EMRFD Message Archive 1324

Message Date From Subject
1324 2008-01-14 02:32:41 Heikki Ahola (OH... Variable gain in linear amplification
Hi group,

There was a lengthy discussion about linear amplification chains in
this group some time ago. One topic, however, did not come up, i.e.
gain variation/adjustment within the chain. During my experimentations
I have found this feature quite useful in compensating the unavoidable
(?) gain reduction with frequency. Another application Ihave in mind
is ssb speech processing (rf clipping) where the clipping level could
be adjusted with variable gain. I have already used a broadband MOSFET
amp for this purpose and also built the cascode hybrid amp (one stage
only). In latter case I found the signal levels in tx amplification
exceeding those the amplifier can handle.

Question: Has anyone experience of or suggestion for this type of
circuits. What I have considered already include dual-gate mosfets,
bipolar broadband amps and pin diode attenuators. The amplifier should
comfortably handle signal levels up to 1 V (50 Ohms. I would imagine
that a bipolar amplifier is just the right thing if the gain
adjustment can be worked out somehow (bias current, emitter
degeneration ?).

I am currently constructing my second SSB exciter very much according
to the guidelines given in EMRFD. If any solutions for the gain
variation can be found, those will be readily included in my design
and applied to the equipment.


Heikki (OH2LZI)
1325 2008-01-14 05:45:22 Allison Parent Re: Variable gain in linear amplification
1326 2008-01-14 06:43:25 Heikki Ahola (OH... Re: Variable gain in linear amplification
Dear Allison and the group,

Tks fer the comments and advice ! I agree that we should aim at flat
response in the linear amplification chain but at least in my case the
outcome has not been that optimal yet...

I would like to clarify my question regarding rf clipping. The
circuitry in EMRFD by Wes works OK, but it has no control on clipping
level. The old info from the sixties with oscilloscope images
(duplicated in many editions of ARRL Handbook) show the effect of this
level. Although just one constant level might be sufficient,
experimentation calls for a variable gain amplifier in front of the
clipping stage. This has nothing to do with flattening the response
with ALC etc., just some means of varying the gain.

Heikki (OH2LZI)
1329 2008-01-14 18:33:37 Allison Parent Re: Variable gain in linear amplification