Again, the common emitter amplifier gives you 180deg of phase change. If
this were to be applied directly to the base of the transistor through only
a resistor, you would get direct cancellation at a level determnined by the
size of the resistor. The feedback network throws in a phase change
proportional to the tangent of the reactance and the resistance part of the
network. I think the tangent function for the feedback will look something
like:

w(k1) / [k2 + w(k3)]

where w is the radian freq. and k1, k2, and k3 are constants made up of R1,
R2, and C. k1 = C(R2)**2 or something like that.

As the freq goes up the phase change gets larger - i.e. it moves the phase
difference between the collector and the base away from 180deg., which
means less negative feedback. The maximum phase change contribution from
the RC network would be 45deg as w gets very, very large (arctan 1 = 45).
As the frequency goes down, the phase change contribution from the RC
circuit gets to be smaller and smaller which means the feedback moves
closer and closer to being 180deg - which means more negative feedback. (as
w approaches zero arctan 0 = 0)

This is probably only good for RF frequencies. Say, above 1Mhz. I would have
to actually write out the transfer characteristic and graph it to see
exactly what happens. Conceptually, it makes sense though.

tim ab0wr



wrote:



Thanks to all who responded to my question.

I went back and took a closer look at other editions of the Handbook,
and at Solid State Design for the Radio Amateur (SSDRA).

The Handbooks all seem to suggest that the shunt feedback networks
using a resistor and a cap in series will somehow result in more
negative feedback at lower frequencies. Given that capacitive
reactance varies inversely with freq, I still can't see how this
happens. Tim's message seems to provide one possible explanation, but
I'm kind of surprised that I haven't seen mention of this in the ham
literature.

When SSDRA discusses shunt feedback, (Chapter 8, page 188) they have a
resistor, a blocking cap, and an inductor all in series between the
collector and the base in a common emmiter amp. "The inductor has the
effect of decreasing the feedback at high frequencies, while the 470
ohm resistor is the dominant element at low frequencies." That's easy
to see. But without the inductor (no inductor in the Handbook
presentations) it is hard to see how this works.

Thanks again to all,

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

tim gorman wrote:
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?

Help! 73!

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

I think to actually figure this out you will need to write the gain
transfer equation for the amplifier. Many of the replies here are talking
about an RF amplifier but are analyzing the feedback at DC.

Remember that the output of a transistor is NOT in phase with the input.
As the base voltage goes UP the collector voltage goes DOWN. (think of a
transistor used as a switch - when the base is biased off the no current
flows and the collector is at power supply potential - when the switch is
biased on then current flows and the collector is driven toward the
potential of the emitter -- i.e. the collector voltage goes down) For RF
the actual phase difference between input and output is dependent upon
the input and output impedances of the transistor as well as the gain
transfer characteristic (things like transit times of the current
carriers and junctions widths and all sorts of stuff figure in here).
*THEN* you have to consider the phase contribution of the RC network in
the collector to base network. At some RF frequency the collector is
180deg out of phase with the input so a direct feedback link from the
collector to the base would be *negative*. It is quite possible that the
phase relationships of this particular amplifier are such that the
negative feedback increases as the frequency goes down.

You would just have to write the equations and see where they take you.

tim ab0wr