> I am working on common Emitter configuration (transistor) or Common source
> configuration (for MOSFET). and i trying to amplify 14Mhz 100mw input to
> 4 W output. Class A configuration.

Quite difficult to get 40x from one transistor at 14Mhz... maybe you
have more stages ?

OK, you have a RF amplifier. I was thinking to an ordinary audio one.
The input and output impedance should be fixed by application (usually
at 50 or 75 ohm) but not necessarily.
For the A class, the load will take both DC and AC current. The major
contribution in load heating will have the DC.

>
> The common thumb rule used  for output impedance is (Vcc * Vcc )/(2 * Po).
> (Vcc - supply voltage , Po - output power )

Do you have equal loads Zo on both collector and emiter ?
The emiter load is AC decoupled and the output is the collector ?
There are two transistors ?

Assuming the whole voltage is contributing to the output power (no
lost on Vce or Vds, no lost on the emiter resistor, with load in the
collector) then:

1. Po = Vcc * I   where Po = power, Vcc= supply, I = load current
2. from the Ohm law  I = Vcc/Zo
taking Zo from eq2.  and replacing in eq 1. will result  Zo = Vcc^2 / Po

Differences between this equation and yours shows that only half from
Vcc is contributing to the output power, so either you have Zo in
emiter and colector, or the amplifier liniarity is good only from half
of the supply output swing (which sound resonable) or your output
stage have two transistors and the output swing is Vcc/2.

Showing the whole schematic will make us more happy in changing our
opinions with you.

greetings,
Vasile
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