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Design >> RF Design >> RF Power Amplifier: input impedance
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Message started by crhu on Feb 28^th, 2008, 3:44pm

Title: RF Power Amplifier: input impedance
Post by crhu on Feb 28^th, 2008, 3:44pm

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.

Title: Re: RF Power Amplifier: input impedance
Post by nanion on Mar 22^nd, 2008, 9:58am

Hi There,

I think you are trying to get everything out of one stage. First of all you need to choose the device size to match it for 50 ohm. Depending on broadband or narrowband you can either use a shunt-shunt feedback or common gate approach. For shunt feedback to make it look like resistive feedback resistor has to be comaparable to the capacitive impedance of your input device.

I mean do you have a restriction of using a single stage thats the main question?

Thanks,

Title: Re: RF Power Amplifier: input impedance
Post by khouly on Mar 24^th, 2008, 6:53am

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

khouly