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Oscillator

Oscillator is the electronic  circuit that serves to produce the electrical signal, in the waveform of a sinusoidal, square wave, or the other.

Oscillators are used in transmission systems and radio communications receiver, musical instrument, or in digital circuits and computers.

The basic principle is to work oscillator amplifier in accordance with the principle of positive feedback. With positive feedback the output will oscillate with a frequency that can be adjusted in accordance with the parameters of the electronic circuit, usually a component of RC or RL, or RLC.

The Armstrong oscillator

A common-emitter or common-source amplifier can be made to oscillate by coupling the output back to the input through a transformer that reverses the phase of the fed-back signal. The phase at a transformer output can be inverted by reversing the secondary terminals. The schematic diagram of Figure 1 shows a common-source amplifier whose drain circuit is coupled to the gate circuit via a transformer. In practice, getting oscillation is easy. If the circuit won’t oscillate with the transformer secondary hooked up one way, you can just switch the wires.

Figure 1. The Amstrong Oscillator Basic Circuit

The frequency of this oscillator is controlled by means of a capacitor across either the primary or the secondary winding of the transformer. The inductance of the winding, along with the capacitance, forms a resonant circuit. If you’ve forgotten it, now is a good time to review it. The oscillator of Figure 1 is known as an Armstrong oscillator. A bipolar transistor can be used in place of the JFET. It would need to be biased, using a resistive voltage-divider network, like a class-A amplifier.

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