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I understand that the Radio Frequency Choke at the collector of a Common emitter is suppose to block the radio frequency from reaching the transistor and therefore interacting with the output. So by adding a resistor Rc in parallel to the RFC don’t we essentially nullify the RFC?

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I understand that the Radio Frequency Choke at the collector of a Common emitter is suppose to block the radio frequency from reaching the transistor and therefore interacting with the output.

Your understanding is flawed. The collector is handling the oscillator RF currents in order to sustain the oscillations at the right amplitude. The RFC serves two purposes (and it isn't always necessary to have one at all): -

  • It can allow the output voltage p-p value to be nearly double what the DC supply voltage is.
  • It blocks RF currents from the oscillator getting back on to the power supply lines and interfering with other sensitive circuits.

So by adding a resistor Rc in parallel to the RFC don’t we essentially nullify the RFC?

No, not exactly. A resistor can be added in parallel with the RFC in case the RFC's self-resonant-frequency (SRF) is particularly troublesome should the frequency of oscillation of the Colpitts circuit be too close.

And, as I said earlier, a Colpitts oscillator of any type (CE, CC or CB) doesn't always require an RFC in fact, I've never designed a Colpitts oscillator that used one.

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