Q1Analog Electronics

Question

(e) Discuss how does the circuit of Clap oscillator is different from that of Colpitt oscillator?

Answer 1

The Colpitts oscillator uses a tank circuit with one inductor L and two capacitors C1 and C2 in series (voltage divider feedback). The oscillation frequency is:

f_{0} = \frac{1}{2 π L C _{e q}}, C_{e q} = \frac{C _{1} C _{2}}{C _{1} + C _{2}}

The Clapp oscillator adds a third capacitor C3 in series with the inductor L in the tank circuit. C3 is made much smaller than C1 and C2, so $C_{e q} \approx C_{3}$ (dominated by C3). This makes the oscillation frequency primarily dependent on C3 only:

f_{0} = \frac{1}{2 π L C _{3}}

Since C1 and C2 (which are influenced by transistor parasitic capacitances) do not affect frequency, the Clapp oscillator has better frequency stability than the Colpitts oscillator. This is the main difference.

Q1Analog Electronics

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