I found this oscillator at a Theremin project pdf. I simulated the circuit, and found that the output (left wire) oscillates at 236.5 kHz.

enter image description here

This circuit is referred to as "Variable Pitch Oscillator", but that doesn't seem to be the best reference for it.

So I'm wondering: what circuit is this, and how do I calculate the oscillation frequency??


3 Answers 3


The oscillation frequency is determined by C1 and L5: -

\$f_O = \dfrac{1}{2\pi\sqrt{LC}}\$

And, for values of 3900pF and 100uH, it should oscillate at 254.8 kHz theoretically but there will be parasitic capacitance in the inductor and miller capacitance in the transistors that make the actual capacitance bigger hence lowering the resonance to 236kHz.

I'd call it an LC oscillator.

  • 1
    \$\begingroup\$ Oh forgot to mention - it's a class C bias arrangement on the feedback - there is no standing DC bias on Q2 because it is fed via C3 - the reason it still produces a decent sinewave is the Q of the tuned circuit. It's quite neat actually and it's similar to an oscillator I designed once but I used diode shaping in the feedback to get really, really good sinewave purity and amplitude stability. \$\endgroup\$
    – Andy aka
    Commented Aug 12, 2014 at 19:08

The working principle of this oscillator is as follows:

We have a classical tank circuit (L5||C1) which is connected in a closed loop with a two-stage non-inverting amplifier. This amplifier consists of an emitter follower (Q2) driving a common base amplifier stage (Q1). Hence, there is no phase inversion within the loop (condition for positive feedback).

The gain around the whole loop (loop gain LG) satisfies the oscillation condition (LG>1) at one single frequency only. This frequency was given already in Andy aka´s answer.

Remark (edit): The large value of the coupling capacitor C3 has no influence on the oscillation condition (at the frequency range of interest) and the value of R3 determines the feedback factor - and, thus, the magnitude of the loop gain which should be (slightly) larger than unity (0 dB).


This is known as a differential pair oscillator. One advantage is that it doesn't require a tapped tank circuit like a Hartley or Colpitts oscillator.


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