Skip to main content

LC oscillator calculator

What Clapp has over Colpitts

Not the frequency — that is the same. The difference is how strongly the transistor's own capacitance affects it, and that capacitance wanders with temperature and operating point. In a Colpitts it adds to C1 and drags the frequency with it; in a Clapp the frequency is set by a small C3 in series with the coil, and the influence of everything else falls several times over.

C1to the active deviceC2
Colpitts: a capacitive divider provides the feedback, and the point between C1 and C2 is ground
Circuit
Tank inductance (µH)
C1 — upper divider capacitance (pF)
C2 — lower divider capacitance (pF)
Driven element capacitance (pF)
Transconductance of the active device (mA/V)
Equivalent tank resistance (Ω)
Result
Oscillation frequency
Equivalent capacitance
Feedback ratio
Tank reactance
Loaded tank Q
Condition for oscillation
gm·R must be at least
EU
Oscillation

📐 The formulas​

Colpitts:

C_eq = C1·C2 / (C1 + C2)
f = 1 / (2π·√(L·C_eq))
β = C1 / C2
condition for oscillation: gm · R_tank ≥ C2/C1

Clapp is the same Colpitts with a third capacitor in series with the coil:

1/C_eq = 1/C1 + 1/C2 + 1/C3

C3 is much smaller than C1 and C2, so it is what sets the frequency.

Hartley:

L_eq = L1 + L2 + 2·M
f = 1 / (2π·√(L_eq·C))
β = L1 / L2

A check point:​

L = 1 µH, C1 = 100 pF, C2 = 1000 pF → C_eq = 90.91 pF, f = 16.69 MHz. The same circuit with C3 = 47 pF (Clapp) → C_eq = 30.98 pF, f = 28.59 MHz.

One picofarad of transistor capacitance moves the Colpitts by 75 kHz and the Clapp by only 44 kHz.

Where this calculator stops:
  • The formulas ignore the input and output conductance of the active device, which lower the tank Q
  • The condition for oscillation is necessary but not sufficient: a real circuit also needs correct biasing
  • Stray wiring capacitance adds to C1 just as the transistor's does — at VHF it is often the larger of the two
  • The calculation says nothing about phase noise: that needs the tank Q and the device's operating point

🎯 In practice​

  • A VFO for a homebrew transceiver — and an honest comparison of which circuit is steadier.
  • The local oscillator in a simple direct-conversion receiver.
  • Checking somebody else's circuit: is the condition for oscillation even met.
Practical advice:
  1. For a VFO use Clapp: it is several times less sensitive to the active device.
  2. Keep two or three times the margin for oscillation, no more: tighter feedback drives the oscillator into saturation.
  3. The tank capacitors should be NP0/C0G and the coil on a ceramic former, rigidly fixed.
  4. Thermal drift is compensated with a negative-coefficient capacitor, chosen by experiment.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «LC oscillator calculator: Colpitts, Clapp, Hartley». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/lc_oscillator (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

Last updated on