Colpitts Oscillator Calculator

Edwin H. Colpitts, AT&T engineer, invented the capacitive-divider LC oscillator in 1918 while working on long-distance telephony and vacuum-tube amplifiers. His US patent 1,624,537 (filed 1919, granted 1927) documented the circuit that bears his name — a tuned-collector/tuned-base oscillator with capacitive feedback to the emitter.

The Colpitts topology dominated radio-receiver local oscillators from the 1920s through the 1970s because capacitor tolerances were easier to match than coil taps (Hartley) and the circuit had reliable startup across temperature. Ham-radio VFOs and early FM transmitters all used Colpitts with variable caps for tuning.

Modern integrated VCOs (PLLatino, MAX2622) inherit Colpitts topology in silicon — the LC tank is on-chip (spiral inductor + varactor diode + tap caps), with BJT or CMOS transistor for the active element. Phase noise targets of -120 dBc/Hz at 10 kHz offset are routine for modern RF synthesizers using this topology.

Enter inductor L and the two divider capacitors C1 (tap to emitter/source) and C2 (tap to ground). The tool computes series-combined capacitance C_s = C1·C2/(C1+C2), oscillation frequency f = 1/(2π·√(L·C_s)), feedback ratio k = C1/(C1+C2), and inductor reactance X_L at f.

For reliable startup with BJT amplifier: target k ≈ 0.1-0.3 (i.e., C2 > C1 by factor of 3-10). Loop gain A_trans · k > 1 required; for JFET or MOSFET buffer, higher k needed. Inductor Q directly sets phase-noise floor — use silver-plated air-core coils for lowest noise. Everything runs client-side.