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Class AB PA Design - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


SDO
Post subject: Class AB PA design Posted: Thu Aug 13, 2009 7:02 am

Captain


Joined: Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Hello,
I have been involved in most of the class A amplifiers some of which at some stage were changed to class AB by changing the bias and slightly changing (if required) match as well.
My question here is if we want to design a Class AB amplifier biased at say 28V@60mA (60mA quescent current), which impedance do we take into account to match to 50ohm load:
1-Impedance at 60mA?
OR
2-Impedance at 250mA where we get max power?
Note:Class AB current is not fixed and varies with drive power.
Thank you.


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RDO-RF
Post subject: Re: Class AB PA designPosted: Fri Aug 21, 2009 2:19 pm

Captain


Joined: Wed Apr 08, 2009 7:53 am
Posts: 12
Hello SDO,

I fully understand your way of thinking the problem.. But, have you ever questioned if, even though your impedance characteristic is 50ohm, you should really need to match the amplifier output to 50ohm?
This is a big difference between a class A design and a class AB design. The output and input match impedance selection depends on the functionality you want your amplifier to have.
You can match your amplifier to maximum gain, to max p1db, to max IP3 .. and in all cases your matching circuit will be different. This is one of the reasons why these kind of amplifiers usually use an isolator on it´s output, so if you place a load sensitive device connected to the amp output, it will work correctly.. (lets say an Amp connected to a cavity filter, if you don't use an isolator between them, your filter calibration will be modified by the non 50ohm output impedance of your amp).

It is true that your output match changes with the output power, so this is why you cant measure the amplifier output impedance with a network analyzer. For these kind of amplifiers you should do loadpulling. With loadpull techniques what you do is to test the amplifier performance for a given output match and bias point. For ex, if you need max IP3, then with a tuner you should find the optimum impedance that makes your amplifier have max IP3, then, you measure with a network analyzer your tuner and find the correct load that your amplifier will need to match the performance you need.

Take care

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