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The feedback network used in the Colpitts oscillator is an LC circuit is shown below.

enter image description here

I understand that the phase difference between the terminals 1 and 3 is 180 degrees. Also the amplifier output is fed at terminal 1 and voltage at terminal 3 is fed back to the amplifier. In that case won't the feedback signal will always be in phase with the input of an inverting amplifier? Then why does the Colpitts oscillator oscillate only at one frequency?

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  • \$\begingroup\$ You are contradicting yourself. You say "phase difference btw the terminals 1 and 3 is 180 degrees" then you argue "fedback signal will always be in phase with the input of the amplifier" - these are contradictory and make your question hard to answer because we don't know what you mean! \$\endgroup\$ – Andy aka Apr 24 '15 at 18:40
  • \$\begingroup\$ Sorry.I meant when used with an inverting amplifier which already has a phase shift of 180 degrees. \$\endgroup\$ – Newstein Apr 24 '15 at 18:46
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    \$\begingroup\$ Because it's also a filter, and only a few frequencies will have a \$\pi\$ shift. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 24 '15 at 18:50
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I'm not answering this in the classical "colpitts explanation way" because I think certain things need to be said about the driving source to pin 1 and the off resonance of L and C2 (ignoring C1's effect initially)....

If you apply an oscillating voltage at pin 1, L and C2 form a resonant low pass filter with pin 3 being the output. See below: -

enter image description here

Depending on the Q of the circuit there may be voltage amplification but there will be round about 90 degrees of phase shift between pin 1 and pin 3.

However, this isn't quite how the colpitts works. C1 plays a role and pin 1 isn't a driven by hard voltage source but by a weak voltage source. This adds up to an overall phase shift that can become 180 degrees; the output resistance of the driving source produces a phase shift with C1 that may be (say) 70 degrees and at the right frequency (slightly higher than resonance of L and C2), L and C2 produce another 110 degrees. All this adds up to 180 degrees and the oscillator then oscillates at this frequency.

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  • \$\begingroup\$ Wikipedia of course answers in the classical way, with useful connections to the related Pierce and Hartley oscillators. Worth reading. en.wikipedia.org/wiki/Colpitts_oscillator \$\endgroup\$ – cuddlyable3 Apr 24 '15 at 23:10
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    \$\begingroup\$ I think wiki should focus more on concepts rather than facts \$\endgroup\$ – Newstein Apr 25 '15 at 5:35

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