US2298960A - Amplification with high efficiency - Google Patents

Description

Oct. 13, 1942. J. w. M RAE AMPLIFICATIQN WITH HIGH EFFICIENCY 3 Sheets-Sheet 1 Filed Aug 15, 1940 INVENTOR I J W MC RAE frz w w A TTOR/VEV Oct. 13 J. W. I AMPLIFICATION WITH HIGH EFFICIENCY Filed Aug. 15, 1940 3 Sheets-Sheet 2 MODULATED AVERAGE GRID VOLTAGE MODULATE'D UNMODULA TE D RECTIFTIER nine-z PHASE SUPPLY INVENTOR J n. MC RAE Arron/Er Oct. 13, 1942. J, w, MGRAE 2,298,960

AMPLIFICATION WITH HIGH EFFICIENCY Filed Aug. 15, 1940 3 Sheets-Sheet 3 FIG. 7

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I INVENTOR t J. W MCRAE ATTORNEY Patented Oct. 13, 1942 AMPLIFICATION -WITH HIGH EFFICIENCY James W. McRae, Neptune, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 15, 1940, Serial No. 352,722

a 8 Claims. (01. 119-171) The present invention relates to power ampliiier circuits for radio transmission or similar purposes, with'special provision for reducing distortion and increasing efficiency of operation.

A feature of the invention is the use of an electron multiplier type of amplifier as a power v amplifier, with circuit provisions for reducing distortion and increasing efficiency.

Electron multipliers, as is known, permit of very large gains and have the advantage of not requiring electrodes of individual stages to be maintained at alternating current potentials. This makes them especially suited to high frequency amplification. Moreover, one of their characteristics is that the anode potential of the final or output stage has practically no effect on the anode current, at least in the normal operating range. This fact is favorable to the transmission through the multiplier of short pulses of current which retain their form under nearly complete control of the input circuit. The use of the short pulses permits high efliciency to be obtained in the output stage which couples to the load. I

Wherethe input signal to the first stage is a modulated carrier wave or where modulation takes place anywhere ahead of the output multiplier stage, a feature of the invention is a control of the final anode voltage in accordance with the envelope of the modulated .wave in such a manner as to keep the average anode voltage approximately the same as the instantaneous peak -alternating voltage in order to reduce dissipation of energy in the space path of the final stage. The invention also comprises as features the use of overall negative feedback at radio frequency and, therewith or separately, envelope frequency feedback around the initial stage to reduce noise and distortion.

These and the other features and the various objects of the invention will appear more fully from the following detailed description taken in connection with the accompanying drawings in which:

Fig. 1 is a schematic circuit diagram of one form of electron multiplier system incorporating the invention;

- Figs. 2, 3, 4 and 5 show wave forms to hereferred to in describing the operation;

Fig. 6 is a partial diagram of a circuit, which may be substituted for that portion of Fig. 1 to the right of the broken line 88;

Fig. 'l is a schematic circuit diagram of two electron multipliers in a-systemembodying he invention in one form; and 1 Fig. 8 is a partial diagram of a circuit, which may be substituted for that portion of Fig. 1 to the left of the brokenline 8-8.

Referring to Fig. 1, an electron multiplier tube is connected to amplifywaves received through an input connection 2 and to-impress the amplifled output on a tuned output or tank circuit 3 which may comprise a load or to which any' suitable load I is coupled, such as, an antenna or sending circuit.

The electronmultiplier tribe I of itself forms no part of the present invention but may be of any suitable construction, the prcferred 'construction being in accordance with the disclosure in my copending application Serial No. 371,124, filed December 21, 1940.

As illustrated herein the multiplier comprises a succession of suitably arranged and suitably treated emitting surfaces 5, 8, etc. with an ina put or starter section co prising cathode control grid 8, accelerating grid 9 and first emitter plate 5, and with an output section or anode region. comprising final emitter i0, positive grid ii and anode 12. A suitable direct current supply source shown in the form of battery l3 ap plies steady positive voltage to the emitters, increasing in value from emitter to emitter throughout the tube. A voltage divider or potentlometer H may be used for applying the potentials to some of the emitter plates. In the case of the later stages which draw more power from the supply it may be advantageous to use sections of the battery as indicated in the diagram. It would-be obvious to employ other types of voltage supply in view of known practice. The

- emitter plates are by-passed to ground through large condensers i9.

The starter section includes an input circuit comprising the control grid 8, input coupling l5, bias battery It and cathode resistor I! which is shown shunted by capacity I! (which may be omitted where not necessary) and cathode 1.

This section includes an output-circuit comprising first emitterplate 5, portion of resistor l4,

cathode network ll, I8 and cathode l. The battery it applies such a large negative potential to the grid that class B or. class C operationv is secured.- That is, transmission through the starter section consists of pulses of half cycle or smaller duration.

In the case of a modulated highfrequency in- .put wave at 2, the cathode network I1, I8 is,

7 preferably adjusted to varythe control grid bias at the envelope or signal frequency. Rectificafiii tion in the plate circuit of the starter section causes a detected current of the signal frequency to flow through resistance I1, thus producing a varying grid bias which permits only the positive tips of the high frequency wave to be transmitted. For this. use, condenser l8. acts as a radio frequency by-pass. At the same time, resistance l1 and condenser i8 provide negative feedback at the signal frequency. This negative feedback compensates for distortion in the starter section and the varying bias provides the short impulses that are favorable for securing highly eilicient operation of the output or anode section as will be described later.

Grid 9 is biased to a positive potential to accelerate electrons toward the emitter 5. Condensers 20, 2| are blocking or by-pass condensers In the output or anode section the final emit-- ter Ill is at direct current ground. Screen H is positive with respect to emitter I3, and anode I2 is maintained at average potential more positive than that of screen-H by the lead 24 extending to the power supply rectifier circuit generally indicated at 25. Lead 24 includes filter choke 28 and filter condensers 21. 21.

A negative feedback connection is made ode l2 to control grid 8 for producing large negative feedback for reducing noise and distortion in the multiplier in accordance with the teachings of Black Patent 2, 0 71, December 21, 1937.

The power supply circuit 25 comprises a sixphase rectifier system including mercury vapor rectifier tubes 29 having their cathodes connected in, common to filter choke 28 and their anodes connectedto individual windings of a six-phase secondary 30 provided" with a three-phase pri mary 3| connected to 60 cycle three-phase supply line 32, in accordance with usual power circuit practice. In order to control the voltage that is applied to the anode I2 01' the multiplier,

there is provided aphase controlling system,

comprising the two phase shifting tubes 33 and 34 which serve to control the phase of the 60- cycle grid excitation of the tubes 29 by way of the three-to-six phase transformer 35. The grids of tubes 29 are individually connected to windings of the six-phase secondary through limiting resistors 36.

The use of tubes such as 33 and 34 in combination with tubes 29 to control a terminal voltage is, in and of itself, old and well known in the art. In the present case the phase shifting tubes 33 and 34 are under control of the output current of direct current amplifier 38 or 38' depending upon the position of switch 55, in a manner and for a purpose to be described presently. .As the voltage from amplifier 38 varies, with switch 55 in the position shown, the internal resistance of tubes 33, 34 is varied and the magnitude of this resistance relative to the fixed value of capacity 39 or 46 causes a shift in phase of the Waves applied through transformers 4| and" with respect to the waves appearing across retubes 29 for a greater or less fraction of the positive half cycle-of plate potential. The effect of this is to vary the current flowing through .inductance 26 into the capacity 21, which has a capacity value with respect to the other circuit constants such as to permit voltage changes of The screen II in the multiplier is connected through inductance 41 and resistance 43 to battery section 49 and as stated has a positive voltage of average value less than that of the anode l2. swing of the voltage of anode l2, if the anode falls to sufiiciently low voltage, a portion of the current emitted from plate It will flow to the screen ll producing a drop of potential across elements 41 and 43 which is applied to the input of amplifier 33 after filtering at 33. In this way the phase shift tubes 33 and 34 are controlled to, in turn, control the average voltage of the anode l2 through the medium of the rectifier circuit 23 as described. The proper number of stages must be used in amplifier 33 to produce the correct phase relation between the voltage drop appearing across 41 and 43 and that-applied to the grids of tubes 33 and 34.

Referring to Fig. 2, the radio frequency voltage of the anode i2 is shown and also the average voltage of anode l2 in the absence of the control exerted through the rectifier 25 by the phase controlling tubes 33, 34. The average value remains the same for unmodulated and modulated waves and is indicated at E, Fig. 2. Fig. 4 shows the anode current pulses which are indicated as short sharp impulses occurring at the negative point of the anode voltage swing.

'Three values are shown corresponding to (1) the trated by Fig. 3 where the average anode voltage changes in proportion to the radio frequency peak voltage or approximately so and is shown as having the three values E, E" and E'. The instantaneous voltage at the time the current is flowing is seen to be about the same in all cases and of about the value E: of Fig. 2, for illustration. The average voltage of grid H is shown as E0 and it will be noted that the average voltage of the anode I2 is always adjusted so that the minimum peaks'dip just below the value Es. During the instants when the anode voltage is lower than the grid voltage, current flows as indicated in Fig. 5, of an average magnitude such as to determine the corresponding average anode voltages E, E" and E' for the three illustrated conditions. In the absence of any grid current, the rectifier 25 would tend to supply minimum averageanode voltage and this would result in production of grid current flow which would in turn tend to raise the average anode voltase 'until a condition of balance were reached. Increase of grid current causes increase of anode average voltage and decrease of grid current causes decrease of anode average voltage. The resultis a condition somewhat similar to that indicated in Fi 3 such that the minimum value of the anode voltage swing for allconditions'of modulation and for the unmodulated condition is held to about the same value, I

During the negative radio frequency lated radio frequency waves or other type, are

just below the average'voltage of screen I l. The

tube losses in the output section are therefore kept' to a minimum.

Since a voltage of the signal frequency is developed across the cathode resistor ll, this voltage may be used to control the phase shifting tubes 33 and '38 if desired. For this purpose switch 55 is thrown to its alternate contact connecting the output of amplifier 38 to the grids of tubes 33 and 34. The input of amplifier 38' is connected across resistor ll. A reverse poapplied across the input coupling resistor 61.

tentlal may be applied toa suitable part of the amplifier. 38', if necessary, to counteract to the required extent the oil-ground steady potential of the resistor (1. It is necessary to use the proper number of stages in amplifier 38 so that the correct phase relation will be obtained between the voltage drop appearing across resis or i1 and the voltage applied to the grids of tubes 33 and 36.

With this circuit modification, as the pulses of current flowing through resistor l I increase in amplitude under control of the input wave, the

' rectifier circuit is controlled to increase the average anode voltage, and as these pulses decrease in amplitude the average anode voltage is decreased in corresponding manner. The result is similar in general to that described above in connection with the curves in Figs. 2, 3 and 4 but in this case no use is made of the grid current pulses of Fig. 5.

An alternative form of circuit for controlling the final anode voltage is shown in Fig. 6 in which the circuit portion (not shown) at the left of the broken line G6 is assumed to be 1 the same as the corresponding portionof Fig. 1. The three-phase supply 56, which could, if de- From this they are applied to the multiplier inputs in parallel, a quarter wave-length line 68 being inserted in the input to multiplier 66, inv

accordance with the Doherty disclosure. Selfbias is shown for the starter section of each multiplier comprising cathode resistor l1 and bypass condenser I 8, and the input circuit is coupled in each case through a series capacity 69, the inductance I0 being a radio frequency choke.

The operation of the bias in giving signal frequency negative feedback around the starter sired, be a single phase supply instead, is connected to rectifier S'Ithe positive output terminal of which is connected through reactor 58 to the anode I2 01 the multiplier output section. The reactor 58 may form part or all of the smoothing filter, additional series inductance be are provided as before. The reactor includes saturating windings 59 and secondaries 60 which are included in series in the primary leads. With small current in windings 59 large inductance is ing provided if necessary. Filter condensers .21

developed in the input of the rectifier and with large current in the saturating windings the seerable not to allow the minimum instantaneous voltage of anode l2 to fall low enough-to produce current flow in the circuit of grid 8 I. This method of control has the advantage of simplicity, however, over that of Fig. 1-.

In Fig. v'I two electron multipliers 65 and 6B are associated together to form a Doherty amplifier of the general configuration disclosed in British Patent 471,423 and in U. S. Patent 2,210,-

028, granted August 6, 1940, but using electron multipliers in place of triodes. These may be of any suitable type but are shown as of the same type as those of Fig. 1 except that in this case the circuits for controlling the anode voltage at the speech rate are omitted. I

The input waves, which may be signal modupropriate type.

section is similar to that of Fig. 1. It is also fwithin the invention to use fixed bias obtained.

for example, from a battery.

The outputs of the multipliers 65 and 66 are connected to the output circuit-or load 12 which may be a tuned circuit, such as an antenna, or may be of other type. Alternatively this load may be theinput of a succeeding amplifier stage and may be untuned. The voltage for the final anode of each multiplier is obtained from} suitable supply 13 such as a rectifier to which connection is made by way of radio frequency choke 14. The quarter wave-length line H is inserted between the output of multiplier 65 and the load in accordance with the teachings of the Doherty patent. Feedback connection 5l provides negative feedback around the two multipliers for increasing the linearity of the circuit.

The electron multipliers are especially suited to usein a Doherty circuit because of their high gain and ability to operate with short pulse ex citation. 1

The high gain makes possible modulation at low power level and the short pulse operation results in high efficiency. Moreover, aspointed out above, the lack of tuned interstage circuits in the multiplier makes possible large gain with preservation of. the wave form of the pulses and the negative feedback also increases the linearity of the amplifier.

The circuit of Fig. 7, therefore, also represents a system in which there is a control circuit, in this case in the form of auxiliary multiplier 66, whereby the eiiiciency of the main amplifier is prevented from varying as the amplitude of the modulating signal changes, at least for a considerable portion of the modulation cycle.

A further way of compensating distortion in the multiplier is by use of predistortion of ap- One circuit that may be used for this purpose is shown for illustration in Fig. 8'. In this figure the signal input is at 82 and the signal passes through input coil 83 to the grid of tetrode 84 by which it is amplified. The output of tube 84 is coupled by a resistance-capacity circuit to the grid 8 of the multiplier l. The sries coupling capacities are at 86 and 89 -and the shunt resistance consists'of 85, 88, 98

and the internal resistance of diode 81, which latter resistance is variable depending upon the voltage applied across it. Batteries 9| and 92 supply steady potentials to the anode and screen grid of tube 84 and to the diode, respectively.

The distortion in the starter section of the multiplier is due to its curved grid voltage-plate current characteristi By adjusting resistances 85 and 88 to the right values the non-linearities of the tetrode and diodecan be made to compensate the distortion in the multiplier, especially that occurring in the starter section.

The scheme is not limited to the use 01' a tetrode and a diode. Apparatus of reasonably equivalent performance may be used. For example, solid element non-linear resistances such as rectifiers may be used'in place of the diode. One such element would be copper-oxide.

The invention is not to be construed as limited to the detailed: circuit arrangements disclosed since the invention itself may take on widely diflerent embodiments. The scope of the pulses, to vary in amplitude in accordance with informationto be transmitted, a source of anode voltage, and means controlled by said impulses of current for varying the voltage of said source in correspondence with the variations in amplitude of said impulses.

' 2. In combination with an electron multiplier having a primary or starter section and an output section, means to apply a signal modulated high frequency wave to said starter section, distortion compensating means comprising means, to produce negative feedback at the signal frequency aroundsaid starter section and distortion reducing and noise reducing means comprising an over-all negative feedback path for the high frequency waves from the output side of said output section to the input sideof said starter;- section.

3. The combination according to claim 2 in which said starter section has a non-linear transmission characteristic tending to produce distortion, comprising in combination therewith a predistorting circuit included in said means to apply the signal modulated high frequency wave to said utilization circuit, .a source of positive voltage connected to said anode, means biasing said multiplier to cause the modulated wave to be transmitted by said output stage in short impulses of current of a duration small compared with the high frequency wave period whereby the chiciency of said stage is high for large ratio of high frequency to direct'voltage between said emitter and anode, and means controlled by said signal modulated wave for increasing the eiiiciency of said output stage when the high frequency voltage on the anode has amplitudes corresponding to the unmodulated carrier and smaller amplitudes.

'6. In combination with an electron multiplier having a succession of multiplier sections and an output section comprising the last emitter, a screen and an anode, means to apply positive potentials to the screen and anode, means to transmit high frequency pulses of current through said multiplier and to modulate said pulses in accordance with signals before they are impressed on said output section, circuit means for deriving from electrodes of said multiplier a rectified component of said modulated pulses, means to isolate the signal component thereof and means controlled by said isolated signal component for varying the positive potential applied to said anode to increase the operating emciency of said output section.

7. In combination with an having a succession of multiplier sections and an output section comprising the last emitter plus a screen and an anode, a source of direct current supply for energizing said multiplier sections including means to apply positive potentials to said screen and anode, means to transmit high frequency pulses of current through said multiplier and to modulate the pulses so transmitted in acand said screen draw current from said supply starter section, said predistorting circuit having a transmission characteristic of such form as to counteract said distorting tendency of said startin: section. a

4. In combination, a space discharge device having an electron emitting cathode, an anode and a grid, means to apply steady positive potentials to said anode and grid with respect to saidcathode, a tank circuit coupled to said anode and cathode, means to control said device-in ac cordance with an input wave to produce signal modulated pulses of current between said cathode and anode at thefundamental frequency of, said tank circuit, each pulse of a duration short compared with a half-period of the fundamental frequency waveof said tankcircuit, said positive grid receiving a portion of the space current at the instant of minimum voltage swing, and means controlled by said grid current for varying the average voltage of said anode to improve the eiiiciency of said device. a

'6. In combination, a source of signal modu- "iated high frequency-input waves, a utilisation circuit, an amplifier-comprising a multistage source in pulses of varying amplitude, and means controlled by the current drawn by certain of said electrodes for varying the voltage applied to said anode at the signal frequency, said last means causing an increase in the current so drawn tov emitter and an anode coupled to said utilization circuit, a source of positive voltage connected to said anode, means biasing said multiplier to cause the modulated wave to be transmitted by said output stage in short impulses of current of a duration small compared with the vhigh frequency wave period, whereby the eiiiciency of said stage tends to vary throughout the modulation cycle dependentumn the instantaneous amplitude of the modulating B ind. and space discharge means controlled by said signal modulated electron multiplier an input terminal.

coupled to said source and an-output stage comprising anlemitter and-an anodecou'pled tosaid modulation cycle.

wave for the voltage across said output stage to maintain the emciencypf said output stage constant'for at least a portion of the sums w. .ldcR-AE.

ielectron multiplier 4

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