Colpitts Oscillator Calculator
Design a Colpitts LC oscillator with capacitive-divider feedback. Solves for oscillation frequency from L and the series-cap pair C1, C2.
How to Use
- Enter inductor L and the two tap capacitors C1 and C2.
- Tool computes oscillation frequency f = 1/(2π·√(L·C_series)) and feedback ratio C1/C2.
Show Work
Formulas
History
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.
About This Calculator
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.
About the Colpitts Oscillator Calculator
Whether you are at a desk or on your phone, the Colpitts Oscillator Calculator makes electronics and circuit design easy — and it is completely free. Design a Colpitts LC oscillator with capacitive-divider feedback. Solves for oscillation frequency from L and the series-cap pair C1, C2.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Colpitts vs Hartley?
Colpitts: capacitive divider sets feedback ratio (C1 to emitter, C2 to ground). Hartley: inductive divider. Colpitts is easier to build with matched caps than matched coil taps, so it dominates in RF oscillator design.
Feedback ratio?
Voltage divider C1/C2 sets the feedback. For reliable startup: k = C2/(C1+C2) > 1/A (A = amp gain). Typically pick C1 < C2 by factor 5-10.
Why LC?
RC oscillators are limited by phase-noise and Q of the RC network. LC tanks with Q = 50-200 provide low phase-noise for RF synthesis; quartz extends this to Q > 10,000 in crystal oscillators.
How do I use the Colpitts Oscillator Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
VFO for Receiver
Ham radio VFO: variable C1 with L fixed sweeps across band.
RF Carrier
100 MHz FM transmitter local oscillator in 1970s car radios.
PLL VCO
Discrete LC VCO tuned by varactor across C1 or C2.
Last updated: