Looking to understand Inductor in Class C Amplifier

[Henry57]

I'm trying to get a better understanding of how an inductor (rfc) works in the collector of an NPN class C amplifier working around 100MHz.

A simple understanding is that it simply "contains", or impedes the RF from traveling further in the circuit, but it obviously does more than this. I would expect that larger values of L would give larger voltage swings, since V = L(di/dt), but this is not the case, as smaller values of L tend to give the largest voltage swings.

I would expect to see a more negative voltage on the collector when the transistor becomes active, in order to oppose the current flow, and then a more positive voltage at the collector near the end of the active period of the transistor, while the inductor field is collapsing and attempting to keep the current flow in the same direction.

When I simulate Class C circuits, my expectations are not met. I have read that at higher frequencies, there can be a phase difference between collector current and base current, but still - rarely do I see the sort of "shapes" I am looking for. Any help in furthering understanding will be appreciated.

Thanks.

 

[k7elp60]

lAre you talking about theRF choke that is there to keep the rf out of the powersupply, or the inductor and capacitance to resonate at the operating frequency?

 

[Henry57]

Yes, the inductor acts as an RF choke to keep the power supply clean, but my understanding is it also provides for a higher voltage swing at the collector (the intricacies of this and the timing by which it happens is what my original question is about).

Not all Class C amps have a tuned circuit at the collector -- some simply have an inductor in the collector, with a low-pass filter further on in the circuit.

 

[ronsimpson]

but my understanding is it also provides for a higher voltage swing at the collector (the intricacies of this and the timing by which it happens is what my original question is about).

The output voltage will have a larger peak to peak voltage with a collector inductor. The DC voltage across the inductor is near zero. (the average voltage is about 0) The average output voltage voltage needs to be about the same as the supply for there to be no average voltage across the coil. The collector can pull the output to about 1 volt above ground. The collector voltage must be above supply for a time to get the average voltage to be near the supply voltage.

You can get near 2x the output voltage using a coil.

 

[audioguru]

I think the inductor at the collector resonates with stray capacitance and/or capacitance in the circuit or at the antenna.
Then the collector voltage swing can be much more than the supply voltage.