Royer oscillator

A Royer oscillator is an electronic relaxation oscillator that employs a saturable-core transformer. It was invented and patented in 1954 by George H. Royer. It has the advantages of simplicity, low component count, rectangle waveforms, and easy transformer isolation. By making maximum use of the transformer core, it also minimises the size and weight of the transformer. The classic Royer circuit outputs square waves; a modified version, essentially by adding a capacitor, turns it into a harmonic oscillator, outputting sine waves. Both versions are widely used, mainly as power inverters.

The circuit consists of a saturable-core transformer with a center-tapped primary winding, a feedback winding and (optionally) a secondary winding. The two halves of the primary are driven by two transistors in push-pull configuration. The feedback winding couples a small amount of the transformer flux back in to the transistor bases to provide positive feedback, generating oscillation. The oscillation frequency is determined by the maximum magnetic flux density, the power supply voltage, and the inductance of the primary winding.

The basic Royer generates square wave output.

The need to use a saturable-core limits the choice of core material. Some common materials are:

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