COLPITT’S OSCILLATOR Electronic devices and circuits lab manual - Computer Programming


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Wednesday, October 5, 2011

COLPITT’S OSCILLATOR Electronic devices and circuits lab manual

AIM: To study and calculate frequency of oscillations of colpitt’s oscillator.
APPARATUS:  Transistor   BC 107
                           Capacitors  0.1µF      - 2Nos
                                             10µF        - 2Nos
                                              47µF       - 1No
                           Resistors    6.8kΩ, 1kΩ,100kΩ
                           Decade Inductance Box       (DIB)
                           Decade Resistance Box      (DRB)
                           Cathode Ray Oscilloscope (CRO)
                           Regulated Power Supply   (0-30V)
                           Connecting Wires

                  The tank circuit is made up of  L1,C4 and C5 .The resistance R2 and R3 provides the necessary biasing. The capacitance C2 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 / (2p (CTL1)­­­­1/2­) Where CT = C1C2 / ( C1 + C2)
The energy supplied to the tank circuit is of correct 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.
1.    connections are made as per circuit diagram.
2.    Connect CRO output terminals and observe the waveform.
3.    Calculate practically the frequency of oscillations by using the expression
                             f = 1 / T  ( T= Time period of the waveform)
4.    Repeat the above steps 2,3 for different values of L, and note down the practically values of oscillations of the collpitt’s oscillator.
5.    Compare the values of oscillations both theoritically and practically.

Inductance ( mH )
Theoretical Frequency
( Hz )
Practical Frequency
( Hz )


1.     The connections should be correct.
2.    Transistor terminals should be identified properly.
3.    Readings should be taken without parallalox error.

RESULT: Frequency of oscillations of colpitts oscillator is measured practically and campared with theoritical values .