Class AB PA design - RF Cafe Forums
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Post subject: Class AB PA design Posted: Thu Aug 13, 2009 7:02
Joined: Tue Aug 26, 2008 5:18 am
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
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 max power?
current is not fixed and varies with drive power.
Post 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 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.