# Why does the tank circuit of Coplitts oscillator produce 180 phase shift?

http://www.circuitstoday.com/colpitts-oscillator

I would like to understand how Coplitts oscillator works. This link says that the tank circuit of Coplitts oscillator produces 180 degrees phase shift. I Can't understand how a combination of a coil and two capacitors produce 180 degrees phase shift, Would you explain it,please ?

• Are your two questions somehow related to each other? If they are, could you make the connection more clear? If they aren't, please post the second one as a separate question. – The Photon Oct 29 '14 at 15:52
• The two questions was related to each other but I deleted the second question to prevent confusion. I will know the answer of it, if the first question is answered. – Michael George Oct 29 '14 at 19:33

You must discriminate between Colpitt oscillators in common base or common emitter configuration, respectively. And - most important - you must realize that the LC tank contains a capacitive divider circuit.

The mid point between the two capacitors is either grounded (common base) or is connected to the base node (common emitter). In the first case (common base without signal inversion) the LC tank - indeed - produces 360 deg phase shift and this signal is fed back to the emitter.

In the second case (common emitter with signal inversion) the frequency dependent circuitry could be seen as a third order lowpass that consists of an first order RC lowpass followed by an LC lowpass. Such a third-order lowpass produces 180 deg phase shift at the desired frequency. The remaining 180 deg are produced by the inverting characteristic of the common emitter amplifier (between base and collector).

(Comment: The first RC lowpass function is produced with the help of the finite output resistance at the collector node. For opamp realizations (zero output resistance) you must, therefore, add a an additional resistor. Sometimes, This resistor is forgotten in in some papers).

You have two caps in series + in parallel with the inductor so that gives you an overall phase shift of 360 deg / 0 deg (whatever you wish). The transistor is common emitter which means the output is 180 deg. out of phase in relation to its input.

• In total, this would result in a phase shift of 180 deg. - and the circuit would NOT be able to oscillate. – LvW Oct 30 '14 at 16:41
• @LvW ?!? The common emitter's output is 180 deg. out of phase and the additional phase shift resulting in the capacitors || inductor contributes an extra 180 deg. needed for oscillation. – user34920 Oct 30 '14 at 17:48
• But you didn`t mention in your answer any additional 180deg phase shift (You only spoke about 360deg). As I have explained in my answer - there is a third-order lowpass producing additional 180deg. – LvW Oct 31 '14 at 8:37