Here is a repeat of my previous input on this topic.

Bill W0IYH

"William E. Sabin" wrote in message
news:IlM5f.445451$x96.139037@attbi_s72...
The 1994 ARRL handbook, page 4-23, Fig 53C shows the feedback amplifier
that you describe. However, it is a common-emitter circuit, not a
common-base circuit. The 3300 ohm resistor returns to +12 V DC and
provides DC base current for the 2N5109. The 0.01 uF bypass makes the 560
ohm resistor the main source of this feedback at radio frequencies.

There are two kinds of feedback in the circuit. One is the 560 ohms in
the base circuit. The author calls this "negative feedback". I call this
"voltage feedback". This feedback does not change very much over the HF
region.

The other feedback (he calls it "degenerative feedback") is a 10 ohm
resistor in the emitter in series with a 100 resistor which is shunted by
0.01uF. I call this "current feedback". This feedback increases at low
radio frequency because the impedance from emitter to ground increases at
low radio frequencies. This is the feedback that the text is referring to
in the text and it is correct. This kind of feedback increases at low
frequency. If the 0.01 uF were replaced by a 1.0 uF this increase in
feedback would be a lot less at low radio frequencies.

The author, probably DeMaw, got his terminology slightly mixed up but he
is referring to the emitter to ground current feedback, not the collector
to base voltage feedback.

If you have a copy of the 2004 Handbook, chapter 17 has a sidebar
discussion of negative feedback that is interesting. Later editions may
have deleted it.

Bill W0IYH

wrote in message
oups.com...
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?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr