# Why does a Colpitts oscillator oscillate at a particular frequency?

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

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?

• 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! – Andy aka Apr 24 '15 at 18:40
• Sorry.I meant when used with an inverting amplifier which already has a phase shift of 180 degrees. – Newstein Apr 24 '15 at 18:46
• Because it's also a filter, and only a few frequencies will have a $\pi$ shift. – Ignacio Vazquez-Abrams Apr 24 '15 at 18:50