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Power amplifier 8.1 MHz for symmetric loop antenna

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Junior Member level 1
May 4, 2006
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Czech republic
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Hello everybody, I'm quite new to this branch, so I have a lot of questions :D .
I have to construct the PA amplifier with two IRL510A mosfets ( rather modify old one to better performance ). The amplifier has supply 24 V and max. current has to be 300 mA. PA is used for the loop antenna in 7.5 - 8.7 MHz band.
You can see on pic below an old PA including drivers. The input is differencial sinus signal with swept frequency from 7.5 MHz to 8.7 MHz. The resonant circuit with loop antena is coupled to the output of PA by pot core transformer.
I've found some App notes from Phillips with such amps, but it has to be modified for my case and I'm not sure with modifications, cos I don't have 50Ω input and output.
So, my questions are:
1) Is it possible to match the input of MOSFET, which is driven from OpAmp? Can this OpAmp be treated as current source?
2) For what impedance could be the PA matched? Is it the impedance of resonant circuit ( capacitor and loop antenna )? If so, how can I determine the Q of loop antenna? I can measure inductance of antenna with RLC meter, but only at max. frequency 200 kHz - L = 6.4 uH, Q = 35, R = 0.184Ω.
3) How could I choose an inductance of transformer's windings? What is the influence of inductance of both windings on the rest of circuit? Is it better to use a pot core trasformer or toroid in this case?

I thanx for any answers and comments.

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