Referenced phase RF feedback linear amplifier

A feedback network for a linear amplifier provides for sampling the input signal and removing the modulation from the input signal while maintaining a 180.degree. phase relationship with the input which is essentially the process of converting it into a square wave. The output signal is also sampled and the envelope separated from the sampled output signal and used to drive a modulator which modulates the square wave. This modulated square wave is then mixed with the input signal prior to amplification. In addition, automatic gain control is provided by a level comparator that compares the output signal with the input signal and the results of the comparison is used to attenuate or to control the level of the modulated square wave.

BACKGROUND OF THE INVENTION 
This invention relates to feedback networks for linear amplifiers. One of 
the major disadvantages of linear solid state amplifiers is that 
intermodulation distortion performance is poor in comparison to their 
vacuum tube counterparts. It is generally difficult to apply RF feedback 
around multiple stage of broadband solid state linear amplifiers. The 
phase shift associated with these amplifiers varies too widely to consider 
any type of phase compensation network in the feedback loop. Without 
feedback, of course, the amplifier performance is limited to the 
capabilities of the transistors or tubes in the amplifier gain chain. The 
performance of an amplifier, in the Radio Frequency (RF) range, is 
measured by the intermodulation distortion produced by the application of 
two equal amplitude signals or tones, generally separated in frequency by 
1 KHz, to the amplifier. Acceptable performance of the amplifier is when 
the intermodulation distortion is greater than 25 to 35 db below either 
tone and gain variation on the order of 6 db. It is generally accepted 
that application of feedback will improve the performance and reduce gain 
variation depending upon the amount of feedback employed. Traditionally, 
10 db of feedback should improve the intermodulation distortion level by 
10 db. 
The problem was addressed in U.S. Pat. No. 3,777,275 in which non linear 
amplifying devices were used to produce bandpass linear amplification of a 
signal having amplitude variation. The input signal was transformed into 
two constant amplitude phase modulation components which together contain 
in their phase fluxuation the total information content of the input 
signal. The components were amplified separately by devices which 
preserved phase, and the recombination of the amplified components 
reproduced a linear amplified replica of the original signal input. This 
technique was primarily useful at high frequency, but also could be 
modified to provide a frequency translation. 
Similarly in U.S. Pat. No. 3,909,742, hybrid combiners, nonlinear 
amplifying devices, phase varying devices and an attenuator were used to 
produce a linear amplified replica of a bandpass analog signal having 
amplitude variations. The original analog input signal and a feedback 
signal are applied to a hybrid combiner which subtracts the feedback 
signal from the original input signal to produce one of two components of 
equal amplitude and adds a feedback signal to the original input signal to 
produce the other component. The components are amplified separately by 
nonlinear amplifying devices and the amplified components then are 
recombined by a second hybrid combiner which produces two outputs. One 
output, the difference between the two components is the linearly 
amplified replica of the original signal. The other output, the sum of the 
two components, is the feedback signal which is applied through a series 
combination of the phase varying devices and the attenuator to the first 
hybrid component. 
The problem of distortion in the use of feedback circuits to reduce or 
eliminate distortion was discussed for tube type amplifiers in Chapters 13 
and 14 of the book entitled, "Signal Sideband Principles And Circuits" by 
E. W. Pappenfus, W. B. Bruene and E. O. Schoenike, published by 
McGraw-Hill. 
SUMMARY OF THE INVENTION 
A feedback network for a linear amplifier provides for sampling the input 
signal and removing the modulation from the input signal while maintaining 
a 180 degree phase relationship with the input which is essentially the 
process of converting it into a square wave. The output signal is also 
sampled and the envelope separated from the sampled output signal and used 
to drive a modulator which modulates the square wave. This modulated 
square wave is then mixed with the input signal prior to amplification. In 
addition automatic gain control is provided by a level comparator that 
compares the output signal with the input signal and the results of the 
comparison is used to attenuate or to control the level of the modulated 
square wave. 
The circuitry disclosed utilizes a hybrid component as well as 
semi-conductors and can be used on essentially any power amplifier. 
However, the preferred embodiment discloses the circuitry for utilizing 
Radio Frequency feedback (RF) for a linear radio frequency amplifier. 
Many advantages of the present invention may be ascertained from a reading 
of the specification and claims in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference should now be made to FIG. 1 wherein the input signal is applied 
to the input terminal 9 of the linear amplifier 10 which includes a mixer 
stage 1, an amplifier stage 2 and a feedback loop 3 that includes a level 
comparator 7 for comparing the output of the amplifier 2 as it appears on 
terminal 5 with the input signal present on input terminal 9. Not only is 
the input signal, as mentioned earlier, applied to the level comparator 7, 
the mixer 1 but is also applied to the square wave generator 21. The 
output signal of the level comparator 7 results from the comparison of the 
input signal level present at terminal 9 and the output signal level 
present at terminal 5 and is utilized as an error signal which is applied 
to the attenuator 15 by conductor 13 to control the level of the mixing 
signal applied to the mixer 1. The mixing of the two signals distorts the 
input signal so as to compensate for the non linearities of the amplifier 
2. 
The mixing signal that is applied to mixer 1 is developed from the input 
signal at terminal 9 and the envelope of the output signal present at 
terminal 5. The input signal that is present on terminal 9 is converted to 
a square wave by the square wave generator 21 while maintaining a 180 
degrees phase relationship between the input signal and the square wave. 
The square wave is then applied to the modulator 23 where it is modulated 
by the modulating signal on conductor 25. The modulation signal is 
essentially the envelope that is present on the output signal at terminal 
5 that has been separated from the output signal by the envelope detector 
27. The modulating signal modulates the square wave which is then applied 
to the attenuator 15. The attenuator adjusts the level of the mixing 
signal in relationship to the direct current level or error signal that is 
provided by the level comparator and applies the modulated square wave or 
mixing signal to the mixer 1 which mixes the mixing signal with the input 
signal. The resulting product is then amplified by amplifier 2. The mixing 
signal that is present on the output of the attenuator 15 has ideally a 
180 degree phase relationship with the input signal that is present at 
terminal 9. It is important to note that although the amplifier 2 is shown 
as a single unit, it can represent a multi-state amplifier, which for 
optimum performance, the feedback loop should cover the whole amplifying 
chain. However, the mixing signal applied to the mixer 1 should have a 180 
degree phase relationship with the input signal and the feedback network 
would thus reduce the intermodulation distortion that is usually present 
in solid state amplifier circuits, especially ganged amplifiers. 
In FIG. 2 there is provided a simplified schematic diagram of a linear 
amplifier 10 with a reference RF feedback network 3 that includes the 
amplifier 2 which can include a plurality of amplifier stages. An RF input 
signal is applied to the input terminal 9. The RF signal is a signal such 
as that represented by the waveform 109. The RF input signal is applied to 
the mixer 1 and also to a high gain amplifier 21. The high gain amplifier 
21 is a limiter that converts the input signal into essentially a square 
wave such as that illustrated by waveform 121 which is applied to the 
modulator 23. The modulation signal that modulates the square wave is a 
feedback signal, represented by the waveform 105 and is picked off from 
the output terminal 5 by conductor 13. 
The feedback signal is applied to the envelope detector 27 which separates 
the envelope from the signal and provides the envelope for modulation 
purpose to the modulator 23. The envelope signal is illustrated by 
waveform 127. The square wave is then modulated and applied to the 
attenuator 15. The attenuator is used to control the signal level of the 
mixing signal and is driven by the output of the level comparator 7. 
The level comparator samples either the input power or voltage level that 
is applied to terminal 9 and compares it with the output power or voltage 
level that is present on terminal 5 and uses the results of the comparison 
to adjust the attenuator 15. 
The output of the attenuator is filtered by the harmonic filter 14 to 
remove unwanted harmonics and spikes and applied to the feedback mixer 1 
with a 180 degrees relationship with the input signal through resistor 12. 
Resistors 12 and 13 isolate the RF input signal source from the mixing 
signal through the low base impedance of transistor 16. 
In summation, the above described invention obtains a sample of a radio 
frequency input signal to a broadband amplifier and feeds it to a high 
gain amplifier 21 the output of which is a square wave signal that is 
representative of the input signal. Because the square wave signal is 
limited, it contains no amplitude modulation component. The square wave 
output is connected to a modulator 23 which mixes the square wave input 
signal with the output of an envelope detector 27 which has separated the 
envelope from the output of the amplifier 2. The square wave is thus 
modulated with the envelope and fed into an attenuator 15 controlled by a 
level comparator 7. The level comparator 7 samples either the input power 
or voltage level and compares it with the output power or voltage levels 
and uses the results of the comparison to control an attenuator 15 which 
in turn controls the amount of feedback that is provided to the input of 
mixer 1. The function of the attenuator is to increase or decrease 
feedback in the mixer 1 in order to keep the gain of the overall amplifier 
constant. The output signal of the attenuator is filtered to remove 
harmonics and applied to the mixer 1, 180 degrees out of phase with the 
input signal. 
In general, a sample of the signal that is applied to the linear amplifier 
is taken, modulation removed but the sample retains a phase relationship 
of 180 degrees with the input signal. The sample is modulated with the 
output signal's modulation envelope and level control with the controlled 
modulated sample signal being used as a feedback signal into the input. 
Many changes and modifications in the above described embodiment of the 
invention can, of course be carried out without departing from the scope 
thereof. Accordingly, to promote the progress and science in useful arts, 
the invention is disclosed and is intended to be limited only by the scope 
of the appended claims.