Colpitts_50MHz_Stabilizing PUBLIC

Exploring how a Colpitts oscillator can self-stabilize its amplitude (AGC).
Along the way, discovering how some simulated oscillators can "spontaneously" start.

This is a classic Colpitts oscillator, original simulated design from:

Colpitts_01 https://www.circuitlab.com/editor/#?id=sk783g

The positive feedback is from Collector to Emitter of Q1 in a common base configuration.

The nominal frequency is set by L1 and C1seriesC2. 150nH and 50pF > 58MHz. At this frequency, internal transistor capacitances will have an effect. The simulated frequency is approx. 52MHz.

SW1 and R4 working together shorten simulation times, by giving Q1 a 1/2mA jolt to get things going more quickly. Otherwise its a (simulated) 1uS wait while the amplitude ramps up.

There are a couple of questions on the circuit diagram with detailed answers underneath. By altering the plot variables you can see the inner workings of this classic circuit.