Hi, experts,
I am having hard time to to handle the 50 ohm input impedance for my RF Power Amplifier.  The PA works fine connecting the voltage source
directly to the gate of the large transistor.  However, if an 50 ohm resistor is inserted between the voltage source and the gate of the transistor, I could not get the expected power out.  I probed the nodes and I found that the input signal is significantly reduced at the gate of the gate due to the low pass filter formed by the resistor and the gate capacitance.

To solve the problem, I use an inductance to tune out gate capacitance, and vgs=Qin*Vsrc

However, I found that I could not get large enough rf power with this input network:
to maintain high enough bias current, I need to use a large transistor -> large input capacitance -> small input inductance -> small Qin -> small vgs swing (=Qin*Vsrc) -> not enough rf power

If instead, I increases Qin, thus larger vgs swing; but it means small gate capacitance -> small transistor->small gm -> not enough rf power.

It seems to me that there is no way out?!


Any idea how I can solve the problem?

Thanks a bunch.

u need to design a complete input matching network , to conjugate match the input impedance of the transistor

khouly