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Re: [Amps] Transceiver Output Impedance

Subject: Re: [Amps] Transceiver Output Impedance
From: peter.chadwick@Zarlink.Com
Date: Sun, 9 Mar 2008 10:46:44 +0000
List-post: <">>
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.

If the PA stage is about 50% efficient, then you would expect to get 
something like a conjugate match. Look at the curves for the 6146, and you 
see the plate resistance is about 2000 ohms, which is roughly the correct 
load impedance, efficiency is about 50% and the correct load is something 
like a conjugate match..

Consider a transistor PA. It has a very low collector resistance, but 
wants a load given by Vcc squared divided by 2 Pout. Here we start getting 
to the point where other resistances start to play a part - emitter 
ballasting, bond wire, even the connecting tabs. This modifies the 
practical case.

In general, I think most linear PA stages do look something like a 
conjugate match because they are 50% efficient, give or take a bit. 

Finally. Suppose a 12 volt 1 amp power supply. For  load change between  0 
and 1 amp, the voltage sags by 1 millivolt, so the internal impedance is 1 
milliohm. If conjugately matched, it would try to deliver 12,000 amps! So 
maximum power in that case is not when conjugately matched  - just as the 
electricity supply network isn't. If you try, you blow fuses - we had a 
chap in college who did just that. Except it was on 1200 amp phase of the 
415 volt three phase supply!


Peter G3RZP

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