On 19 Oct 2005 22:29:03 -0700, wrote:

Allison: Thanks.

Obviously you are right. But I see the same kind of confusing language
in other editions of the handbook. In the 1980 edition (otherwise one
of my favorites) they have (pg 6-19) a collector-base feedback network
with just a cap and a resistor in series. Text reads: "C1 and R3
provide negative feedback which increases progressively as the
frequency is lowered." It calls for a cap of values between 220 pf and
.00015 for hf band amps, IN SERIES with a resistor of from 51 to 5600
ohms.

Is this just a case of some confusing language that has kind of worked
its way in the handbook DNA and is passed down from generation to
generation? Or is there really something about these cap and resistor
in series networks that cause them to increase the amount of feedback
as freq drops?

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

That reference is definately an error. The only way that would be
expected to occur is when the C is sufficiently large enough to be
useful at lower frequencies. Often enough it was.

Amplifiers of that type and time use transistors of say FT 150mhz
and a betacutoff of 1 to 3 mhz. What happen is as you go up in
frequency from 1 to say 30 mhz the effective AC beta goes from
about 100 down to 5. The amplifier gain also has that same
downward curve. It's that rising gain when progressing down in
frequency your working against with the described network.
So when that allowed for and the series C and R it's is possible
to say that as the feedback increases as the gain increases.
Even then without putting boundaries on the values and using
a real (working) circuit it's not a good explanation and borders
on error.

The "yabut" of amplifiers (that one being a class C poweramp)
is that EVERYthing around the transistor interacts due to the low
impedences and high currents. So by observation that circuit
may actually work somewhat as described but likely there are
onter factors at work that are not well explained there.

Examples of this (same circuit P6-19) that can cause grief. The
bypass on the collector circuit needs to be effective at audio through
RF and yet the schematic only shows one cap. The likely circuit
when built would not be stable.

I think again that explanations suffers from trying to be too brief
and leaving out detail. When examined without full context
appears to be an error can still have validity. The RAH is a good
text, lots of information and relatively few errors. However it's
only one source and even they cite other sources for greater
detail. Failure to take advantage of those citations and delve
deeper can lead to believeing in lore rather than studied
consideration.


Allison