Date: Thu, 25 May 2017 21:58:43 +0000
From: Manfred Mornhinweg <manfred@xxxxxxxxx>
To: amps@xxxxxxxxxxxxxx
Subject: Re: [Amps] SWR and amplifiers
<On the subject of whether an amplifier will absorb power applied to its
<output connector - "reflected power" or otherwise:
It does not depend fundamentally on the class of operation. Instead it
depends on the amplifying device used, and the circuit it's used in.
Transistors (FETs and BJTs), pentodes, and to a large extent also
tetrodes, have a very high output impedance, much higher than their load
impedance. That means that these devices behave as controlled current
sources. The gate, base or grid drive signal defines what current they
will conduct. Any RF voltage applied via the output to their
drain/collector/plate will not cause a significant RF current there, so
they will not absorb power.
Triodes instead have a much softer curve, in which plate voltage has a
strong effect on plate current. So these do have a much lower plate
impedance, and do absorb power in this situation.
But that's for the devices alone! Installed in an amplifier circuit,
it's crucially important to understand how much negative feedback the
amplifier has. If there is no feedback, we get the behaviour of the
device proper. Instead if there is very strong negative feedback, then
the amplifier tends to act as a voltage source, that is, it has very low
internal impedance. In this case even a slight RF voltage applied to the
output terminals will result in a large current. That is, the amplifier
behaves much like a short circuit to this reverse power, and short
circuits don't absorb power...
Instead if the amplifier uses a medium amount of negative feedback, it
can actually have an internal output impedance that's close to the
nominal load impedance. In that case the amplifier will absorb (and
dissipate) a significant amount of power in this situation.
Any usual impedance matching circuitry between the amplifying device and
the output port, be it broadband or tuned, will function both ways. It
will not "reflect power before it reaches the device", unless that
signal is on an out-of-range frequency.
If we introduce circulators and such, we open a new can of worms, but
they are not commonly found in ham HF power amplifiers...
Manfred
## The tube cant source anything. It can only sink current.
Jim VE7RF
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