Class AB PA Design - RF Cafe Forums
Post subject: Class AB PA design Posted: Thu Aug 13, 2009
Joined: Tue Aug 26, 2008 5:18
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?
2-Impedance at 250mA where we get
Note:Class AB current is not fixed and varies with
subject: Re: Class AB PA designPosted: Fri Aug 21, 2009 2:19 pm
Joined: Wed Apr 08, 2009 7:53 am
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
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.