Clapp Oscillator

Clapp Oscillator

AU: Dec.-03

• In Hartley and Colpitt's oscillator, the frequency stability is poor because,

i) The transistor internal capacitance affects the of feedback network equivalent value capacitance.

ii) The transistor parameters depend on temperature which changes its internal capacitance and affects the frequency.

• To provide good frequency stability, Clapp oscillator is used.

• The tank circuit in Clapp oscillator is modified as shown in the Fig. 10.11.1.

• In addition to C,, C2 and L, one additional capacitor C3 is introduced in series with the inductance.

The value of C₃ is much smaller than C₁ and C₂.

• The equivalent capacitance is now given by,

• The oscillating frequency is given by,

• But as C3 is much smaller than C, and C2, Practically, C_eq is almost equal to C₃.

• Across C₃, there is no transistor parameter and hence internal transistor capacitance does not affect C₃ and thus the frequency of Clapp oscillator is stable.

• The transistorised Clapp oscillator is shown in the Fig. 10.11.2.

• Keeping C₃ variable, frequency can be varied over wide range.

1. Advantages

1. The frequency is stable and accurate.

2. The good frequency stability.

3. The stray capacitances have no effect on C₃ which decides the frequency. 

4. Keeping C₃ variable, frequency can be varied in the desired range.

Ex. 10.11.1 Calculate the frequency of oscillation for the Clapp oscillator with C₁ = 0.1 µF, C₂ =l µF, C₃ = 100 µF and L = 470 µH.

Sol. :

Review Questions

1. Draw the circuit diagram of Clapp's oscillator and state the expression for frequency of oscillations.

2 With circuit diagrams, explain why Clapp oscillator is preferred compared to that of Colpitt's oscillator.

3. In a Clapp oscillator, C₁ = 0.01 µF, L = 5 µH, C₂ = 0.001 µF and C₃ = 63 µF. Calculate frequency of oscillations.

[Ans.: 8.967 MHz]