Class F amplifier

126

02:03:19

Class F amps add some odd harmonics to shape the output a bit more like a square wave. It's suitable for driving class E amps, since they work best with abrupt on/off control. 

published 8 years ago

jason9

8 years ago

This seems pretty imperfect in efficiency. Is the goal of this to somehow achieve near 100% efficiency while also making the output a square wave version of the input? If so, I’m pretty sure this is wrong, because when the transistor activates the wave is high and the transistor has to dissipate the energy of the capacitors that are maintaining the hight of the wave.

thebugger

8 years ago

It's in a typical class C configuration. A very low portion of the input wave activates the transistor, so efficiency levels can be as high as 80%. The output impedance is not matched and this is mostly what leads to the apparent inefficiency. There's a workaround, where the harmonic injection filters are added to the output after the LC transformation network, so the efficiency gets boosted, but I went with this option, just for the example's sake.

hurz

8 years ago

@thebugger, check the class E amp @jason9 published and learn how it works proper.

thebugger

8 years ago

This is a class F amp, not E

jason9

8 years ago

You mean my modification of @thebugger’s Efficient Amplitude Modulator? Or the class-Es I posted a while back?

hurz

8 years ago

For example this one http://everycircuit.com/circuit/4709847533879296

thebugger

8 years ago

This is class F, nothing to do with class E. It adds a little bit of extra harmonics to the signal to shape it like a square wave. It can be used to drive class E amps at high frequencies

hurz

8 years ago

Yes, yours is just F. The mess in traps at collector does not work as you expect in real. Remember all caps see all coils and wise versa. So there is e.g. also serial resonance.

thebugger

8 years ago

Yes, but this is an amplifier, not an oscillator. The input signal will trigger only the matched resonances. The off tuned resonances will be quite damped.

hurz

8 years ago

And all other resonances will attrac other input frequencies and noise and harmonics generated by the anplifier which might make it very unstable.

thebugger

8 years ago

Hmm, I doubt it. At these frequencies every sensible engineer will add proper shielding and filtering against these things

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