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Re: [Amps] Transceiver Output Impedance

To: "" <>
Subject: Re: [Amps] Transceiver Output Impedance
From: "Jim Brown" <>
Date: Sun, 09 Mar 2008 12:56:02 -0800
List-post: <">>
> For maximum power transfer the output impedance and the load must be the
> same.

NO!  This is a major misunderstanding of the maximum power transfer 
theorem, which says that if a generator has a fixed value of resistance 
(that is, we can change ONLY the load resistance) AND IS LINEAR AT ALL 
transferred when the resistances are equal. But power amplifiers are NOT 
LINEAR FOR ALL CONDITIONS -- they get very NON-LINEAR when heavily loaded 
(low values of load impedance), so the maximum power transfer theorem does 
not apply to power amps!  

But as Manfred correctly observes, power amplifiers are generally designed 
to work into impedances much larger than their own AC output impedance, 
become non-linear when more heavily loaded (lower impedance) and have more 
dissipation when more heavily loaded (it's a simple voltage divider with 
their own output R and the load).  

Now, the output Z of an RF power amp DOES include the output network, 
which is often a low pass filter. But we don't want those components 
overheating either!  And has been correctly observed, the response of that 
network, like all passive networks, depends on the values of driving 
source impedance and load impedance. That is, the network will be designed 
to work with the actual output Z of the transistors in their circuitry, 
and with the specified output Z of the load (the antenna). 

Bottom line -- optimum load Z of a power amp is the impedance at which the 
amplifier, including its output filter circuitry, is happy. And the value 
of Z load is unlikely to be equal to the value of Z out. 


Jim Brown K9YC 

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