Absolutely correct Tom. We choose to use 50 ohms as a convenience because
transmission
lines are usually 50 ohms today.
However consider a high power audio amplifier driving a load of 4 ohms. The
output Z of
the amp isn't 4 ohms - more like approaching zero ohms to be able to deliver
the required
current!
I think where some folks are going wrong is that they have lost sight of what Z
actually
is. You can (within reason) make a 50 ohm Z from a myriad of combinations of
L, C and R
but in only case - the series resonant condition - the resistive portion is
actually 50 ohms and the reactance of the L & C cancel out. For lots of others
the value
of the apparent R is very low - like the audio amplifier example above. Power
cannot be
dissipated in a reactance, only in a resistance. So the "dissipative Z" theory
immediately falls flat on its face because the R portion isn't 50 ohms!
Clearly in a
practical world ALL components have some value of R - due to loss mechanisms.
So
defining the efficiency of a rf PA isn't easy. I am going through that
scenario at work
at the moment!
I had this exact discussion with Prof Abrey of Pretoria University some years
ago and he
explained that depending on the configuration of the amplifier the output Z
could either
very low or very high. Transistor amplifiers are predominately low Z and have
very low
collector impedances. Tube amplifiers on the other hand usually have highish
impedances
- as witnessed by the load-line resistance.
One shouldn't lose sight of all the stray reactances in a tube, the grid to
kathode and
plate to grid capacitances can be fairly high and at high frequencies these
come into the
equations. Similarly the inductive reactances, although not often considered
at hf, are
still there.
Hope this helps.
John ZS5JF
----------
> From: Tom Rauch <w8ji@contesting.com>
> To: amps@contesting.com
> Subject: [AMPS] Impedance of amplifiers
To: <amps@contesting.com>
> Date: 16 July 1999 04:11
>
>
> If the impedance of amplifiers is a "low one or high one", as
> credited to professor Areby?:?, then you could prove it with this
> simple test.
>
> Put various resistor on the output, and measure the power
> delivered. If the PA delivers increasing power as the resistance is
> decreased, it is indeed a "low one".
>
> If it delivers increasing power if the load is made higher, it is indeed
> a "high one".
>
> My money is on the table that maximum power will occur near 50
> ohms in an amp designed to drive 50 ohms near rated output.
>
> That's a simple test that separates the bologna from the truth.
>
>
> 73, Tom W8JI
> w8ji@contesting.com
>
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