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Project Notes

#180 ColpittsOscillator

Testing a basic Colpitts Oscillator circuit

Breadboard Build


A Colpitts oscillator uses a combination of inductors (L) and capacitors (C) to produce an oscillation at the resonant frequency of LC circuit.

The distinguishing feature of a Colpitts oscillator is that the feedback to gain device (here an NPN BJT) is taken from the midpoint of the two capacitors.

With C1,C2=100nF and L1=1mH, I should expect oscillation at 22.5kHz, however on my first breadboard build I’m getting around 33kHz:


So more experiments required..

A Protoboard Build

Protoboard Build

Putting the circuit on a protoboard and using some better quality capacitors makes all the difference!

I’m seeing an almost perfect 22.9kHz compared to the theoretical 22.5kHz.

The frequency can be tuned (reduced) with a capacitor in parallel with the inductor, up to about 100nF when the oscillation will be lost.

Here’s an AC plot showing Vout on CH1, and TP1 on CH2. Some things to note:

  • there’s some distortion at the lower end of the Vout cycle; I tried twaeking the transistor bias to eliminate this but to no effect
  • an almost perfect sine at TP1 (midpoint of the capacitor bridge/transistor emitter)
  • 4.2V swing on Vout - not bad for a 5v supply
  • 2V swing on TP1


Here’s the DC-coupled plot, with both channels shifted down -5V

  • Vout is oscillating around 5V
  • TP1 oscillates ~2.8V




The Schematic

Breadboard Build

Protoboard Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.