Clapp oscillator

The Clapp oscillator is one of several types of LC electronic oscillator constucted from a transistor (or vacuum tube) and a feedback network. Referring the notional circuit in the figure, the feedback 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.

Clapp circuits are often preferred over the Colpitts circuits for constructing variable frequency oscillators (VFOs) that use variable capacitors to adjust the frequency. In a Colpitts VFO, the voltage divider network contains the variable capacitor (either C1 or C2). This causes the feedback voltage to be variable as well, thus making the Colpitts circuit less likely to achieve oscillation over some portion of the 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.