AGC to Which Stages? - Comments Welcome - RF Cafe Forums
Post subject: AGC to which stages? Posted: Mon Aug 21,
2006 7:58 am
Joined: Fri May 19, 2006
Looking at old schematics of tube radios, the AGC is usually
wired to all stages: RF preamp, RF mixer, and IF stages (sometimes
just the IF stages). Would I be able to get away with hooking it
to just the RF preamp - possibly (DC) amplfied?
they used to connect the AGC to all stages to get more negative
feedback out of a low voltage swing: -1V on one grid only brings
gain down by so many dB, but -1V on 4 grids brings it down by much
Thanks in advance,
Post subject: Posted: Mon Aug 21, 2006 3:39 pm
Joined: Sun May 14, 2006 8:53 pm
Do you have a particular application in
AGC is mainly there to keep everything linear, so if
the first thing to non-linear is the RF-amp that's where you need
it the most. Usually though the first thing is the IF. And because
sometimes you have strong unwanted signals within your passband,
you may want to start introducing loss (i.e. AGCing) in the IF so
as to not affect your NF (and thus not heavily impacting a low-power
As I said before, it depends in your application.
Post subject: AGCPosted: Mon Aug
28, 2006 11:11 pm
Joined: Wed Feb 22,
2006 3:51 pm
Just a quick note - it's
actually very hard to design an amplifier with a gain that's variable
over a wide range. That's why many circuits apply AGC to several
stages - to get a wider control range.
However, there are
other considerations: intermodulation and noise figure, for example.
That leads designers to use "delayed AGC", where some stages receive
the AGC at levels that other stages don't receive it at. There are
different nonlinearities used to get this, some not very visible.
subject: Posted: Tue Aug 29, 2006 12:01 pm
Joined: Fri May 19, 2006 5:01 am
Thanks fred, that's very helpful. Confirms my suspicions!