US2306749A - Amplifying system - Google Patents

Description

Dec. 29, 1942. J. 1.. POTTER AMPLIFYING SYSTEM J Filed May 22, 1941 2 mumuAZSjE 0E0 moub a u 3 HARHONIC 2 HARMONIC SIGNAL VOLTS Rns.

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Patented Dec. 29, 1942 NT oF cE AMPLIFYING srs'rim James L. Potter, New Brunswick, N. J., assignor to General Electric Company, a corporation of New York Application May 2 2, 1941., Serial No. 394,572

2,,Claims. (Cl. 179-171) My invention relatesto an amplifying system and particularly to a multi-stage amplifying system of the feed-back type.

In certain applications of signal amplifiers, it is desirable to have the voltage gain through the amplifier substantially independent of variations in the impedance of a load device coupled to its output. For example, a class B amplifier, i. e., one in which anode current fiows in the individual tubes only during half cycles of the applied voltage, presents a-load impedance to the driving source which varies appreciably over each cycle due to the varying grid currents drawn during portions of. each cycle. Therefore, if the proper grid excitation is to be supplied to its input, itis necessary for the driver stage to have low internal impedance. For perfeet voltage regulation of the driver stage under these conditions, its apparent internal impedance should be exactly equal to zero. If such is the case, the output voltage of the driver stage will always be proportional to the input voltage regardless of the value of its external load impedance.

An approach has been made to these conditions by employing a driver amplifier of considerable power output, and in some .cases by loading the secondary of the output transformer with a relatively low resistance comparable to the minimum grid impedance of the class B amplifier. sorbs a considerable amount of power which-is not effective in driving the grids of the class B amplifier.

In order to decrease the apparent internal resistance of the driver stage without sacrificing eillciency of operation, it has been proposedv to use feedback in thedriver amplifier. In accordance with the presentinvention, a combination of positive and negative feedbacks is utilized in a multi-stage amplifier for the attainment of this result. Briefly, in a preferred embodiment of my invention, a particular form of impedance network is interconnected with two cascaded amplifying stages. This network functions to provide a degenerative feedback between the grid and anode circuits of each stage controlled by its own anode current, and a regenerative feedback between the stages which is controlled by the anode current in the second stage. By a proper design of the circuit elements the apparent anode resistance to the second stage may be made as low as desired. It may-be reduced substantially to zero, or even made negative-as will appear more fully hereinafter.

It is therefore a primary objectof my invention to provide an improved multi-stage feedback amplifier which is particularly adapted to supply power to a variable impedancev load dece. Another object of my invention isto provide a simple and efllcient feedback amplifier in which the apparent anode impedance of the output stage may be given a desired value differing materially from its, actual resistance.

A further object .of my invention is to provide an improved: amplifier of this general, type H wherein the apparent anode impedance of the output stage may be made substantially equal to .zero,.and the voltage gain through the amplifier thereby made independent ofvariations in the output load impedance.

ily adjusted.

Still another object of my invention is to provide" an improved multi-stage amplifier which has a combination'of positive and negative feedbacks and which is stable in operation and readclas B amplifier or the like.

This loading resistance necessarily ab- The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 diagrammatically represents aelass B push-pull amplifier with a two-stage driver amplifier embodying my invention; and Figs. 2-5. are curves representative of the operating characteristics of a particular experimental amplifying system embodying my invention. 1

In the amplifying system represented in Fig. 1, signal voltages from a suitable source, not

shown, are impressed upon the primary winding of the input transformer ill. For example in an audio system, these voltages may be audio frequency signals. The secondary winding. of the 1 transformer I0 is coupled to the input of a-voltage amplifier having two stages H and I2. The

amplified signal voltages are supplied-from the output of this amplifier to the input circuit of a push-pull class B amplifier B through, a coupling transformer The output of the amplifier." is supplied to any suitable utilization device, not shown, through the output coupling transformer It. For example, the output of the amplifier l3 may be supplied to a sound reprowill further be observed that the resistor R3 is ducer, or the amplifier l3 may be used as a class common to the grid and anode circuits of both B modulator for a radio transmitter. amplifiers Ti and T2. Therefore, a voltage is fed The first stage II comprises an electron disback to the grid circuit of Ti which is proporcharge amplifier T1 which is illustrated as a 5 tional to the anode current in T2. This voltage triode having a cathode 16, control grid 11, and is regenerative and tends to increase the gain of anode l8. Signal voltages e. are impressed bethe amplifier. tween the grid I1 and ground from the secondary By adjusting the values of the various resistors, of the input transformer In. Cathode I5 is conthe apparent anode impedance of the amplifier nected to ground through two resistors R1 and T: may be given various desired values. If the Rain series. The anode I8 is connected to ground positive and negative feedbacks are properly prothrough a coupling resistor R0 and a suitable portioned, the efiective internal impedance of the source of anode operating potential, represented amplifier T: can be made just equal to zero. conventionally by a battery l9. Under these conditions the gain through the am- Signal voltages appearing on the anode 18 are plifier ll, l2 will be independent of the effective supplied to the input circuit of the second stage load impedance seen looking into the terminals I2 through a coupling capacitor 20. The stage of the primary winding of the transformer l4. l2 comprises a second electron discharge ampli- An exact analysis of this system is complifier T2, which is represented as a pentode having cated, since all tubes are non-linear devices. a cathode 2|, control grid 22, screen grid 23, sup- 20 However, by making the assumption that the pressor grid 24 and anode 25. In the particular tubes Ti and Ta behave as linear impedances system illustrated, the suppressor grid 2i is inover their operating ranges, a close approximaternally connected to the cathode 2| in the usual tion may be made. First solving for i i, the manner and the screen grid 23 is connected to anode current of the tube T1, and considering the the anode 25 so that the device T: effectively 25 second tube T2 and all feedback eflects, functions as a triode amplifier. A grid resistor e R 26 is connected from the junction of the coupling i,,,= (1) capacitor 20 and the control grid 22 to ground. h 1+ The cathode 2| is connected to ground through w a resistor R: and the previously-mentioned re- 30 amplificatmn fact T1 sistor R3. The anode 25 and screen grid 23 are amde current of and connected to ground through the primary wind- RP1=ande resistance T1 m3 the P ansml'mel' and the P Likewise, solving for in, the anode current of the tial source IS. The output voltage as is impressed second tube upon the transformer ll.

As shown at Fig. 1, the push-pull amplifier la fl@i. (2) comprises two duplex triodes T: and T4 connected R1 2+ZO+ i+ a) (m+ 1) in parallel, on one side of the amplifier, and simi- Where ar duplex triodes T5 and To connected in parallel, on the other side. It will, of course, be 0 52:3: $5 5 fg'lgfigggggg: gi' i g into apparent to those skilled in the art that by using the primary winding of transformer a plurality of amplifiers connected in parallel increased power output may be supplied from the Equations 1 and 2 may now be solved in terms of transformer I5. The grids of these amplifiers i -i, eliminating 1, from which it follows that in: mmRoe. (3) afi- 0? (R2+RI) (firip1+ 0+ H- a) (#i'i' )]-#1#z o a are biased approximately to cutoif for class 13 Since the gain of the amplifier is equal to the operation from a suitable bias source, represented quotient of co divided by e., conventionally by the battery 21. Suitable anode Gain= I HiM o o o:+ o+ i) (m+ pl+ 0+ l+ 3) (m+ )]mmRc t operating potentials are supplied from a high Equation 4 may also be written in the following potential source, represented by the battery 28. form:

Considering now particularly the driver am- Gain= mm o o (5) 0[ pl+ 0+( 1+ 3)(Pl+ p2+ z'i a) (1 2+ ni'io-i- H- a) 0 1+ )]#im o s plifier ll, l2 of Fig. 1, it will be observed that If the amplifier T2 is to appear as though it the common cathode circuits of the devices Ti had zero anode impedance, thereby making the and T: are interconnected by means of the netgain of the amplifier independent of the load imwork comprising the three Y-connected resistors pedance Zo, then p2+ (RH-R1) (2+ H m'i' o'i' (R1 3) (1 1+ ml i o s R1, R2 and Rs. It will further be apparent that Solving for Ra:

the anode current for the am lifier T must fio through the resistors R1 andpm ,1 the ,1 R3 iRe+ Rl+Ro (2+ 1 1IR.1+RO+ R.+R, (m+ sistors R1 and R3 are unbypassed, this provides umz o a degenerative feedback to the grid l1 which is proportional to the anode current in tube Ti and The quantities of (Ril-I-RB) and (RH-Rs) are the substantially uniform at all signal frequencies in required bias resistances for each amplifier. the operating range. Similarly, the flow of anode Hence, their value is known for any given types current in the device T: through the resistors R2 of tubes. However, it must be taken into considand R3 provides a uniform current-controlled eration that the anode current of both amplifiers degenerative feedback to its control grid 22. It flow through the resistance R3. Therefore, in

. Gain= Mil-2 (8) pl+ 0+ (R1+R3) (P'l+ It follows from Equation 8 that the gain of the amplifier is now independent of the magnitude and phase angle of Z0. It may further be noted that the gain of the amplifier ll, l2 without feedback is expressed by the following: Gain:

- manner is especially suited for working into a non-linear impedance, such as the grids of the class B amplifier IS in the illustrated embodiment. Furthermore, since the gain is independent of the phase angle of Zn, this type of amplifier is also advantageous when working into an impedance which varies with frequency, because it tends to flatten the frequency response.

By making the anode impedance of the driver 65 amplifier T2 in Fig. 1 appear nearly equal to zero, a reduction in harmonic distortion in the class B amplifier I3 is also secured. It may even be found desirable in somecases to make the anode impedance of the driver T2 appear somewhat negative so that an actual rise in voltage is obtained when the grids of the class B amplifier 13 are at maximum positive values. This will tend to compensate for the curvature generally occurring in the plate characteristic in this region and, also, to compensate for the losses in the coupling transformer l4.

In a particular apparatus embodying my invention, a type 6L7. tube was used for the amplifier T1, a triode-connected type '6F6 tube for the amplifier T2 and type 6N7 tubes for the amplifiers Til-T6. The curves of Fig. 2 are'actual test curves taken with this apparatus showing the harmonic content at the output transformer l5 plotted against the feedback resistance Rs, which controls the current regeneration between the stages II and I2. The power output from the amplifier 13 was held constant at about 33.2 watts. In this particular instance the apparent anode impedance of the amplifierTz was equal to zero when the feedback resistance R; had a value of about ohms. For values of R above 45 ohms, the apparent anode resistance was negative and below 45 ohms, positive.

Fig. 3 shows experimental curves for this same apparatus, with R3 equal to about 45 ohms, and illustrates the variation in anode current I, and grid current I; for the amplifier l6 plotted against signal volts input to the first stage II of the driver amplifier. Figs. 4 and 5 show the 70 tion:

corresponding variations in harmonic content and power output.

In this particular apparatus, the driver tube T2 was working at maximum output. The harmonic distortion could have been reduced still further by utilizing a driver tube of greater capacity. 7

It will thus be apparent that an amplifying system embodying my invention has low harmonic distortion, high efilciency, is stable in operation and is easy to adjust. While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover anysuch modifications 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. In a signal amplifying system, first and second thermionic amplifiers, means for coupling said amplifiers in cascade relation comprising a coupling resistance R0, said amplifiers havin internal anode resistances Rpl and R z respectively and amplification factors Ill and 2 respectively, degenerative feedback resistances R1 and R2 common to the grid and anode circuits of the respective amplifiers, and a regenerative feedback resistance R3 common to the grid and anode circuits of both amplifiers, said resistances having values determined substantially by the rela- 'll-2 0 whereby the gain through said amplifiers is substantially independent of the value of output load impedance -in the anode circuit of said second amplifier.

2. In a driver amplifier for a class B amplifier or the like, the combination comprising first and second thermionic amplifying devices T1 and I: each having an anode, cathode and control grid, said devices having internal anode resistances R131 and Rp2 respectively and amplification factors n and 12 respectively, resistance coupling means between said devices comprising a load resistor R0 for T1, a degenerative-regenerative feedback network in said amplifier comprising three resistors R1, R2 and R3 connected in a Y, the free terminalsof R1 and R2 being connected to the cathodes of T1 and T2 respectively and the free terminal of Rev being connected to a point of reference potential, connections from the grids of Ti. and T2 to said point through respective grid coupling impedances, a connection from the anode of T1 to said point through Ro, a connection from the anode of T2 to said point through an output load impedance, and means for supplying anode operating potentials to said devices, said resistors being proportioned substantially in accordance with the relation:

Pim o whereby the apparent anode impedance of T2 is substantially equal to zero.

JAMES L. PO'I'IER.

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