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The Colpitts Oscillator

The Colpitts oscillator (named for its inventor, Edwin H. Colpitts) is sort of a reverse on the Hartley oscillator. Instead of a tapped coil, it uses two capacitors in series to provide the feedback point. The result is shown to the left.

Because feedback is through the capacitive leg of the LC tank circuit, the total capacitance of this leg is the series combination of C1 and C2, so that C = C1C2/C1+C2. The operating frequency is controlled by the tank circuit: ω = 2πf = 1/.

Because the transistor cannot be biased through the capacitors, we need a separate dc biasing circuit for the transistor itself. That is the purpose of the source resistor in this circuit. Of course, parasitic capacitance in the resistor and the transistor will have a small effect on operating frequency. However, this can be balanced out by careful adjustment of C in the tank circuit.

The circuit above is based on a transistor configured to operate as a source follower (also known as the common drain configuration). However, because the capacitors in the tank circuit do not affect the dc bias conditions of the transistor, we can easily adapt the Colpitts oscillator to other transistor configurations, such as the common base version shown to the right.

As with the first circuit, the resistors are all for the dc bias of the transistor, and either bipolar transistors or FETs will work fine. Indeed, the earliest versions of this circuit were designed around triode vacuum tubes.

As far as the signal is concerned, L is still in parallel with the series combination of C1 and C2, because the power supply has no resistance or impedance for ac signals. In most cases, the power supply is bypassed with appropriate capacitors specifically to make sure signals will not appear on the power supply rails.

One drawback of the Colpitts oscillator circuit is that it has problems when we want to make the operating frequency variable. We can make C1 variable, but then the feedback ratio depends on the operating frequency. This can cause oscillations to cease due to insufficient feedback.

We need a better way to make this oscillator have a variable oscillation frequency.

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