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The Clapp Oscillator

The circuit shown to the right is a variation of the Colpitts The Clapp oscillator circuit. oscillator first introduced in 1948 by James K. Clapp. A number of circuits of this type were introduced by different authors at around the same time, but Mr. Clapp's name stuck, and this circuit is now commonly known as the Clapp oscillator.

The Clapp oscillator is distinguished from the Colpitts oscillator by the addition of C0 in series with L. This additional capacitor has several effects on the oscillator circuit. The signal voltage divider formed by C1 and C2 is unchanged, and these capacitors are fixed in value. Therefore the amount of energy fed back to sustain oscillations is the same at any frequency, so the amplitude of the signal is not affected much by a change in frequency. In addition, stray capacitances in the transistor have less effect on the operating frequency, which now depends on the series combination of all three capacitors.

The circuit still oscillates at a frequency of ω = 2πf = 1/, but now C0 is part of C in that expression. If C0 is made variable, it can control the frequency of oscillation without changing the feedback ratio or the stability of the overall circuit.

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