`
`ttt)
`
`3,900,823
`
`i45] Aug. 19, 1975
`Sokal et al.
`
`[57]
`
`ABSTRACT
`
`[54] AMPLIFYING AND PROCESSING
`APPARATUS FOR MODULATED CARRIER
`SIGNALS
`
`[76]
`
`Inventors: Nathan O. Sokal; Alan D. Sokal,
`both of 4 Tyler Rd., Lexington,
`Mass. 02173
`
`{22}
`
`Filed:
`
`Mar. 28, 1973
`
`{21] Appl. No.: 345,509
`
`A power amplifying and signal processing system for
`modulated carrier signals separately processes the am-
`plitude component of the system input signul and the
`component of frequency or phase or both frequency
`and phase, and later recombines the separately pro-
`cessed components to provide an output signal. The
`amplitude and phase transfer functions of the system
`can be accurately controlled. The input signal is fed to
`a power amplifier whose output provides the output
`for the system. The input and output signals of the sys-
`tem are fed by separate paths to a comparator which
`compares those signals and emits an error signal to a
`controller. The controller regulates the amplitude and
`phase, or both, of the power amplifier’s output to null
`the error signal. One or both of the signal paths to the
`comparator may have in it a non-linear function gen-
`References Cited
`erator which acts upon the signal fed by that path to
`UNITED STATES PATENTS
`the comparator. The system may also have a fre-
`2,298,930 10/19429Decino....... eee 332/18 X
`
`quency translator and phase shifter mterposed be-
`tween the system input terminal and the power ampli-
`
`
`2,705,775 4/19559Crosby....... . 332/37 RX
`2,708,717
`5/1955
`Holmesetal...
`we 328/162 X
`fler’s input to shift the frequency or phase or both of
`3,241,077
`3/1966)
`Smyth et aloo. 328/155 X
`the signal applied to the power amplifier’s input.
`3,274,492
`9/1966
`Van Kesser et al..
`. 332/37 RX
`3,486,128
`12/1969
`Lohrmann..................... 332/37 RX
`
`[52] U.S. Cho, 330/149; 330/129; 332/37 D
`[St]
`Int. Cheee HO3y 3/00
`(S&|
`Field of Search ......0..0000000. 330/149, 127-129,
`332/18, 37 R. 37 D; 328/162, 163, (55;
`325/475, 476
`
`{56]
`
`
`
`
`Primary Examiner—James B. Mullins
`Attorney, Agent, or Firmr:—Wolf, Greenfield & Sacks
`
`20 Claims, 24 Drawing Figures
`
`POWER
`o OUTPUT
`
`AMPLIFIER
`
`
`AMPLITUDE
`DETECTOR
`
`
` ATTENUATOR
`
`
`
`
`
`POWER
`
`OUTPUT
`
`CONTROL
`
`AMPLITUDE
`DETECTOR
`
`
`
`APPLEET AL. EXHIBIT 1005
`
`APPLE ET AL. EXHIBIT 1005
`
`1
`
`
`
`PATENTEDAUG 1 91975
`
`3,900,823
`
`Sil 1 uF 7
`/
`
`POWER
`AMPLIFIER
`
`0 OUTPUT
`
`
`
`
`
`
`AMPLITUDE
`POWER
`
`FIG 1
`DETECTOR
`QUT PUT
`
`
`CONTROL
`
`
`
` VARIABLE
` POWER
`
`FREQUENCY
`OUTPUT
`PHASE -SHIFT
`TRANSLATOR
`
`
`AMPLIFIER
`NETWORK
`
`(OPTIONAL }
`
`
`
`
`
`AMPLITUDE
`
`POWER
`DETECTOR
`
`
`OUTPUT
`
`CONTROL
`
`
`
`DIFFERENTIA
`AMPLIFIER
`
`MECHANISM
`
`
`
`CONTROL SIGNAL TO
`VARIABLE PHASE -SHIFT
`
`NETWORK /7
`
`
`
`
`AMPLIFICATION
`AND CONTROL
`
`
`CIRCUITRY
`
`
`
`FIG. 3
`
`
`
`
`PHASE
`
`DETECTOR
`°
`FROM
`
`FROM INPUT
`OUTPUT
`OR FROM
`
`FREQUENCY TRANSLATORIF ANY
`OUTPUT OF
`
`2
`
`
`
`PATENTED AUG 1 91975
`
`3,900,823
`
`SHEET 2 ur
`
`2?
`
`
`he
`OUTPUT
`
`
`FREQUENCY
`PCWER
`
`TRANSLATOR
`
`AMPLIFIER
`
`
`( OPTIONAL)
`
`
`
`AMPLITVCE
`DETECTOR
`
`POWER
`OUTFUT
`
`
`CONTROL
`
`
`AMPLITUDE
`FUNCTION
`
`DETECTOR
`
`
`-|GENERATOR
`
`5
`
`
`
`“SIGNALS
`(OP TIONAL)
`
`FIG. 4
`
`35
`
`
`OUT PUT
`
`
`
`POWER
`VARIABLE
`COUPLING
`
`
`
`PHASE-SHIFT
`AMPLIFIER
`
`iNPUT
`OUTPUT
`NETWORK
`NETWORK
`
`
`
`POWER
`
`
`
`OUTPUT
`
`CONTROL
`ATTENUATORL.
`
`
`
`3
`
`REFERENCE PHASE
`INPUT
`
`
`
`
`
`PHASE
`26
`DETECTOR
`
`
`
`3
`
`
`
`PATENTED AUG 1 91975
`
`3.900,823
`
`
`es
`
`
`
`-5f
`
`TO POWER
`
`
`FROM
`meet
`ATTENUATOR J
`
`
`LIMITER/
`3-STATE
`
`DIGITAL
`SUBTRACTOR
`
`FIG. 7
`
`;
`INPUT
`
`MIXER
`
`/
`
`
`7
`-
`POWER
`3
`AMPLIFIER
`
`
`POWER
`OUTPUT
`
`CONTROL
`
`#0
`TO POWER 0 ITPUT
`CONTROL
`
`
`LIMITER /
`3-STATE
`
`
`ADDER:
`PHASE INPUT
`FIG 6
`onan|REFERENCE
`
`
`
`
`197
`e208
`-£/0+9
`12
`
`
`FROM
`FROM
`SUBTRACTOR
`INPUT
`ATTENUATOR 7
`
`SUMMER
`LIMITER/
`OUTPUT
`
`3-STATE
`SYNCHRONOUS
`CONTROL
`
`
`
`
`
`DETECTOR
`DIGITAL
`REFERENCE
`
`ADDER
`
`PHASE INPUT
`
`
`
`
` ATTENUATOR
`
`
`5
`OUTPUT
`
`FIG. 8
`
`RF
`SUBT RACTOR
`
`37
`
`
`DETECTOR
`
`REFERENCE
`PHASE INPUT
`
`
`4
`
`
`
`PATENTED AUG 1 91975
`
`3.900,823
`
`Sneel 4 OF 7
`
`FIG. IA,
`
`e2:
`
`7
`
`uOAD
`
`LOAD
`CURRENT
`
`13
`
`|
`
`FIG. 9B —
`
`7
`
`FIG. IC
`| SAMPLE/CYCLE
`25%OF MAX.
`RF OUTPUT
`
`FIG. 9D
`
`90% OF MAX.
`
`RF OUTPUT
`
`|
`
`|SAMPLE/CYCLE eee
`FIG. JE An
`
`E3
`2 SAMPLES/CYCLE
`25 % OF MAX.
`RF OUTPUT
`
`FIG IF
`2] saMpLes/cycLe
`90% OF MAX.
`RF OUTPUT
`
`5
`
`— +E2
`
`——+E2
`
`—— —El
`
`—+€2
`
`—+E2
`
`—-E€
`
`5
`
`
`
`PATENTED AUG 1 S975
`
`3,900,823
`
`SxcET S OF 7
`
`FIG. 10A
`RF INPUT
`SIGNAL
`
`
`
`FIG8
`
`FIG. 10C ee
`
`FIG 10D
`TRIANGLE
`WAVEFORM
`
`
`
`
`
`FiIG./OFee OOTic
`
`6
`
`
`
`[POWER QUTPUT CONTROLvA
`
`TO POWER AMPLIFIER 1
`
`i | |
`
`aC
`
`| | i|
`
`— - a
`
`
`
`ERROR SIGNAL
`FROM COMPARISON
`MEANS
`
`|\ | | ||| || ||
`
`FIG. UB
`7}
`
`—-—|---—---4
`POWER OUTPUT COnTROL
`|
`
`
`Dissipative|2? |
`
`DIRECT-COUPLED |
`|
`PEGULATOF:
`
`PATENTED AUG 1 91975
`
`3,900,823
`
`6.001 6 OF 7
`TO POWEn AMPLIFIER 1
`
`FIG. 11A
`
`&.
`
`
`
`en
`
`} j i | | | | \ {
`
`! |q
`
`a \
`|
`
`oc,
`JOL TAGE
`SUPPLY
`
`ee ee eee __!
`
`SWITCHING REGULATOR
`|
`aA
`|
`by
`Ly
`Po
`
`
`
`
`
`|
`
`I |
`
`| | | | | |L
`
`oLf lk
`
`7
`
`
`
`PATENTED AUG 1 91975
`
`3,900,823
`
`CctT 7 OF 7
`
`FIG. HC
`
`&e
`
`POWER AMPLIFIER !
`
`COMBINATION SWITCHING
`ANO
`DISSIPATIVE REGULATOR
`
`
`
`
`&
`
`
`
` Oc.
`VOLTAGE
`SUPPLY
`
`ERROR SIGNAL
`FROM COMPARISON
`MEANS
`
`POWER AMPLIFIER1
`
`FIG 11D
`
`
`
`
`
`
`COMBINATION
`
`AC-COUPLED MODULATOR
`AND
`SWITCHING REGULATOR
`
`
`
` 0.C.
`
`VOLTAGE
`SUPPLY
`
`
`
`ERROR SIGNAL
`FROM COMPARISON
`MEANS
`
`8
`
`
`
`3,900,823
`
`1
`AMPLIFYING AND PROCESSING APPARATUS
`FOR MODULATED CARRIER SIGNALS
`
`TABLE OF CONTENTS
`
`FIELD OF INVENTION
`LINEAR AMPLIFIER
`AMPLIFIER WITH OTHER THAN LINEAR AM-
`PLITUDE TRANSFER FUNCTION AND/OR
`OTHER THAN CONSTANT TIME DELAY
`OBJECTS OF THE INVENTION
`BRIEF DESCRIPTION OF DRAWINGS
`DETAILED DESCRIPTION OF THE INVENTION
`A. IMPROVED LINEAR AMPLIFIER
`1. Linear Amplitude Transfer Function, Input and
`Output Frequencies the Same
`2. Linear Amplitude Transfer Function, with Fre-
`quency Translation Between Input and Output
`3. Linear Phase Transfer Function
`B. AMPLIFIER WITH ARBITRARY TRANSFER
`FUNCTIONS
`C. SUPERIOR EMBODIMENTSOF THE LINEAR
`AMPLIFIER
`D. ALTERNATE FORM OF A SYSTEM WHICH
`INCLUDES FREQUENCY TRANSLATION IN-
`SIDE THE FEEDBACK LOOP
`E. EQUALITY OF PROPAGATION DELAY
`F. POWER OUTPUT CONTROL
`L. Control! of DC Power Supply with Switching
`Regulator
`2. Control of DC Power Supply with Dissipative Di-
`rect-Coupled Regulator
`3. Control of DC Power Supply with AC-Coupled
`Modulator
`4. Control of DC Power Supply with Switching
`Regulator and AC-Coupled Modulator
`5. Control of RF Cycle Duty Ratio
`6. Combinations of the Above Methods
`G. MODIFICATIONS OF THE INVENTION
`CLAIMS
`
`FIELD OF INVENTION
`
`10
`
`20
`
`25
`
`30
`
`35
`
`The presentinvention relates in general to power am-
`plifiers for modulated carrier signals which, for exam-
`ple, can be employed as radio-frequency power ampli-
`fiers for radio communications or phased-array direc-
`tional
`transmitting systems, or ultrasonic-frequency
`power amplifiers for underwater communications or
`sonar transmitters. In its most general form, the inven-
`tion relates to a signal processor as well as a power am-
`plifier inasmuch as the invention can be embodied in
`apparatus having accurately-controlled but arbitrary
`amplitude and phase transfer functions. Sometimes
`such controlled functions are referred to as “shaped
`gain characteristics” and “shaped phase characteris-
`tics”.
`An amplifier embodying the invention can provide
`powerefficiency together with accuracy of the ampli-
`tude and phase transfer functions which are signifi-
`cantly improved over those of prior-art power amplifi-
`ers. In particular, the invention permits both higheffi-
`ciency (approaching 100%) and high accuracy to be
`obtained simultaneously, whereas “prior art’? power
`amplifiers require a compromise to be made between
`these two characteristics because an improvement in
`one is achieved only at a sacrifice in the other. The in-
`vention, in addition, permits the amplitude and phase
`transfer functions to be made dependent upon oneor
`
`45
`
`50
`
`55
`
`65
`
`2
`more external control signals. The invention can be
`embodied as a power amplifier that
`is particularly
`suited for high-efficiency linear amplification of the
`amplitude-modulated and single-sideband signals em-
`ployed in radio communications. Other embodiments
`of the invention can be constructed to be particularly
`useful as high-level high-efficiency high-accuracy am-
`plitude modulators, phase modulators, and amplitude
`compressors.
`Considering the frequency spectrum which results,
`the high accuracy which can be attained with the inven-
`tion in amplitude and phase transfer functionsis partic-
`ularly advantageous in that it allows the output spec-
`trum of the amplifier to have a greatly reduced spurious
`content as compared with prior-art amplifiers. That
`spurious output is undesirable for two reasons. First,
`the spurious output can interfere with other uses ofthe
`frequency band in which spurious componentslie. For
`example,
`the spurious output can cause crosstalk
`among channels in a frequency-division-multiplexed
`system. As another example, the spurious output can
`cause interference in the sideband on the otherside of
`the carrier frequency in a single-sideband system, thus
`comprising the utility of the single-sideband system in
`which onesideband is intentionally removed to allow
`that portion of the frequency spectrum to be available
`for other transmitters. Second, the spurious output can
`distort the signals being communicated, leading to a
`loss of accuracy in the received signals. Considered in
`the time domain rather than as a frequency spectrum,
`high accuracy in amplitude and phase transfer func-
`tions makes possible such results as accurate control of
`phased-array antenna beam shape and direction of
`transmission by rf signals (and, optionally, the time-
`varying control signals) applied to the plurality of
`power amplifiers which drive the plurality of antenna
`radiating elements.
`The high efficiency attainable with the invention is
`especially desirable in applications where any of the
`following are important: low power consumption; low
`equipment temperaturerise; high equipmentreliability,
`small equipmentsize; low weight. Because ofthat high
`efficiency, only a small amount of power is wasted in
`the form of heat, whereby the requirementto dissipate
`heat by heat-transfer means such as heatsinks or air-
`blowersis substantially reduced.
`LINEAR AMPLIFIER
`
`Theideal linear amplifier reproducesat its output the
`exact form of the input signals. Regardless of the type
`of modulation employed in the input signal (single-
`sideband, amplitude modulation, phase mdulation,
`etc.), a carrier which has been modulated can be con-
`sidered to be, at any instant of time, a carrier wave
`characterized by a frequency, an amplitude, and a
`phase with respect to a reference such as the unmodu-
`lated signal of the carrier-frequency oscillator. The in-
`stantaneousvalues of frequency, amplitude, and phase
`of the modulated signal change as time proceeds.If the
`frequency, amplitude, and phase are reproduced accu-
`rately at the amplifier’s output, then the amplifier has
`reproduced the input wave substantially without distor-
`tion. The reproduced wave is considered to be undis-
`torted in the usual sense if the amplifier output is equal
`to the amplifier input multiplied by a constant factor
`(the gain of the amplifier) or if that output is delayed
`by a constant time (the propagation delay). The gain of
`
`9
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`
`3.900.823
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`20
`
`30
`
`35
`
`40
`
`45
`
`3
`the amplifier can be positive or negative (a phase inver-
`sion) and its magnitude can be larger or smaller than
`unity depending on the source and load impedances
`and the power gain. A constant time delay can also be
`considered to be a phase lag which is proportional to
`frequency.
`A switching-type power amplifier (c.g. Class D, Class
`S or Class C drivento full output) can be more efficient
`than conventional linear amplifiers of the Class A, AB,
`and B types. However, because the output amplitude of
`the switching-type amplifier is fixed by the voltage of
`the pewer supply, switching-type amplifiers have gen-
`erally been deemed suitable only for applications such
`as cw or fm, where the output signal is of constant am-
`plitude when the amplifier is delivering output power.
`The frequency and phase components of a modulated
`wave are substantially preserved by a switching amph-
`fier if the amplifier is driven by an input obtained from
`a hard limiter which slices the input signal within a
`small amplitude interval centered on the zero axis, The
`present invention permits anyresidual phase distortion
`in the switching amplifier (e.g. that resulting from non-
`linear impedances) to be substantially climinated by
`introducing into the amplifier a controlled phase shift
`which compensates for the phase distortion. Negative
`feedback may advantageously be used in this regard.
`The present
`invention further causes the amplitude
`component of the modulated wave to be preserved by
`controlling the rf power output of the final stage ofthe
`switching amplifier ina manner such that the amplitude
`of the output rf signal is proportional to the amplitude
`of the input rf signal, This control can be effected in nu-
`merous ways, the simplest of which is to control the de
`power supplyvoltage for the final stage of the switching
`amplificr. Although switching of the amplificr causes
`considerable distortion of the individual rf cycles of the
`input signal, the amplitude of the output rf signal is not
`distorted where the rf power output of the switching
`amplifier is properly controlled. The distortion of indi-
`vidual rf cycles produces rf harmonics and,
`in some
`cases, baseband modulation frequencies, but these can
`be readily eliminated by conventional
`low-pass or
`band-pass filtering, respectively, of the output,
`inas-
`much as these harmonics and baseband frequencies
`generally do not extend into the frequency spectrumof
`the modulatedcarrier signal.
`
`4
`
`OBJECTS OF THE INVENTION
`The principal object of the present invention is to
`provide ahigh efficiency power amplifier, phase modu-
`lator, or amplitude modulator capable of providing any
`desired amplitude and phase transfer functions to a
`high degree of accuracy.
`A further object of the present inventionis to provide
`a linear power amplifier which is particularly suited for
`the amplification of amplitude-modulated and single-
`sideband signals and which has increased efficiency
`and decreased distortion as compared with prior-art
`power amplifiers.
`
`BRIEF DESCRIPTION OF DRAWINGS
`FIG. 1 shows the scheme of a rudimentary embodi-
`ment ofthe invention.
`FIG, 2 depicts an embodiment of the invention.
`FIG. 3 schematically depicts a mechanism suitable
`for employment in the FIG. 2 embodiment.
`FIG. 4 shows an embodimentof the invention which
`permits its use as a signal processor in addition to its
`use as a power amplifier.
`FIG. 5 schematically depicts the invention embodied
`in u linear amplifier.
`FIG. 6 is a block diagram ofa circuit involved in the
`evolution of the circuit of FIG. 7.
`FIG. 7 is a block diagram ofa portion of the pre-
`ferred linear amplifier of the invention.
`FIG. 8 depicts the scheme of the invention as embod-
`ied in a power amplifier which maintains an accurate
`amplitude transfer function while providing frequency
`translation of the inputsignal.
`FIG. 9A is a circuit diagram of one embodiment of
`the power output control means of the invention, and
`FIGS. 9B thru 9F depict the associated waveforms.
`FIGS. 10A through 10F depict waveforms occurring
`in the generation of the pulse-width modulated Signal
`used to drive the circuit of FIG. 9A.
`FIG. 11A depicts, in block diagrammatic form, the
`employment of a typical switching regulator in the
`power output control.
`FIG. 11B schematically depicts the use of a dissipa-
`tive direct-coupled regulator to control the dc power
`supplied to the power amplifier.
`FIG. 11C illustrates the employment in the power
`Output contro] of the combination of a switching regu-
`lator and a dissipative regulator to control the dc power
`supplied to the power amplifier.
`FIG. 11D illustrates, in schematic form, the employ-
`ment in the power output control of the combination
`of an ac-coupled modulator and a switching regulator
`to control the de power supplied to the power ampli-
`fier.
`
`AMPLIFIER WITH OTHER THAN LINEAR
`AMPLITUDE TRANSFER FUNCTION AND/OR
`OTHER THAN CONSTANT TIME DELAY
`
`50
`
`With the invention, the rf power output of the switch-
`ing amplifier can be controlled to cause the output am-
`plitude to be any desired function of the input ampli- -
`tude. For example, accurate amplitude compression
`can be obtained by arranging the apparatus to provide
`the desired nonlinear amplitude transfer
`function.
`Likewise, phase control apparatus may be provided to
`cause the output phase to be a desired function of the
`input phase. The amplitude and phase transfer func-
`tions may also be made dependent upon one or more
`external control signals, thereby converting the appara-
`tus into a signal processor as well as a high-efficiency
`amplifier. The dependencies onthe input signal and the
`external control signal(s) can be linear or nonlinear, as
`desired.
`
`40
`
`DETAILED DESCRIPTION OF THE INVENTION
`A. Improved Linear Amplifier
`1. Linear Amplitude Transfer Function, Input and Out-
`put Frequencies the Same
`A linear power amplifier constituting a rudimentary
`embodimentof the inventionis illustrated in FIG. 1. A
`modulated rf (radio frequency) signal from a signal
`source (not shown) is applied via lead 7 to the input of
`an rf power amplifier 1. Power umplifier Lemay be sub-
`ject to amplitude nonlinearity, and may in fact include
`an amplitude limiter or have an amplitude transfer
`function of the type generally associated with alimiter.
`
`10
`
`10
`
`
`
`3.900.823
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`20
`
`25
`
`30
`
`35
`
`6
`5
`ator connected across the system output port. Other
`In particular, power amplifier 1 may be a switching-
`embodiments of attenuator 3 are discussed below.
`type amplifier, although this is not necessary to the
`A modification of the FIG. 1 arrangement can be
`proper functioning of the present invention. The output
`made by interchanging the positions of attenuator 3
`13 of power amplifier 1 is coupled to an attenuator 3.
`and amplitude detector 2. The modified arrangement
`The amplitude of the attenuated rf output signal emit-
`mayat first be thought to be completely equivalent to
`ted from the output 12 of the attenuator is detected by
`that of FIG. 1; it is in fact usable but greatly inferior to
`an amplitude detector 2. The output of the amplitude
`the FIG. 1 scheme. In the modified system, the signals
`detector2is, in turn, applied to an input of a differen-
`at inputs a and b of differential amplifier 4 are very
`tial amplifier 4, The differential amplifier 4 may be a
`nearly equalto the signal at the output of amplitude de-
`conventional comparator which provides an output in-
`tector 5 multiplied by a constant factor (viz. the attenu-
`: dicative of the difference between two input signals.
`ation of attenuator 3). Ideally, the linear relation be-
`The differential amplifier 4 may additionally include
`tween the outputs of amplitude detectors 2 and § is ac-
`compensation networks of well-known character and
`companied by a correspondinglinear relation between
`function (e.g.
`lead-lag or amplitude limiter), for the
`their inputs, viz. the output at 13 and the input at 7 of
`purpose of improving the performance of the feedback
`the linear amplifier system.In an ideal realization of the
`loop of which the differential amplifier 4 comprises a
`modified system, the transfer characteristics of ampli-
`part. The otherinputa ofdifferential amplifier 4 is con-
`tude detectors 2 and 5 are both perfectly linear and
`nected to the output of an amplitude detector 5 which
`free of offset, and the aforesaid linear relation between
`detects the amplitude of the input signal impressed on
`the output and input rf amplitudes obtains. In practice,
`terminal 7. The output 10 ofdifferential amplifier 4 is
`however, amplitude detectors 2 and 5 are not perfectly
`employed by a power output control means 8 to vary
`linear noroffset-free, although those detectors may be
`the output amplitude of the rf power amplifier 1 in such
`nearly identical. In the modified system, amplitude de-
`a way as to cause it to be directly proportional to the
`tector 2 operatesat a different signal level from detec-
`amplitude of the input rf signal 7. The control means
`tor 5, due to the attenuation introduced by attenuator
`8 may be implemented in numerous ways and several
`3 to obtain near-equality of the signals at a and 6. Be-
`embodiments thereof are described later in this exposi-
`cause the two detectors operate at different signal lev-
`tion. The sense of the inputs a and b relative to the out-
`els, the transfer characteristic of the signal path from
`put 10 ofthe differential amplifier is chosen according
`input terminal 7 to input a is not necessarily matched
`to the input-output control sense of control means 8
`to that of the path from output 13 to input 5 even if the
`and power amplifier. 1 such that an increase in ampli-
`two detectors are identical, if those detectors are not
`tude of the signal at input a causes control means8 to
`perfectly linear and offset-free. The circuit designer
`increase the signal at output 13 of the power amplifier
`employing the modified system, therefore, attempts to
`1.
`provide amplitude detectors with highly linear and off-
`set-free transfer characteristics. Insofar as the nonlin-
`earity and offset of the two detectors can be made to
`approachzero, the modified system can be madeto ap-
`proach distortion-free operation. However, nonlineari-
`ties and offsets in the transfer characteristics of detec-
`tors 2 and 5 always occurin practice inasmuchasideal
`detectors are not realizable in actuality.
`The system of FIG.1 is less subject to distortion from
`the above causes, because the amplitude detectors 2
`and 5 are made to operate at the samesignallevel. In
`this arrangement, the transfer characteristics of the two
`detectors 2 and 5 need only be monotonic and matched
`in order to eliminate the above distortion, they need
`neither be linear nor free of offset. Identical nonlineari-
`ties and offsets in the two detectors do not significantly
`degradethe linearity of the full system; they only cause
`the system open-loop gain to vary somewhatwithsignal
`amplitude.
`Other embodiments of attenuator 3 are applicable in
`various circumstances. Attenuator 3 can operate with
`voltage or current input and with voltage or current
`output, dependent on the types of circuits used for
`feedingits input and for using its output signal. Voltage
`input refers to an attenuator whose input impedance is
`high compared with its source. Current input refers to
`an attenuator whose input impedanceis low compared
`with its source. Voltage output refers to an attenuator
`whose output impedance is low compared to the im-
`pedanceof its load whereas current output refers to an
`attenuator whose output impedance is high compared
`to its load impedance. A resistive attenuatoris the type
`of device most commonly used for attenuataor 3 and is
`usually designed for voltage input and voltage output.
`
`The system as a whole constitutes a negative-
`feedback system operating on well-knownprinciples. If
`the gain around the feedback loop (“‘open-loop gain”’)
`is large, the signals at inputs a and 4 ofdifferential am-
`plifier 4 are caused to be very nearly equal. The outputs
`of amplitude detectors 2 and § are therefore very
`nearly equal. If the transfer characteristics of amplitude
`detectors 2 and 5 are matched, there is a corresponding
`near-equality of amplitude ofrf signals 7 and 12. As the
`amplitudeofthe rf outputsignal 13 is merely a constant
`multiple of the amplitude of the rf signal 12 (assuming
`the attenuator 13 is linear), output signal 13 is then
`very nearly proportional to the input signal 7, and the
`amplifier system is very nearly linear.
`Attenuator 3 can be any of many well-known types.
`The requirements for the desired linear performance
`are (1) the attenuator should have a linear transfer
`function, i.e. H(s) of the attenuator, as operating be-
`tweenits source andits load, should be independent of
`the magnitude and phase ofthe signal applied to the at-
`tenuator input, and (2) the transfer function ofthe at-
`tenuator should be the inverse of whatis desired for the
`entire system. Attenuator 3 may optionally be variable
`for purposes of gain adjustment. Because it usually is
`desired that the output voltage of the system be larger
`than the input voltage, attenuator3 will usually have a
`voltage attenuation from input to output. In some cir-
`cumstances, as where the load being driven by the am-
`plifier is of low impedance, it may be desired to have
`the system output voltage not larger than the system
`input voltage. In such circumstances, attenuator 3 may
`in fact be designed to have an output voltage equal to
`or larger than its input voltage. The most commontype
`of device used for the attenuator3 is a resistive attenu-
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`Attenuator 3 can also be, for example, an inductive
`pickoff coupled to the magnetic field in the vicinity of
`the load (e.g. the antenna) or the load-current-carrying
`conductor(s) feeding the system output port or con-
`necting that port to the load (e.g. the transmission line
`between the amplifier output and the load). Attenuator
`3 can also be a capacitive pickoff coupled to the elec-
`tric field in the vicinity of the load or the system output
`port. It could also be a step-down (or step-up) trans-
`formeror autotransformer connected across the system
`output port, or a Capacitive or inductive voltage divider
`there, etc.
`Moreelaborate versions ofattenuator 3 can use com-
`binationsofresistive, capacitive and inductive coupling
`to the output 13, to the load, or to one or more ofthe
`load-current-carrying conductors. For example, capac-
`itive and inductive couplings can be combined so as to
`sense separately the “forward” (‘‘incident’’) signal and
`the “reverse” (‘reflected’) signal in a system which
`has a load whichis not purely resistive, i.e. a standing
`wave ratio (SWR) greater than 1.0. Such a circuit is
`shown, for example, as a SWR monitor in The 4.R.R.L.
`Antenna Book, 8th Edition, pages 132-134, published
`by American Radio Relay League, Newington, Conn.
`The circuit shown there includes, additionally, diode
`detectors which rectify the carrier-frequencysignals to
`detect their amplitudes for presentation on a dc meter.
`Taking the difference between the forward and reverse
`signal magnitudes yields the componentofthe signal in
`which the voltage and current are in-phase with each
`other, i.e. that component whichresults in powerbeing
`delivered to the load, as distinguished from stored en-
`ergy being exchangedperiodically between electric and
`magnetic fields.
`In general, attenuator 3 can includeresistive, capaci-
`tive and inductive couplings to the power amplifier out-
`put 13 orits load or the load-current-carrying conduc-
`tors between them. Additionally, the attenuator can in-
`clude provisions for sensing the phase difference be-
`tween voltages and currents at the output or load or in
`the conductors, and for combiningthe resulting signals
`in various ways to sense voltage, current, in-phase com-
`ponents, quadrature-phase components, and functions
`thereof.
`For the case of an amplifier which delivers mechani-
`cal power to its load via an electromechnical trans-
`ducer(e.g. to excite acoustic waves in water for sonar
`or underwater communication applications), the pres-
`sure, velocity, or displacementor functions thereof, of
`the medium (the water, in this example) can be sensed
`by an appropriate sensor, instead of sensing an electric
`or magnetic field. This sensor converts the sensed me-
`chanical quantities to an electrical signal which is fed
`to the input of attenuator 3. Similarly, other applica-
`tions can involve conversion of the amplifier output
`powerto light (e.g. in an optical communication sys-
`tem), heat (e.g. in an rf induction heater system), or
`other forms of output. Appropriate sensors are then
`employed to sense and process those outputs and pro-
`vide electrical signals for the attenuator 3. The system
`inherently acts to reduce the effects of nonlinearities
`introduced by the electromechanical, electroptic or
`electrothermal energy conversions from the amplifier
`output to the load inasmuch as the system provides
`high linearity between the input 7 and the output 12 of
`attenuator 3. In view of the various forms which the
`output of power amplifier 1 may take, it is intended
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`that the block labeled attenuator 3 shall include appa-
`ratus which senses the output of the power amplifier in
`whatever form it may take and provides appropriate
`electrical signals at the output of the attenuator.
`The linearity of the entire system transfer function is
`governed directly by the linearity of the attenuator 3
`transfer function. Therefore the components chosen
`for use in the attenuator should be sufficiently linear to
`assure the desired overall system linearity, For exam-
`ple, appreciable nonlinearity can be found in some ce-
`ramic capacitors, some inductors with ferromagnetic
`cores, and some carbon composition or screenedthick-
`film resistors. In addition, the inductance, Capacitance
`and resistance values of these attenuator components
`can change during the modulation of the poweroutput
`of power amplifier 1. This can occur because the Cc,L
`and R values can be functions of the component tem-
`perature. The component temperatures can vary in re-
`sponse to the varying powerdissipation in the compo-
`nents, resulting from the modulation-caused variation
`in power output of power amplifier 1.
`Similarly, certain types of components generate more
`electrical noise than others; such excessive noise, when
`present, causes spurious noise modulation of the sys-
`tem output signal via the feedback control system. In-
`adequate shielding of the attenuator 3 from sources of
`interfering signals may allow such interfering signals to
`be picked up by attenuator 3. The introduction of such
`signals into the feedback control loop could likewise
`cause spurious modulation of the system outputsignal.
`Having been informed of these potential sources of
`distortion, the engineerskilled in the art can design the
`attenuator 3 to be sufficiently free of distortion, noise,
`and pickupofinterfering signals for the purposesofhis
`equipment design.
`2. Linear Amplitude Transfer Function, with Fre-
`quency Translation between Input and Output
`It is common for amplitude-modulated Signals to be
`generated at a frequency other than the outputfre-
`quency and later be heterodyned to the output fre-
`quency. Theinitially generated signal is often fixed in
`frequency while the output frequencyis variable, e.g.
`in a tunable single-sideband transmitter employing a
`fixed-frequency crystal filter in the modulator. Such
`frequency translation involves the use of one or more
`mixers,filters, and buffer amplifiers, any orall of which
`may have nonlinear transfer characteristics, resulting in
`distortion. Where the system of FIG.1 is employed,all
`frequency translation must be made in the input signal
`prior to applying that signal at terminal 7. That system
`may be modified to provide for frequency translation
`within the system by utilizing the scheme depicted in
`FIG. 2 where a frequency translator 9 is interposed be-
`tween input terminal 7 and rf power amplifier 1, and
`the inputof amplitude detector 5 is fed from the output
`of the modulator (now shown) or from a subsequent
`point in the signal path. Any amplitude distortion due
`to nonlinearity in the frequency translator is now re-
`duced by the feedback action of the system. Amplitude
`detectors 2 and 5 are designed so thattheir amplitude
`transfer characteristics are well matched when the de-
`tectors operate at different rf frequencies, as is the case
`here.
`3. Linear Phase Transfer Function
`Spurious phase modulation is sometimes a significant
`source ofdistortion in linear amplifiers. This phasedis-
`tortion mayresult, for example, from nonlinear loading
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`of tuned circuits (e.g. by a transistor or vacuum-tube
`input impedance) or from nonlinear reactances (e.g.
`reverse-biased semiconductor junctions or partially-
`saturated inductors). Just as the system of FIG. 1 em-
`ploys feedback to reduce amplitude distortion, feed-
`back can be employed to reduce phase distortion. In
`the system of FIG. 2 this is accomplished by the control
`mechanism 11 which is arranged to control a variable
`phase-shift network 17 so as to cause the phase of the
`signal at the output 13 of the system to be equal to the
`phase of the signal at lead 19 (plus perhaps a constant
`phase shift).
`An implementation of the contro] mechanism 11 is
`schematically illustrated in FIG. 3. In that scheme,
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