Colpitts oscillators are similar to the shunt fed Hartley oscillator circuit except the Colpitts oscillator, instead of having a tapped inductor, utilizes two series capacitors in its LC circuit.

With the Colpitts oscillator the connection between these two capacitors is used as the center tap for the circuit. A Colpitts oscillator circuit is shown at Figure 2-5, and you will see some similarities with the Hartley oscillator.

Colpitts Oscillators

The simplest Colpitts oscillator to construct and get running is the “series tuned” version, more often referred to as the “Clapp Oscillator.” Because there is no load on the inductor, a high “Q” circuit results with a high L/C ratio and of course much less circulating current.

This aids drift reduction. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits.

The total capacitive reactance of the parallel combination of capacitors depicted as series tuning below the inductor in a series tuned Colpitts oscillator or “Clapp oscillator” should have a total reactance of around 200 ohms.

Not all capacitors may be required in your particular application. Effectively all the capacitors are in series in a Colpitts oscillator, i.e. they appear as parallel connected but their actual values are in fact in series.

Ideally, your frequency determining components L1 and the parallel capacitors should be in a grounded metal shield. The FET used in the Colpitts oscillator is the readily available 2N4416A.

Note, the metal FET case is connected to the circuit ground. The output from the Colpitts oscillator is through output capacitor 47 pF; this should be the smallest of values possible, consistent with continued reliable operation into the next buffer amplifier stage.

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