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October 19th 05, 05:45 PM

On 18 Oct 2005 22:58:55 -0700, 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?

In the 1994 RAH the same reference is in section 4 figure 53C.

There are TWO feedback networks. One in the emitter circuit
and the Collector Base feedback. Ther is also the transformer
that will limit low frequecy response as a secondary action
which also limits Collector feedback as well.

However, they are refering to the RC pair found on the leg of the
emitter. For that case the statement would be correct.

Typically those amps (unless higher power) the emitter has a bias
resistor. That resistor is bypassed with a RC of .1 or .01uF and
series R of a few ohms (typically less than 20 ohms). That C is
fairly small for low RF and audio to effectively bypass the emitter
circuit. so at say DC and audio the Emitter resistance is 110ohms.
At some greater frequency the capacitor looks like a very low
reactance and the emitter resistance is effectively 10 ohms at RF.

That emitter network allows setting DCbias (operating point) and also
the AC feedback at the Emitter Base leg. Since the capacitor
has greater reactance at low frequencies the feedback is greater.

The RAH has only a very terse description and lacks design detail.
If you look at Solid State Design for the Radio Amateur (ARRL press)
and/or Exprimental Methods in RF Design (ARRL press) you will
have a far more detailed explanation of how that amplifier works and
how to calculate values for design.

Hope that helps.

Allison