The Colpitts circuit

Chapter The Colpitts circuit

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • more than a fraction of a watt of power. If more power is needed, the signal can beboosted by one or more stages of amplification.It’s important to use only the minimum amount of feedback necessary to get oscil-lation. The amount of feedback is controlled by the position of the coil tap.The Colpitts circuitAnother way to provide RIF feedback is to tap the capacitance instead of the induc-tance in the tuned circuit. In Fig. 25-4, NPN bipolar (at A) and N-channel JFET (at B)Colpitts oscillator circuits are diagrammed.The amount of feedback is controlled by the ratio of capacitances. The coil, ratherthan the capacitors, is variable in this circuit. This is a matter of convenience. It’s almostimpossible to find a dual variable capacitor with the right capacitance ratio betweensections. Even if you find one, you cannot change the ratio of capacitances. It’s easy toadjust the capacitance ratio using a pair of fixed capacitors.Unfortunately, finding a good variable inductor might not be much easier than get-ting hold of a suitable dual-gang variable capacitor. A permeability-tuned coil can beused, but ferromagnetic cores impair the frequency stability of an RF oscillator. A rollerinductor might be employed, but these are bulky and expensive. An inductor with sev-eral switch-selectable taps can be used, but this wouldn’t allow for continuous fre-quency adjustment. The tradeoff is that the Colpitts circuit offers exceptional stabilityand reliability when properly designed.As with the Hartley circuit, the feedback should be kept to the minimum necessaryto sustain oscillation.In these circuits, the outputs are taken from the emitter or source. Why? Shouldn’tthe output be taken from the collector or drain? The answer is that the output can betaken from the collector or drain circuit, and an oscillator will usually work just fine. Butgain is not important in an oscillator; what matters is stability under varying load condi-tions. Stability is enhanced when the output of an oscillator is taken from the emitter orsource portion of the circuit.To prevent the output signal from being short-circuited to ground, an RF choke(RFC) is connected in series with the emitter or source lead in the Colpitts circuit. Thechoke lets dc pass while blocking ac (just the opposite of a blocking capacitor). Typicalvalues for RF chokes range from about 100 µH at high frequencies, like 15 MHz, to 10mH at low frequencies, such as 150 kHz.The Clapp circuitA variation of the Colpitts oscillator makes use of series resonance, instead of parallelresonance, in the tuned circuit. Otherwise, the circuit is basically the same as the par-allel-tuned Colpitts oscillator. A schematic diagram of an N-channel JFET Clapp oscil-lator circuit is shown in Fig. 25-5. The P-channel circuit is identical, except for thepower supply polarity, which is reversed.The bipolar-transistor Clapp circuit is almost exactly the same as the circuit of Fig. 25-5,with the emitter in place of the source, the base in place of the gate, and the collector The Clapp circuit461