[Amps] Transceiver Output Impedance

[Carl Clawson]

This should be measurable. In class A you could just bias up the amplifier
and measure it, no? Other classes are more problematic because they are
non-linear and you'd need to have them producing power in order to measure
them. (And indeed the concept of impedance itself becomes pretty dicey
because impedance is a linear concept.) But I think it's still measurable in
principle. If you're driving a mismatched load, that reflected power either
gets dissipated in the amplifier (conjugate match) or re-reflected to the
load. In the former case your power dissipation for a given net power would
be higher than for a matched load because the amplifier would producing more
forward power, the reflected portion of which is being dissipated.

(The "non-dissipative resistance" concept, IMO, is just sleight of hand to
turn one of these cases into the other and is physically meaningless. The
reverse power is either dissipated or re-reflected. That should be
measurable.)

I lean toward thinking that a conjugate match is not necessary. Here's a
thought experiment: Take an ideal amplifier element that produces an output
current proportional to the input voltage, independent of load resistance.
It has infinite output impedance, yet you can make a dandy amplifier with
it. Increase the load resistance and you get more power as long as the
supply voltage is not exceeded.

Now, didn't I just describe something that looks a lot like a class A
bipolar transistor amplifier?

73, Carl WS7L

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of peter.chadwick at Zarlink.Com
> Sent: Sunday, March 09, 2008 3:47 AM
> To: amps at contesting.com
> Subject: Re: [Amps] Transceiver Output Impedance
> 
> This is an interesting question, and there are two schools of thought. One
> says that you get maximum power when you have a conjugate match, and the
> other says you get maximum power when feeding an optimum load impedance.
> 
> The first school of thought (which includes Tom Rauch and Professor
> Belrose) have measured the output impedance of a tubed SSB transmitter and
> got an answer of about 50 ohms
> 
> The second school of thought (Warren Bruene) argue that if you get maximum
> output power when conjugately matched, then efficiency can never be
> greater than 50%, and Class C stages work at much higher efficiencies. To
> which the first school suggests a non-dissipative resistance in Class C,
> but that one passes me by.

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