# Clapp oscillator

The Clapp oscillator or Gouriet oscillator is an LC electronic oscillator that uses a particular combination of an inductor and three capacitors to set the oscillator's frequency. LC oscillators use a transistor (or vacuum tube or other gain element) and a positive feedback network. The oscillator has good frequency stability.

## History

The Clapp oscillator design was published by James Kilton Clapp in 1948 while he worked at General Radio. According to Vačkář, oscillators of this kind were independently developed by several inventors, and one developed by Gouriet had been in operation at the BBC since 1938.

## Circuit

The Clapp oscillator uses a single inductor and three capacitors to set its frequency. The Clapp oscillator is often drawn as a Colpitts oscillator that has an additional capacitor (C0) 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

$f_{0}={1 \over 2\pi }{\sqrt {{1 \over L}\left({1 \over C_{0}}+{1 \over C_{1}}+{1 \over C_{2}}\right)}}\ .$ The capacitors C1 and C2 are usually much larger than C0, so the 1/C0 term dominates the other capacitances, and the frequency is near the series resonance of L and C0. Clapp's paper gives an example where C1 and C2 are 40 times larger than C0; the change makes the Clapp circuit about 400 times more stable than the Colpitts oscillator for capacitance changes of C2.

Capacitors C0, C1 and C2 form a voltage divider that determines the amount of feedback voltage applied to the transistor input.

Although, the Clapp circuit is used as a variable frequency oscillator (VFO) by making C0 a variable capacitor, Vačkář states that the Clapp oscillator "can only be used for operation on fixed frequencies or at the most over narrow bands (max. about 1:1.2)." The problem is that under typical conditions, the Clapp oscillator's loop gain varies as f  −3, so wide ranges will overdrive the amplifier. For VFOs, Vačkář recommends other circuits. See Vačkář oscillator.