Clapp Oscillator Frequency

f = 1/(2π√(L·Cs)), 1/Cs = 1/C1+1/C2+1/C3

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Result
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Formula

f = 1/(2π√(L·Cs)), 1/Cs = 1/C1 + 1/C2 + 1/C3

Description

The Clapp oscillator is a Colpitts variant with an extra capacitor C3 placed in series with the tank inductor. Because the three capacitors are in series, the small C3 dominates the resonant capacitance and largely decouples the frequency from the transistor’s drifting junction capacitances (which set C1 and C2). This gives the Clapp markedly better frequency stability than a plain Colpitts.

Variables

  • f — Oscillation frequency (Hz)
  • L — Tank inductance (H)
  • C1, C2 — Feedback divider capacitors (F)
  • C3 — Series tuning capacitor (F)

Practical Notes

Make C3 much smaller than C1 and C2 so it sets the frequency and swamps transistor capacitance variation. Tuning C3 sweeps frequency with minimal effect on the feedback ratio.

Related Concepts

All Timing & Oscillators formulas →
555 Timer Astable 555 Timer Monostable Crystal Load Capacitance RC Oscillator Frequency Crystal Frequency Stability PLL Lock Time VCO Tuning Sensitivity Jitter from Phase Noise

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