Colpitt's Oscillator

Colpitt's Oscillator

AU ; Dec.-04, 09, 17, 18, May-15, 16

• It uses two capacitive reactances and one inductive reactance in its feedback network. It is also classified as,

i) BJT Colpitt's oscillator

ii) FET Colpitt's oscillator.

1. Transistorised Colpitt's Oscillator

• The Fig. 10.10.1 shows Colpitt's oscillator using BJT amplifier stage.

• The amplifier stage uses BJT in common emitter configuration providing 180° phase shift. The resistances R₁ R₂ and RE are the biasing resistors.

• The RFC is radio frequency choke providing isolation between a.c. and d.c. operation. The C_C1 and C _C2 are coupling capacitors. In the feedback circuit, as the centre of C₁ and C₂ is grounded, it provides additional phase shift of 180°, satisfying Barkhausen angle condition.

• In this oscillator,

To satisfy magnitude condition of Barkhausen criterion, the hfe of BJT used is given by,

h_fe = C₂ /  C₁

2. Derivation of Frequency of Oscillations

The equivalent circuit and simplified equivalent circuit are shown in the Fig. 10.10.2.

Substituting ro in equation (10.10.2) and equating magnitudes of both sides,

h_fe = C₂ / C₁

Ex. 10.10.1 Find the frequency of the oscillations of a transistorised Colpitts oscillator having Cj = 150 pF, C2 = 1.5 nF and L = 50 pH.

Sol. :

Ex. 10.10.2 In a Colpitt’s oscillator = 0.001 pF, C2 = 0.01 pF and L = 10 pH. Find the frequency of oscillation, feedback factor and the voltage gain.

AU : Dec.-04, Marks 6

Sol. :

Ex. 10.10.3 In Colpitt’s oscillator C2 nF. If the frequency of oscillation is 100 kHz find the value of inductor. Also find the minimum gain required for obtaining sustained oscillations.

Sol. :

Ex. 10.10.4 A 1 mH inductor is available. Choose the capacitor values in a Colpitts oscillator so that f=l MHz and feedback fraction is 0.25.

Sol. :

AU : May-16, Marks 4

3. Advantages, Disadvantages and Applications

• The advantages are,

i) Pure output waveform.

ii) Good stability at high frequency

iii) Improved performance at high frequency

iv) Wide range of frequency

v) Simple construction

• The disadvantages are,

i) Difficult to adjust the feedback

ii) Poor isolation.

• Its main application is high frequency function generator.

Review Questions

1. Draw the neat circuit diagram of Colpitt's oscillator using BJT and explain its operation.

AU : Dec-04, 09, Marks 8

2. In a Colpitt's oscillator C1 = 0.001 pF, C2 = 0.01 pF and L = 10 pH. Find the frequency of oscillation, feedback factor and the voltage gain.

(Ans.: 1.6692 MHz, 0.1,10)

3. A Colpitts oscillator is designed with C1 =100 pF and C2 = 7500 pF. The inductance is variable. Determine the range of inductance values, if the frequency of oscillations is to vary between 950 kHz and 2050 kHz.

(Ans.: 61.07 µH to 284.41 µH)

4. Explain the operation of Colpitts oscillator with neat circuit diagram. Also derive the expressions for the frequency of oscillation and the condition for maintenance of oscillation.