# Oscillator Query

Discussion in 'General Electronics Discussion' started by Lightning, Nov 24, 2013.

1. ### Lightning

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4
Oct 12, 2013
Hello,

I am looking at the different types of transistor oscillator circuits.

I know you need positive feedback with a 0/360 degree phase shift but I do not understand how the colpitts oscillator achieves this?

The circuit above uses an inverting transistor amplifier to give a phase shift of 180 but I cannot figure out how it shifts back into phase in the feedback?

Any ideas?

Thanks.

1,252
184
Apr 4, 2010
3. ### duke37

5,364
772
Jan 9, 2011
The tank circuit is tapped near the middle with the two capacitors. When one end goes positive the other end goes negative, like a see-saw, that is 180 degrees. The transistor adds another 180 degrees.

4. ### Lightning

45
4
Oct 12, 2013
That's a good analogy thanks but I do not understand why C2 is compelled to do the opposite of C1?

5. ### duke37

5,364
772
Jan 9, 2011
The tank circuit has an inductor and two capacitors in series. The current is flowing one way then the other at the resonant frequency. During one half cycle, the current will be flowing out of C1 and flowing into C2 so the polarity will be opposite. If the values of C1 and C2 are the same, the voltage level will be the same but the polarity will be opposite.

The reference voltage is the capacitor tap which is connected to ground.

6. ### Laplace

1,252
184
Apr 4, 2010
A Bode plot of the Colpitts feedback circuit is attached. Representative component values were chosen for LC tank resonance near 50 KHz, mainly to investigate the phase shift behavior. Note that at resonance the circuit becomes simply a resistor divider for magnitude but with -180 degree phase shift. The first Bode plot covers a frequency range of three decades while the second Bode plot zooms in on the frequency range near resonance.

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7. ### Lightning

45
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Oct 12, 2013
Ahh, I understand now.

Thank you.