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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

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Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: Bill Turner <>
Date: Thu, 15 Dec 2005 09:17:01 -0800
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At 09:09 PM 12/14/2005, Tom Cathey wrote:

>It was pointed out to me that the cathode resistor for negative feedback is
>to be used only for a grid driven [grounded cathode] amplifier to generate
>degenerative [negative] feedback.


This is a question I've often wondered about.

Does a cathode resistor in a grid-driven amp actually produce 
negative feedback, or does it merely reduce the gain?

My understanding is that for true negative feedback, some of the 
output must be fed back to the input out of phase. In the case of the 
cathode resistor this does not seem to be happening. In fact, the 
output could have a wide variation in gain without affecting the 
input circuit at all. As an extreme example, suppose the plate load 
circuit dropped to a very low impedance for some reason. With true 
negative feedback, the circuit would automatically compensate and the 
output would remain the same (within the capabilities of the feedback 
loop of course). With only a cathode resistor, the output would fall 
and no compensation would occur. To me, this does not sound like true 
negative feedback.

Is this correct?

73, Bill W6WRT

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