Colpitt’s Oscillator

• The tank circuit is made up of C₁, C₂  and L. The frequency of oscillations is determined by:

• When the circuit is turned ON, the capacitor C₁ and C₂ are charged. The capacitors discharge through L setting up oscillations of frequency determined by expression. 1. The output voltage appears across C₂ and feedback voltage is developed across C₁. The voltage across C₁ is 180⁰ out of phase with the voltage developed across C₂ (V_out ). A phase shift of 180⁰ is produced by the transistor and a further phase shift of 180⁰ is produced by C₁-C₂ voltage divider. In this way feedback is properly phased to produce continuous undamped oscillations.

Feedback factor

Feedback factor

Demerits of Oscillator using Tank Circuit

1. They suffer for frequency instability and poor waveform
2. They cannot be used to generate low frequencies, since they become too-much bulky and expensive too.