Most RF power transistors have a highly reactive, usually capacitive at HF,
input impedance.
This is in addition to the resistive part of the impedance being highly non
linear: it takes values from infinity when the transistor base junction is
reverse biased and about 1 ohm or so when biased forward. This resistor-
called a swamping resistor, by the way- does just that: it tends to equalize
the input impedance of the transistor across the range of resistance change.
It can be used as part of the bias network, but it's not usually done, and
definitely not in a class C amplifier.
73 and all the best:
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Herzog
Sent: Wednesday, April 29, 2009 6:05 AM
To: Ham-amps
Subject: [Amps] purpose of 3 ohm resisters between base and emitter?
Parasitic oscillations is a valid guess, and better than to just load
both sides of the sine-wave drive. Biasing could be another valid guess.
Quote:
`Date: Tue, 28 Apr 2009 07:04:31 -0700
From: Dan Sawyer <dansawyer@earthlink.net>
Subject: [Amps] purpose of 3 ohm resisters between base and emitter?
All, I have an ENI power amp with what look like SD 1726 transistors in
a class C configuration. The circuit is pretty standard, however there
are 3 ohm resistors between the base and the emitters. Can some
speculate on the purpose of such a low value? It is common for resistors
to be used in this configuration however they are usually a larger value.
Thanks for your thoughts on this - Dan kb0qil
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