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AGC to which stages? - Comments welcome - RF Cafe Forums

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Charl

Post subject: AGC to which stages? Posted: Mon Aug 21, 2006 7:58 am

Colonel

Joined: Fri May 19, 2006 5:01 am

Posts: 25

Location: Netherlands

Hello all,

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?

I presume 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 more.

Thanks in advance,

Charl

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languer

Post subject: Posted: Mon Aug 21, 2006 3:39 pm

Captain

Joined: Sun May 14, 2006 8:53 pm

Posts: 17

Location: Earth

Do you have a particular application in mind?

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 desired signal).

As I said before, it depends in your application.

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fred47

Post subject: AGCPosted: Mon Aug 28, 2006 11:11 pm

General

Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Hi!

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.

Good Luck!

Fred

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Charl

Post subject: Posted: Tue Aug 29, 2006 12:01 pm

Colonel

Joined: Fri May 19, 2006 5:01 am

Posts: 25

Location: Netherlands

Thanks fred, that's very helpful. Confirms my suspicions! :)

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