On Wed, 19 Oct 2005 13:16:45 -0500, "Steve Nosko"
wrote:


wrote in message
roups.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


In the 70's I worked on 150 MHz PA's and we used what we called a
"banana". It was called that because once one got fried and afterward it
looked like a cooked banana. It was a orange drop (dipped) cap (I think
mylar), value forgotten - somewhere in the .001 - .01 range, with two 1/4
watt resistors in series, one at each end that held it up over the power
transistor.
Though I don't remember if they were ever measured, the rationale was
that these caps had considerable inductance at 150 MHz. and were thus an
"open" there. Down in the 1-20 MHz range, where the regen (regeneration -
oscillation) occurred, they were a "short". Also remember that the
transistor impedances are in the .1-1 ohm range for power devices. I think
we also may have put small, 50 ohm, beads on the resistor leads. If the
bead exploded, you knew you hadn't sloved the regen problem because there
was considerable energy at the regen frequency. and thay became good loads.

For regen there are the "dancing faintlies" and the "christmas tree" types.
(:-)

Forget not those parasitics in the components.
73, Steve, K,9.D;C'i

Having tamed a few solid state power amps I can appreciate that.
However this is not a similar case.

The amplifier in question is of the lower power (under 50mW) type
commonly used for wide band amplifiers and/or RF amps at low levels.
The question really stems from a far too brief description of said amp
in the handbook.

Allison
KB1GMX