Clapp oscillator

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The Clapp oscillator is one of several types of electronic oscillator constructed from a transistor (or vacuum tube) and a positive feedback network. It was invented by James K. Clapp in 1948.

Referring the notional circuit in the figure, the network is comprised of a single inductor and three capacitors, with two capacitors (C1 and C2) forming a voltage divider that determines the amount of feedback voltage applied to the transistor input. The Clapp oscillator is a Colpitts oscillator with an additional capacitor placed in series with the inductor. The oscillation frequency in hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (FET), is

f_0 = {1 \over 2\pi} \sqrt{ {1 \over L} \left( {1 \over C_0} + {1 \over C_1} + {1 \over C_2} \right)} \ .

Clapp Oscillator (direct-current biasing network not shown)
Clapp Oscillator (direct-current biasing network not shown)

A Clapp circuit is often preferred over a Colpitts circuit for constructing a variable frequency oscillator (VFO). In a Colpitts VFO, the voltage divider contains the variable capacitor (either C1 or C2). This causes the feedback voltage to be variable as well, sometimes making the Colpitts circuit less likely to achieve oscillation over a portion of the desired frequency range. This problem is avoided in the Clapp circuit by using fixed capacitors in the voltage divider and a variable capacitor (C0) in series with the inductor.

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