Definition

Colpitts oscillators are commonly used in simple amateur radio projects and are a form of tuned inductor-capacitor or LC oscillator. The Colpitts oscillator uses an amplifier feeding energy back into the tap of a capacitive voltage divider. The oscillator was invented in 1918 by American engineer Edwin H. Colpitts. A typical arrangement of a modern circuit based on an NPN bipolar transistor is shown below.

Colpitts Oscillator

The parallel LC tuned circuit is made up of L1, C1, C2 and VC1. Series connected capacitors C1 and C2 (themselves part of the overall capacitance) form a capacitive voltage divider. The capacitor C3 couples the output of the LC tuned circuit to the input of an amplifier formed by Q1. The output of the amplifier is taken from the top of R3 and fed to the tap formed between C1 and C2. Components R4, R5 and Q2 form an emitter follower. The emitter follower helps to ensure that connections to the output do not load the oscillator.

Uses

This form of oscillator is commonly used for simple morse code / CW (continuous wave) transmitters self-built by amateur radio enthusiasts. In this use, it is important to provide a stable frequency source of a pure sinusoidal voltage. A stable frequency source is one whose fundamental frequency is locked to a particular value that does not change over time. This leads to ease of use with no need for frequent re-tuning. If a circuit can generate a nearly perfect sine wave then it can be used to transmit a very narrow bandwidth signal. Narrow bandwidth transmissions are desirable because they cause little interference on neighbouring frequencies, so allowing a larger number of simultaneous users within a given radio band.

Challenges

Without an amplifier any oscillations of current in a parallel LC circuit will naturally tend to decay over time due to energy losses in the circuit. If the oscillator is connected to other circuitry (as in a transmitter) then that circuitry will also draw a current, no matter how small. This can be thought of as a further energy loss. The trick in designing a good Colpitts oscillator is to design a low-distortion amplifier providing the exact amount of power gain (note: not necessarily voltage gain!) that prevents the oscillations from decaying. A common amplifier is provided by an NPN bipolar transistor connected as an emitter follower (as in the above circuit). The input to the amplifier is at the base of the transistor, and the output is at the emitter. This has a voltage gain of very slightly less than 1 and a large current gain, ie it has power gain. The output at the emitter is fed back into the capacitive voltage divider.

In practice designing such a perfect amplifier is difficult, because if the amplifier has even just slightly more gain than necessary then the oscillations will tend to increase in amplitude (rather than decaying) until eventually the output waveform distorts. A distorted waveform that is not sinusoidal will not provide the necessary narrow bandwidth. One advantage of using a high gain amplifier, however, is that the oscillations will start quickly and reliably when power is first applied (rather than building up slowly over several wave cycles, or not starting at all as in the case of not enough gain).

In practice there are a number of ways to deal with the amplifier gain problem. One is to deliberately provide too much gain (which leads to reliable starting) and allowing the waveform to build quickly until it approaches the limits of the supplied voltage to the circuit and distorts. The distorted waveform is then filtered to restore the sinusoidal shape. Another method is to provide a non-linear, fast acting, gain limiting component (such as a diode) that will quickly begin to conduct once the waveform builds up to a certain amplitude. This has the effect of limiting circuit gain by "clipping" the tops and\or bottoms of the generated sinusoidal waveform. Again, the waveform should ideally be filtered. In some lower frequency oscillators such as the Wien bridge it is common practice to use a slower acting gain control component such as a thermistor that introduces less distortion to the waveform. Unfortunately, achieving reliable startup and avoiding distorted waveform output are not the only challenges.

Frequency Drift and Stability.

There are a number of reasons that the frequency of the circuit does not remain fixed. Any circuit that includes resistance and passes an electrical current will tend to heat up. Also there are natural changes in temperature of the environment that will affect the circuit. Although the temperature changes can be very small indeed, they will tend to change the gain of the amplifying components, the size (due to thermal expansion) of the inductor and capacitor and the values of these components. These changes lead to frequency drift, which is most noticable immediately after power-up. Placing hands near variable components used to tune the circuit can in itself lead to changes in component values and frequency. Sound and\or vibration can cause miniscule changes in the inductance of the inductor component - an effect known as microphony. Another common effect is that the circuitry connected to the oscillator can tend to affect its frequency by introducing changes in current, inductance or capacitance. This is often referred to as "pulling" the frequency of the oscillator.

Designing Good Quality Practical Circuits

For these reasons, Colpitts oscillators are typically constructed in a mechanically sound manner, with the turns of the inductor wound on a solid former (with no "loose" wiring). The power consumption is set to be quite low to avoid temperature effects. It is possible to purchase capacitors of negative or positive temperature coefficients, and the relative mix of these is chosen to balance the other temperature coefficients of the circuit to lead to a near-zero overall temperature coefficient. The supply voltage is regulated and the circuit is often fitted with a buffer circuit and filter following the oscillator to reduce the effects of "pulling". If a variable capacitor is used for tuning a shaft is fitted to reduce "hand effects" that change circuit capacitance. The circuit is enclosed to shield it from the effects of temperature change.

References

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