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Power Amplifier Tuning in Test Bench


Junior Member level 3
Nov 9, 2008
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I have to re-design a power amplifier to meet a set of specifications. The amplifier has to output 5 W of power. The problem is that I don't have a load pull setup to determine the ideal impedance for max Pout and I don't have a non-linear model of the transistor or a simulator to be able to simulate the load pull and extract the required impedance by itself. I do have a large signal setup in which an RF generator + driver amplifier provide the required input power to the PA and then I can look in the spectrum analysis the frequency spectrum.

How can I optimize the design experimentally and efficiently?

Load pull rigs are basically a mechanically tuned LC,
right? You can acquire mechanically variable capacitors
(scrounge old tube radios if need be for air-plate low
values) and you can acquire or make adjustable
inductors (continuous, ot tapped-gang). An orderly
set of stepC by stepL and you can map it out rough
for where peak may reside, and maybe another pass
fine-fiddling with some of the gross variations taken
off the table.

That would be old school.

Now it's somewhat surprising that a transistor you can
physically get, does not have a viable circuit design
model somewhere behind it. Nor a "close relative" to
pretend is "good enough" (perhaps with Cdg fiddled).
But "I don't have" doesn't really mean anything - it
might be for lack of looking.
If you could find a Sliding Tuner @Operating Frequency, it would have been easier.
Principally, the Output Matching Circuit is the main key to get the desired Output Power.
So, you will observe the Output Power with its' harmonics while tuning the OM components. It's not an effective way of doing but no way out.

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