# 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.[1] According to Vackář, oscillators of this kind were independently developed by several inventors, and one developed by Gouriet had been in operation at the BBC since 1938.[2]

## Circuit

Clapp oscillator (direct-current biasing network not shown)

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.[3]

The oscillation frequency in Hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (FET), is

${\displaystyle f_{0}={1 \over 2\pi }{\sqrt {{1 \over L}\left({1 \over C_{0}}+{1 \over C_{1}}+{1 \over C_{2}}\right)}}\ .}$

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.[4]

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, Vackář 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)."[5] 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, Vackář recommends other circuits. See Vackář oscillator.

## References

1. ^ Clapp, J. K. (March 1948). "An inductance-capacitance oscillator of unusual frequency stability". Proc. IRE. 367: 356–358.
2. ^ Vackář, Jiri (December 1949). LC Oscillators and their Frequency Stability (PDF) (Report). Prague, Czechoslovakia: Tesla National Corporation. Tesla Technical Report. Archived from the original (PDF) on 2009-01-24. Retrieved 2008-12-20.
3. ^ Department of the Army (1963) [1959]. Basic Theory and Application of Transistors. Dover. pp. 171–173. TM 11-690. Modification of the Colpitts oscillator by including a capacitor in series with winding 1–2 of the transformer results in the Clapp oscillator.
4. ^ Clapp 1948, p. 357
5. ^ Vackář 1949, pp. 5–6