[Amps] grounding grids

[DF3KV]

The un-bypassed emitter or cathode resistor is still causing NFB, the
voltage developed across the resistor is out of phase with the input signal
and in phase with the output voltage.
That kind of NFB is often used with tetrode amplifiers to improve IMD.
IMHO there is nothing like true NFB, probably you are referring to the
classical feedback amplifier by Black.
You can find NFB in many variations, influencing either voltages or
currents.

http://gabevee.tripod.com/nfb.html


73
Peter


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Bill, W6WRT
Sent: Donnerstag, 30. Juli 2009 18:10
To: amps at contesting.com
Subject: Re: [Amps] grounding grids

ORIGINAL MESSAGE:

On Thu, 30 Jul 2009 11:28:38 -0400, Bill Fuqua <wlfuqu00 at uky.edu> wrote:

>vious message applied to the negative feedback in a grounded grid 
>amplifier due to the cathode and drive impedances.Similar to the negative 
>feedback due to an un-bypassed emitter resistor in a common emitter 
>amplifier. I was not referring to a un-bypassed grid resistor at all.

REPLY:

An un-bypassed emitter resistor is not negative feedback, it is just gain
reduction. For true NFB you have to take a sample of the output and feed it
back
to the input, out of phase with the input.

True NFB always reduces the input impedance because the fed-back signal
opposes
the input signal and therefore requires more drive. In the emitter resistor
case, the input impedance is actually  increased. 

For a full discussion of feedback amplifiers, both negative and positive,
see:

http://en.wikipedia.org/wiki/Negative_feedback_amplifier

Caution: This may be more information than you wanted. :-)

73, Bill W6WRT
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