Arf Feed Bark? (RF Feedback) for RF Amps??

[WBear2GCR]

I know that at least one commercial communications mfr has used RF feedback to linearize and reduce IM in power amplifiers.

A while back I did search the internet for sources of information on the way this is done in practical terms
and for textbook/white paper sources on the design aspects. I came up with just about nothing.

In audio amps one method is to use a diff pair at the input and swing the output back to the inverting input. Another aspect used in some audio applications (and apparently in other industrial applications - maybe servos?) is to pump up the gain and so be able to increase the amplitude of the feedback.

Recently this method was used to bring an already super low distortion audio IC power amp (LM3886) that does 50W with <0.001% THD down to <0.0004%THD (practically impossible to measure accurately).

In the audio world there are a range of feeback methods, including "global loop feedback", "nested loop feedback" and various compensation techniques to extend bandwidth... would love to know if the equivalent exists in the RF world or not?

Of course in solid state audio there is virtually no phase shift in the passband, and no coupling transformers to introduce hysteresis or phase shifts. In a typical solid state RF amp, everything is coupled via ferrite xfrmrs.

What I'm trying to learn is if there are practical RF feedback methods that could be used to improve the performance of a basic solid state linear amp, for example like the EB-104?

Anyone know about this, where to find out, or if there are internet groups that deal with this or have people with expertise in this area??

                                                       _-_-bear

[WA2SQQ]

Isn't this the basis for Anan's Clear Voice feature?

[N2DTS]

Not sure how it would work with transformers in line what with the phase shift.
You might be able to do it with screen modulation, no phase shift, detect the audio at the output and feed it into the audio input out of phase.
I know adding feedback to the tube modulator I built (dx60/w1qix) made things work VERY well, and no need to limit it in range or level.

Did not Collins use RF feedback in some of their stuff?

[N2DTS]

Collins did it:
http://www.collinsradio.org/cca-collins-historical-archives/the-equipment-of-collins-radio/the-grey-boxes/32s-3/

[N2DTS]

If you read the Collins spec's, they got the 3rd order distortion all the way down to -30 db!
Its astounding what inverse feedback can do!
(not)

[WBear2GCR]

RF feedback there chaps, not AF.

Also, the absolute level is not the point to consider as much as the reduction of distortion due to the addition of feedback.

                          _-_-

[Opcom]

From manuals studied over the years, stage by stage feedback has been most often used and rather lightly, both around a stage and inside it as degeneration. Similar schemes that can't really be called simple RF feedback have also been used to make an entire transmitter more linear. The GRC-106 is one example. Totally different schemes seem to apply based on if it is an amplifier or transmitter, and what class of operation.

[WBear2GCR]

At the present my interest is in investigating to find out if it is possible to apply a feedback technique to the typical solid state linear, which I presume is more or less class AB. The EB-104 being one example.

Local degeneration I'm not counting, looking for loop feedback schemes at RF.