FET Fundamentals Unit 7 – Hartley and Colpitts Oscillators
UNIT 7 – HARTLEY AND COLPITTS OSCILLATORS
UNIT OBJECTIVE
At the completion of this unit, you will be able to demonstrate the operation of JFET Hartley and
Colpitts oscillators by using an oscillator test circuit.
UNIT FUNDAMENTALS
An oscillator is a circuit that converts direct current to alternating current with a specific
frequency.
There are four requirements for a circuit to be an oscillator.
- a dc power supply
- an amplifier (active device)
- a frequency-determining network (such as an LC tank)
- positive feedback(in phase) or regenerative feedback which causes a circuit gain greater
than 1
The operating frequency of oscillators can range from less than one cycle per second (< 1 Hz) to
billions of cycles per second (gigahertz, or GHz).
Hartley and Colpitts oscillators usually operate from about 100 kHz to 100 MHz; a Colpitts
oscillator is usually used for the higher frequencies.
The active device (amplifier) of an oscillator may be a bipolar transistor or a FET. The Hartley
and Colpitts oscillators in this unit use a JFET as the active device.