With a tuned output circuit there is a conjugate match between the output
circuit and the feed line. This causes any reflected power to be
re-reflected back to the feed line.
There is no conjugate match between the output circuit and the amplifying
device. It only allows for maximum power transfer.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@xxxxxxxxxxxxxx] On Behalf Of Manfred
> Mornhinweg
> Sent: Thursday, May 25, 2017 4:59 PM
> 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
>
>
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