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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