# Colpitt's Oscillator Working Principle

### How Colpitt’s Oscillator works?

Various commonly used transistor based oscillator circuits are

• Tuned collector Oscillator
• Colpitt’s Oscillator
• Hartley Oscillator
• Phase Shift Oscillator
• Wien Bridge Oscillator
• Crystal Oscillator

All these oscillators have the same function ( ie, generating continuous undamped oscillations). But the major difference between them is the method by which energy is supplied to the tank circuit to compensate the losses. In the post let us discuss about How Colpitt’s Oscillator works?

As shown in the figure, the Colpitt’s oscillator has the tank circuit made up of C1, C2 and L. The two capacitors are placed across a common inductor L and the center of the two capacitors is grounded. This C1-C2-L forms the feedback circuit that produces a phase shift of 180°. The frequency of oscillations is determined by the values of C1, C2 and L. It is given by

where

#### Colpitt's Oscillator Circuit Operation:

• When the colpitt oscillator circuit is turned on, the capacitors C1 and C2 are getting charged.
• Once fully charged they will discharge through inductor L.
• Thus setting up oscillations with the frequency of

• The amplifier output voltage will appear across capacitor C1 and feedback voltage developed across C2.
• As shown in the below figure, the voltage across C2 is 180° out of phase with the voltage across C1. • This C2 voltage is feedback to the base terminal of the transistor which is positive feedback.

Remember about the Positive Feedback amplifier:

A transistor amplifier with proper positive feedback can act as an oscillator. (ie can produce oscillations without any external signal source).
Positive feedback amplifier is one that produces a feedback voltage that is in phase with the original input signal. Phase shift of 180° is produced by the transistor and further 180° phase shift is produced by feedback circuit. So the signal is shifted by 360 and fed to the input.

• In colpitt oscillator circuit, 180° phase shift is produced by the transistor and further 180° phase shift is produced by C1-C2 voltage divider.
• In this way , the feedback is properly phased to the produce undamped continuous oscillation.
• The amount of feedback voltage in colpitts oscillator depends upon feedback fraction mv of the circuit.
Feedback fraction