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Common Oscillator Circuits 395
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24-12 An Armstrong oscillator using an N-channel JFET. This is a
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common-source amplifier with positive feedback through a tuned circuit.
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The Colpitts Circuit The capacitance can be tapped, instead of the inductance, in the tuned circuit of an RF oscillator. Such a circuit is called a Colpitts oscillator, and a P-channel JFET version is diagrammed in Fig. 24-14. The amount of feedback is controlled by the ratio of the capacitances. A variable inductor provides for frequency adjustment. This is a matter of convenience, because it can be difficult to find a dual
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24-13 A Hartley oscillator using a PNP bipolar transistor.
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The Hartley circuit can be recognized by the tapped inductor in the tuned LC circuit.
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396 Amplifiers and Oscillators
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24-14 A Colpitts oscillator using a P-channel JFET. The
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Colpitts circuit can be recognized by the split capacitance in the tuned LC circuit.
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variable capacitor that maintains the correct ratio of capacitances throughout its tuning range. Using fixed capacitors eliminates this problem, and it costs less, too! Unfortunately, finding a good variable inductor for use in a Colpitts oscillator can be just about as hard as obtaining a suitable dual-gang variable capacitor. A permeability-tuned coil can be used, but ferromagnetic cores impair the frequency stability of an RF oscillator. A roller inductor can be employed, but these are bulky and expensive. An inductor with several switch-selectable taps can be used, but this does not allow for continuous frequency adjustment. Despite these shortcomings, the Colpitts circuit offers exceptional stability and reliability when properly designed, and is preferred by some engineers for this reason.
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The Clapp Circuit A variation of the Colpitts oscillator makes use of series resonance, instead of parallel resonance, in the tuned circuit. Otherwise, the circuit is basically the same as the parallel-tuned Colpitts oscillator. Figure 24-15 is a schematic diagram of a series-tuned Colpitts oscillator circuit that uses an NPN bipolar transistor. This circuit is also known as a Clapp oscillator. Its frequency won t change much when high-quality components are used. The Clapp oscillator is a reliable circuit. It isn t hard to get it to oscillate and keep it going. Another advantage of the Clapp circuit is that it allows the use of a variable capacitor for frequency control, while accomplishing feedback through a capacitive voltage divider. Getting the Output In the Hartley, Colpitts, and Clapp oscillators just described and shown in Figs. 24-13 through 24-15, the output is taken from the emitter or source, not from the collector or drain. There s a reason for this. The output of an oscillator can be taken from the collector or drain, just as is done in a common emitter or common-source amplifier to get maximum gain. But in an oscillator, stabil-
Common Oscillator Circuits 397
24-15 A series-tuned Colpitts oscillator, also known as a
Clapp oscillator. This circuit uses an NPN bipolar transistor.
ity is more important than gain. The stability of an oscillator is better when the output is taken from the emitter or source, as compared with taking it from the collector or drain. Variations in the load impedance are less likely to affect the frequency of oscillation, and a sudden decrease in load impedance is less likely to cause the circuit to stop oscillating. To prevent the output signal from being short-circuited to ground, an RF choke (RFC) is connected in series with the emitter or source in the Colpitts and Clapp oscillator circuits. The choke lets dc pass while blocking ac (just the opposite of a blocking capacitor). Typical values for RF chokes range from about 100 H at high frequencies, such as 15 MHz, to 10 mH at low frequencies, such as 150 kHz.
The Voltage-Controlled Oscillator The frequency of a VFO can be adjusted by means of a varactor diode in the tuned LC circuit. Recall that a varactor, also called a varicap, is a semiconductor diode that works as a variable capacitor when it is reverse-biased. The capacitance depends on the reverse-bias voltage. The greater this voltage, the lower the value of the capacitance. The Hartley and Clapp oscillator circuits lend themselves well to varactor-diode frequency control. The varactor is placed in series or parallel with the tuning capacitor, and is isolated for dc by blocking capacitors. In Chap. 20, we saw an example of how a varactor can be connected in a tuned circuit (Fig. 20-9). The resulting oscillator is called a voltage-controlled oscillator (VCO). Varactors are cheaper than variable capacitors or inductors. They re also less bulky. These are the chief advantages of a VCO over an old-fashioned LC tuned VFO. Diode Oscillators At ultrahigh frequencies (UHF) and microwave radio frequencies, certain types of diodes can be used as oscillators. You learned about these diodes in Chap. 20.
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