US2226942A

US2226942A - Neutralized power amplifier

Info

Publication number: US2226942A
Application number: US232482A
Authority: US
Inventors: Pollack Dale
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1938-09-30
Filing date: 1938-09-30
Publication date: 1940-12-31
Anticipated expiration: 1957-12-31

Description

Dec. '31, 1940. b. POLLACK NEUTRALIZED POWER AMPLIFIER Filed Sept. so, 1958 lnvemor Lmprneg Patented Dec. 31, 1940 PATENT OFFICE Daiie Pollack fiaddon Heights, N. J., assignorto Radio Corporation of America, a corporation of Delaware Application Steinem, 1933, Serial No. 232,482 f "'rciann. (01. 179-171) shift in signal voltage' through the channel.

The neutralizing circuit is arranged tolinclude said phase shifting network andthe interelectrode capacity of the tube to prevent energy .interchange between the gridand anode circuits of the amplifyingtube .in thehsaid. signal am.-. plifying channel.

The invention is therefore particularly adapted for use in certain types of power amplifiers comprising parallel-connected amplifier tubescoue pled at the outputxthroughlan impedance in-. verting network, the tubes being .biasedto pro,-

vide normal and peak. signal amplification without overload. w An amplifier of this type is known generally.

as a high efficiency linear ;amplifier,and ,was

3 described by W. 1%. Doherty in :thefieptember 1936 issue of the Proceedings ofthe Institute of,

Radio Engineers, vol. 24,QNo. 9, beginning at page 1163. I I V W This amplifier is a power amplifier foramplitude modulated waves and is provided'with two amplifying channels, in one of which, the main amplifier channel, an amplifier tube is biased substantially to cut-oil", and in the other. of which 4 channels, the peak amplifier channel, the amcomes into operation only on the peaksofl the signals exceeding the carrier level. W p

An impedance-inverting network connected between the output circuitsof the two tubes is arranged to present an impedance at the input end which is inversely proportional to the-terminating impedance thereof. As; soon as the peak amplifier comes into action in parallel with the load, it raises the effective terminating impedance of the impedance-inverting network, with a consequent lowering of the impedance presented to the main amplifier, and resulting in a sharingof the peak load by both tubes. A neutralizing system embodying-the invenplifier tube is biased beyond cut-off so that it tion may be applied to an improved highefficiency linear amplifier of the, type abovereferred to, as shown, described, and claimed in my copending application Serial No. 166,508, filed September 30, 1937, for a High efliciency 5 linear amplifier; and assigned to the .same assignee as this application. In that application a SOip haSe-shifting network andthe impedanceinventing network are employed in'the main channel, the phase shift being arranged to be 10 accumulative, whereby the total carrier phase shift is; substantially 180 between the input and the output circuits of the mainamplifier channel.

cuits.

It is, therefore, an object of the present ,invention to provide an efiective method andrneans for neutralizing the peak amplifier. channel of an amplifier of the type referred to, which may include and advantageously utilize the 180 phaseshifting network and preclude the necessity forltappe'd grid or plate inductances. 'It is a still further object of the presentinvention: to provide a neutralizing circuit for a high eff ciency linear amplifier having two am-. plifying channels, one of which includes a tube biased beyond cut-oiT and theother of which i-n cludes a tube biased to cut-ofi,'wherein the first named amplifying channel includes a phase shifting network and a neutralizing circuit including said network and the inter-electrode capacity of saidtube in said'channel.

Itis also an object of the present invention to provide ahigh efficiencydihear amplifier of the typecomprisi-ng parallel connected amplifier tubes coupled at .the output through .an impedance inverting network, which one of the tubes biased beyond cut-pit for peak amplification is neutralized througha;-circuit including 0;

a phase shifting network and the interelectrode capacityof thetube connected therewith. a

In the drawingto which attention isnow di, rected, the invention is illustrated by way of 55 In the peak amplifier channel, a correspon'd- 15 linear amplifier containing the amplifying tubes 5 and 6, respectively, having a common output anode circuit 1 and a common modulated signal input circuit 8.

The tube 5 is arranged tobe biased'substantially to cut-off, while the tube 6 is biased beyond cut-.oif, the operation being such that the channel B containing the tube 5 provides the carrier wave amplification and the down modulation, while the channel C carries the peaks and the-up modulation, and through its operation'permits the channel B to function to carry an increased load. I

The output circuit of tube 5 is provided with a tuned circuit or tank 9, and is coupled to the output circuit of the tube 6 through an impedance-inverting network II], which may be of any suitable and well-known type. When the impedance of the output circuit 1 increases, resulting from the tube 6 carrying load, the impedance is reflected back tothe output circuit of tube 9 and inverted, that is, lowered to such an extent'that the tube 5 may carry aheavier load.

N This permits the tube 5 to increase its' power output without increasing its plate voltage swing. When the tube 6 comes into operation in parallel with the-load through the circuit 1, it raises the effective terminating impedance of the network I with a consequent lowering of the im-- pedance presented to the tube 5, whereby the tube will carry an increased portion of the load-because of the apparent lowered load impedance presented to it through the impedanceinverting network. It will be seen that-both tubes 5 and 6, or both channels of the amplifier, receive energy from the input circuit 8. From the foregoing description, it will beseen that it is desirable and necessary for the proper operation of the amplifier'that the two halves of the modulation envelope shall be in phase at the output circuit 1. Accordingly, a 90 phase-shifting network H is inserted in the grid or input circuit I2 of one tube and is arranged 'to add an additional 90 carrier phase shift to the 90 carrier phase shift resulting. from the signal passing through the impedance-inverting network I0, so that the output voltages of the two channels will be in phase.

Having obtained 'a 180 carrier phase shift between the modulated signal input circuit and the common output or anode circuit 1, instead of compensating one 90 phase shift by means of the other, a corresponding 180 carrier shift network is provided in the peak amplifier channel C and is'arranged to include means for causing aphase shift of the sidebands and a phase delay in the halves of the modulation envelope which are the same as that caused by the networks and tunedcircuits in the main or .carrier amplifier channel. This 180 phase-shifting network is connected, as indicated within the bracket [5, in the input or grid circuit of the tube 6 and between the tube and the input circuit 8. It will be noted that the 180 phase-shifting network I5 comprises two 90 networks 16 and I! in cascade across the signal channel C containing the tube 6, and that between said networks there is interposed a tuned circuit or tank 18, with a. similar tuned circuit or tank 19 provided adjacent to the tube 6, following the second 90 phase-shifting network ll. The circuits l8 and I9 serve to increase the input impedance of the network at the tube 6.

' In the present example, the two channels are coupled through the common input circuit 8 and an output transformer 22 to a modulator stage comprising an 3.. F. amplifier 20 and a modulator 2|.

The primary 23 of the input transformer is tuned and connected to the output circuit of the modulating stage.

The common anode or output circuit 1 for the two channels is likewise tuned to the carrier and is coupled through a transformer 25 with an antenna or other load circuit 21, for the amplifier. Any other suitable output coupling may be provided which complies with the arrangement for the high eflici-ency amplifier wherein the impedance-inverting network is interposed between the anode circuits of the two amplifier channels.

It will be noted that the networks H and I0 may have a positive or a negative sign in regard to the phase shift, and likewise the networks [6 and I! in the other channel may have a positive or a negative sign, but in any case, whether th shift is positive or negative, the sum 'of the phase shift in the one channel, that is, in the networks II and I0, must be the same as the sum of the phase shift in the networks it and I1, whereby the output from both channels may combine in the common output circuit 1 and may be applied to the load circuit 21 in phase.

By utilizing the 180 network 16 and I! in the peak amplifier. channel C, neutralizing of the system may be accomplished by connecting the input high potential terminal 30 of the 180 phase shift network with the anode circuit 3! of the peak amplifier tube 6 through a neutralizing capacitor 32. This arrangement provides a circuit in which the direction of current flow is reversed in the two capacities provided by the neutralizing capacitor 32 and the inter-electrode capacity 33 of the tube 6 as a consequence of the action of the 180 network I6 and I1. If the magnitude of the capacitor 32 is properly adjusted there will be no fiow of current through the anode circuit! of the tube 6 back to the input circuit and, consequently, no energy interchange between the grid and plate circuits. The flow of current is indicated by the arrows.

The capacity of the capacitor 32 is adjusted to equal cNr-acgp in which 0015 the ratio of the signal input to the signal output voltage of thenet work |6-l'l and the tube 6 and Cgp is the grid to-plate capacity of the tube 6.

The neutralizing connection is such that the input side of the 180 phase shifting netwo rk'is coupled through the capacitor 32 to the anode end of the output circuit of the tube whose grid is supplied with signal energy from the 180 phase shifting network.

Iclaim as my invention:

In a high efiiciency linear amplifier comprising two parallel amplifying channels, the combination with an amplifier tube ineach channel, of means providing a common output anode circuit for said tubes, 2 load circuit coupled to said anode circuit, means providing an impedance inverting network interposed between said tubes in the anode circuit, means providing a 180 phase shifting network in the grid circuit of one of said tubes, and means for neutralizing the amplifying channel including said tube comprising a neutralizing capacitor connected between the input end of said phase shifting network at the high potential side thereof and said common output anode circuit at the output end of said impedance inverting network.

DALE POLLACK.