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I tried simulating two slightly different versions of the Colpitts oscillator in LTspice.

Here is the first one:

enter image description here

This circuit oscillates at 53.8Mhz. It is quite easy to understand how this circuit oscillates just by looking at it.

Then I tried grounding the main resonant capacitor, C4 from the collector, a commonly seen method used in self quenching super-regenerative receivers.

enter image description here

Initially the circuit did not oscillate, but in reality they do in the above-mentioned receivers. When I selected the option "skip initial operating point solution" from the simulation menu, the circuit fired up to life and began oscillating at 52.3Mhz.

Now, my question is how does the above version of the Colpitts oscillator work?

My thought is that there is no a visible LC resonant loop, apparently. Also, C4 easily shunts any oscillations to ground there by not startup any oscillations.

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Regarding what you said about the first circuit: -

This circuit oscillates at 53.8Mhz. It is quite easy to understand how this circuit oscillates just by looking at it.

For the 2nd circuit you said this: -

Now, my question is how does the above version of the Colpitts oscillator work?

Given that you know how the first one works the only thing that needs to be explained about the 2nd one is that the collector capacitor, is still connected to Vcc regarding AC signals so, there is no difference in how they work. In other words, the DC source (V1) has zero AC (and DC) impedance and C4 is still effectively in parallel with inductor L1.

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  • \$\begingroup\$ Thank you. May I ask what might be the reason for different oscillating frequencies? Is it due to the junction capacitances of the transistor? Also in certain cases of relatively high source impedance, such as a weak battery, the second version won't oscillate properly, will it? \$\endgroup\$ – ASWIN VENU Sep 13 '20 at 5:05
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    \$\begingroup\$ Highly likely to be a SPICE thing. If you waited a good length of time in both scenarios, they would converge I'm sure. \$\endgroup\$ – Andy aka Sep 13 '20 at 9:08
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    \$\begingroup\$ Did you get to the point of proving that the frequencies converged @ASWINVENU \$\endgroup\$ – Andy aka Oct 13 '20 at 17:06
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    \$\begingroup\$ Yes I understood that and I have accepted your answer now, I must have missed it yesterday. Apologies. \$\endgroup\$ – ASWIN VENU Oct 14 '20 at 3:15

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