Colpitts oscilattor

OBJECTIVE: To study and calculate frequency of oscillations of Colpitt’s oscillator.


  1. Transistor BC 107 -1 No.
  2. Capacitors 0.1µF - 1 No, 0.01µF - 2 Nos
  3. Inductor 5mH - 1 No.
  4. Resistors  47KOhm, 1kO, 10kO, 1.5K -1 No.
  5. Decade inductance box
  6. CRO(Dual Channel ) (0-20 MHz)
  7. Function generator ( 1Hz to 1 MHz)
  8. Regulated power supply (0-30V)
  9. Connecting Wires


The tank circuit is made up of L1,C1 and C2 .The resistance R2 and R3 provides the necessary biasing. The capacitance C3 blocks the D.C component. The frequency of oscillations is determined by the values of L1,C4 and C5, and is given by

f = 1 / (2(pi) (CTL1)­­­­1/2­) Where CT = C1C2 / ( C1 + C2)

The energy supplied to the tank circuit is of in phase. The tank circuit provides 1800 out of phase. Also the transistor provides another 1800. In this way, energy feedback to the tank circuit is in phase with the generated oscillations.



Connections are to made as per circuit diagram.

Connect CRO output terminals and observe the waveform.

Calculate practically the frequency of oscillations by using the expression

f = 1 / T ( T= Time period of the waveform)

Repeat the above steps 2,3 for different values of L, and note down the practical values of oscillations of the Collpitt’s oscillator.

Compare the values of oscillations both theoretically and practically.


Inductance ( mH )

Theoretical Frequency

( Hz )

Practical Frequency

( Hz )




  1. The connections are to be connected properly.
  2. Transistor terminals should be identified properly.
  3. Readings should be taken without parallax error.

RESULT: Frequency of oscillations of Colpitt’s oscillator is measured practically and compared with theoretical values .


Students are able to Design and analyze Colpitts oscillator.


  1. What are the applications of LC oscillator?
  2. What type of feedback is used in oscillators?
  3. What is the loop gain of an oscillator?
  4. What is the difference between amplifier and oscillator?
  5. What is the condition for oscillations?