A resistor in series with the emitter of a BJT causes negative 'current'
feedback as opposed to negative voltage feedback. In either case, the gain
of the overall circuit is reduced and the IMD performance of the circuit is
improved. One can achieve the same negative current feedback by placing a
resistor in series with the base of the transistor but this is generally not
done, because the beta of the transistor is now part of the equation for the
circuit gain (although the microwave designers do this all the time.) This
causes the gain and IMD of the circuit to vary from one device to another.
These same characteristics would apply to tube circuits in grounded grid or
grid driven and the same variability in performance will result from a
series impedance in the grid circuit as a function of the mu of the tube.
If the open loop gain of the tube is consistent from one to device to
another, one could get away with the series impedance in the grid circuit
and achieve similar performance as a series impedance in the cathode without
all of the thermal losses.
Now, many years later, we are getting tubes from China that may not match up
with the gain or impedance characteristics of the tube that the amplifier
was originally designed. These series components may not behave as
intended.
Steve
KT4FY
---------------------
ORIGINAL MESSAGE:
On Thu, 30 Jul 2009 11:28:38 -0400, Bill Fuqua <wlfuqu00@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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