Operation of an Armstrong Oscillator

Posted on Feb 5, 2014

An Armstrong Oscillator is a type of oscillator that uses a tickler coil which provides feedback from tank circuit. The Armstrong Oscillator is used to produce a sine-wave output of constant amplitude and of fairly constant frequency within the rf range. Inductor L1 operates as a tickler coil, providing feedback from the output circuit [collector]

 Operation of an Armstrong Oscillator
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to the input circuit [base] of the transistor. Inductor L1 is coupled to transformer T1, by mutual inductive coupling. between the tickler and LC tuned circuit. For the circuit to oscillator, the feedback is regenerative. The transistor is identified as a NPN transistor, and could be almost any NPN transistor. The circuit is a basic common emitter configuration. The exact transistor part number would depend on the frequency of operation of the oscillator, and the value of the voltage used as Vcc. The important criteria is that the transistor provides amplification in the frequency range of oscillation. A 2N2222 NPN Transistor offers operation up to 300MHz. Resistor Rc is used to bias the collector of the transistor, a small loss is incurred over inductor L1, applying Vcc to the collector circuit. Resistor Rb is the base bias resistor. Resistor Re is the emitter bias resistor, also called self-bias. The capacitors Cb and Ce are used to by-pass the bias resistors for non-DC voltages. The resistors only set the DC bias, for AC voltages the resistors are shunted out of the circuit. The circuit to the right shows a series-fed armstrong oscillator. The tank circuit is form by variable capacitor C1 and the primary of T1. DC voltage is applied to the tank circuit, making the circuit a series-fed oscillator. Vcc is applied to the bottom side of the tuned circuit and is returned via T1, the collector of Q1 and emitter resistor Re...

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