Matching two RF circuits isn't my strong point yet I need to do this as part of a larger project.
I am trying to match an amplifier stage working over the range 80-120MHz, using a 2N3866 medium power RF transistor to a power stage that uses an MRF138 FET.
I am using a forward biased diode to give fine control of Vbe and so Ic and there is also an adjustable bias on the FET gate.
So far I am delivering a nice 100mV 100MHz sinusoidal input to the base of the transistor and getting an amplitude gain of around 8 at the collector which is fairly OK, though less than I'd hoped.
However when I try to deliver this through an LC network to the gate of the FET I get a rotten-looking second harmonic signal of reduced amplitude.
I reckon the output impedance of the transistor stage consists of the Vce/Ic in parallel with L5 (100nH) which is 140 ohms in parallel with 62 ohms at 100MHz. - or 43 ohms.
And I reckon the input impedance of the FET stage is 27 ohms near enough.
Obviously I am doing something dumb with my matching (please see the schematic) - could anybody please assist?
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