[Amps] Westinghouse BC Ammeter QX-37

[Karl-Arne Markström]

The primary reason why negative feedback reduces IMD is that
it reduces the curvature of the transfer functions of the active devices.

By feeding a portion of the output back to the amplifier input in the proper phase,
some of the inherent curvature in the transfer characteristics of the 
tube or transistor will be cancelled.

There is a limit to how much feedback that can be applied, as the
amplifier gain decreases with increasing feedback.

Also, there is the overall problem of gain and phase stability margins when applying
feedback over many tuned stages.

You can still overdrive an amplifier with feedback, but the relative drive level required
will increase approximately with the amount of feedback applied.

A full treatment can be found in the textbook "Single Sideband Principles and Circuits".

73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "Bill Turner" <dezrat1242 at ispwest.com>
To: "Tom Cathey" <K1JJ at comcast.net>; <amps at contesting.com>
Sent: Monday, January 09, 2006 5:44 PM
Subject: Re: [Amps] Westinghouse BC Ammeter QX-37


> 
> ORIGINAL MESSAGE:
> 
> At 08:39 AM 1/9/2006, Tom Cathey wrote:
> >Speaking of amps, I've started building my new "-55db 3rd order" amp with
> >the 4CX-250 class A driver and 4CX-3000 final using 20 db of RF negative
> >feedback around this chain.
> 
> 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Ok, point taken, thanks.
> 
> One question, though. How exactly does negative feedback reduce IMD? 
> Is it by preventing or reducing overdrive or is it something else?
> 
> Bill, W6WRT
> 
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