particular case is not only undesirable but it is actually harmful, since it will not permit the transfer of even harmonic voltages applied through the feed-back circuit to the grid of the output tubes in order to reduce the even harmonic voltage in the output stage. This simply means that the inverse feedback circuit can only be effective if we eliminate all common couplingbetween the two sides ofthe amplifier in the.

driver output stage. By the use of separate output impedances in the form of output reactors l5 and I6 shown in the drawing, the output circuit of the driver stage has no common impedance since these reactors have separate cores. There is no magnetic coupling between them as would be the case if an output transformer were used. In this manner, the undesired components of even harmonic voltages are impressed, 0n the cathodes of the driver tubes and also transferred through the separate anode circuits in inverse phase to the grids 20 and 20'. In this manner,

rective voltage components to minimize the evenput circuit of the driver stage carefully separated to eliminate all common lmpedaneebetween the fier is eliminated, and an inverse feedback circult between the output 'of said amplifier and a preceding stage.

2. In a multi-stage signal amplifier of the push-pull type capable of large power output having an output transformer of appreciable leakage reactance inherent with its physical size causing distortion due to even harmonic voltages, means for reducing harmonic distortion comprising individual output reactances and individual impedances between amplifier stages whereby coupling between opposite sides of said amplifier is eliminated, and an inverse feedback circuit between the output of said amplifier and a preceding stage.

3. In a multi-stage signal amplifier of the push-pull type capable of large power output having an output transformer of appreciable leakage reactance inherent with its physical size causing distortion due to even harmonic voltages, a driver stage and an output'stage including electron discharge tubes each having anode, cathode and at least one control electrode, a circuit for reducing said distortion comprising individual load inductance elements between anode and cathode electrodes of the tubes in said driver stage, individual impedance elements between the a control electrode and cathode of the tubes in said output stage, capacitive coupling between said stages and a feed-back circuit between the output circuit of said output stage and a portion 01' said driver stage at opposite phase of signal potential with respect to said output stage.

the input circuit of the output stage receives cor- RALPH N. HARMON. FRED W. FISCHER.

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