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Saturday, October 10, 2009

Colpitts Crystal Oscillator Circuit

This is a design schematic of a Crystal Colpitts oscillator can be implemented using a transistor and a parallel mode crystal. This is the figure of the circuit.


In this circuit, the crystal is use as an inductance. A large value capacitive divider is used between gate, source, and ground, and a small series capacitor is placed in the crystal circuit. You should choose the components values so that C2+C3 to C1 ratio has the highest possible value. The ratio of 5 to 10 to 1 is usually used. The schematic shows the typical values. This circuit introduce is a little loading on the crystal. The relatively high value is of C2 and C3 “swamp out” variations and drift caused by variations in device characteristics. Frequency can be fine tuned with C1. A clean enough sine wave appears at the emitter of the transistor.

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