FET Fundamentals Unit 7 – Hartley and Colpitts Oscillators
When the level of the feedback signal is above a critical level and essentially in phase with the
amplifier input signal, the circuit oscillations become self-sustaining, or self-driven.
A practical oscillator uses a frequency-determining network to make it oscillate at a
predetermined frequency.
The frequency-determining network, or tank circuit, could be a resonant LC
(inductor/capacitor) network or an RC (resistor/capacitor) phase shift network.
The frequency-determining network could also be a quartz crystal or a mechanical resonator
Oscillator circuits may be varied over a range of frequencies if part of the tank circuit is
adjustable.
Depending upon the dc power supply configuration, the two general types of oscillator circuits
are series fed and shunt fed. In the series fed oscillator circuit shown, the dc supply current
passes through the amplifier and all or part of the tank circuit.
In a shunt fed oscillator, the dc supply current does not flow through the tank circuit.