RadioBanter - View Single Post

October 19th 05, 05:34 PM

wrote:
I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

I think the capacitor is large enough that you can consider it a short
circuit for all frequencies (above 1/(2*pi*3300*.01*10^6)=4.8kHz). So
the author has confused himself about that. The feedback passes
through an effectively a constant impedance of 560 ohms. The 3300 ohm
resistor and capacitor are just there to set the DC operating point.
As the low frequency gain rises, the feedback current increases as the
output voltage tries to increase, but this has nothing to do with a
change in the feedback path impedance.