palani <palani@orizin.net> wrote:
> One more question regarding output impedance, How to find out
> output impedance of transistor/MOSFET output stage ?
> 
> I found two methods , so got confused.
> Method 1: Use thumb rule Zo =  (Vcc * Vcc)/(2 * Poutput)
> Method 2: By using values of transistor/MOSFET output capacitance,
> Load inductance (inductance between Vcc & collector). I mean , the
> output impedance is calculated by circuit parameters.

I'm sorry, but it sounds like you're way outside your area of expertise
on this one. It's outside my area, too, but I'd start by studying the
ARRL Handbook for Radio Amateurs, which has a lot of very good
theoretical and practical information on RF power amplifier design.

My understanding is that the impedances associated with your active
element (be it bipolar, MOSFET or tube) are mostly dictated by the
design of the device itself, and need to be measured in an environment
that's similar to your final application. It's your job to transform
those impedances as needed to match the requirements of the external
interfaces (signal in, signal out and power supply).

Just as a starting point, most of the designs that I see in the ARRL
book for the frequency and power range you're interested in use a
pair of bipolar transistors in a push-pull configuration with
center-tapped transformer coupling for both input and output.

-- Dave Tweed
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