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The Armstrong Oscillator

The feedback for an oscillator doesn't need to come from an electrical The Armstrong oscillator circuit. connection. In the circuit shown to the left, the feedback comes from magnetic coupling between the coil in the tank circuit and a tickler coil, T.

The frequency of oscillation is still controlled primarily by the tank circuit, so that ω = 2πf = 1/. However, in this circuit there are several factors that make this equation only approximate. Of course, we have the stray capacitances in the transistor and the small inductances of the component leads. But this time we also have that tickler coil, which acts as a load on L due to mutual inductance. All of these factors pull the frequency off a bit.

C can be made variable in order to either adjust the frequency to a specific value, or to make it variable over a range.

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