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I'm looking at the following two circuits:

circuit 1:

enter image description here

and circuit 2:

enter image description here

Both of these circuits are LC oscillators, the thing is that the first one, i.e the one with two inductors and one capacitor, converges nicely to a steady sine wave whereas the second circuit doesn't converge at all, it instead dies out after a few oscillations. This is what I don't understand too well... Both of the circuits run at the same frequency, have the same Q factor (I've added some internal resistance), same feedback mechanism..etc so it doesn't seem too obvious why only one them works.

I've even tried with different values of inductance and capacitance but end up with the same result, which is that only the circuit with the two inductors and one capacitor works.

So why does this happen? Why is it that the "configuration" on the first circuit works out fine as opposed to that of the second circuit?

Thanks

P.S: I've also tried changing the base resistor's value, but to no avail.

rr1303
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1 Answers1

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So why does this happen? Why is it that the "configuration" on the first circuit works out fine as opposed to that of the second circuit?

Because the Colpitts oscillator is idealized and you won't get any bias current through the capacitors for the transistor to work with. Capacitors block DC and hence bias currents are blocked. Try a non-idealized Colpitts oscillator instead (see below for one type of Colpitts oscillator). The Hartley has inductors that readily pass DC bias currents.

enter image description here

Picture from here.

Andy aka
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  • Ahh, I neglected that, thanks! Would a resistor from the source to the base of the transistor make it function? – rr1303 Apr 05 '20 at 17:24
  • Try the more practical version I've just added. – Andy aka Apr 05 '20 at 17:25
  • See also [my answer here](https://electronics.stackexchange.com/questions/355018/colpitts-oscillators-capacitors) for my particular favourite version of the Colpitts oscillator. – Andy aka Apr 05 '20 at 17:29