I found a circuit online about an rf power amplifier working in the 88 to 108MHz FM spectrum. The circuit diagram is shown below enter image description here

I understand that an amplifier needs an inductor and a capacitor in a resonant circuit to build the output waveform without distortion. But why are there 2 inductors over the collector of the transistor and another inductor L2 near the output.

  • \$\begingroup\$ Since you have the simulation, run an AC sweep with different inductor values there \$\endgroup\$ – PlasmaHH Mar 6 '18 at 7:07

This should explain your questions: -

enter image description here


I was clearly wrong about C5 - upon further inspection it has nothing to do with being part of a tank circuit so apologies for that. And L1 is therefore not a tank inductor but just a collector load of about 240 ohms at midband broadcast FM frequencies.

Here's another attempt to fix my earlier stupidity: -

enter image description here

| improve this answer | |
  • \$\begingroup\$ Is the capacitor C5 the tank capacitor? The capacitor seems to be directly connected to the power supply through the RF choke. Even in simulations, C5 doesn't behave like a tank C. \$\endgroup\$ – Kevin Selva Prasanna Mar 24 '18 at 7:01
  • \$\begingroup\$ @kevin yes, I don't know what I was thinking. I'm on a tiny android at the moment so I can't fix the answer but I will do later AND if I forget just leave another comment as a reminder. \$\endgroup\$ – Andy aka Mar 24 '18 at 11:22

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