[Amps] grounding grids

[Bill, W6WRT]

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