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Oscillators
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someone was getting ready to speak over a public-address system. The gain was set too high. The person began to speak; sound from the speakers got into the microphone, was amplified, went to the speakers again, and back to the microphone. A vicious cycle of feedback ensued. The result might have been a rumble, a howl, or a shriek. The system broke into oscillation. The amplifiers became temporarily useless until the gain was reduced. Oscillation can be controlled, so that it takes place at a specific, stable, predictable frequency. An oscillator is a circuit that is deliberately designed to oscillate.
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Uses of oscillators
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Some oscillators work at audio frequencies, and others are intended to produce radio signals. Most generate sine waves, although some are built to emit square waves, sawtooth waves, or other waveshapes. The subject of oscillators, once you understand amplifiers, is elementary. All oscillators are amplifiers with positive feedback. In this chapter, radio-frequency (RF) oscillators are discussed in some detail, and then audio oscillators are examined. In radio communications, oscillators generate the waves or signals, that are ultimately sent over the air. For data to be sent, the signal from an oscillator must be modulated. Modulation is covered in chapter 26. Oscillators are used in radio and TV receivers for frequency control and for detection and mixing. Detectors and mixers are discussed in chapter 27. Audio-frequency oscillators find applications in such devices as music synthesizers, FAX modems, doorbells, beepers, sirens and alarms, and electronic toys.
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Feedback can be in phase or out of phase. For a circuit to oscillate, the feedback must be in phase, or positive. Negative feedback (out of phase) simply reduces the gain. The output of a common-emitter or common-source amplifier is out of phase from the input. If you couple the collector to the base through a capacitor, you won t get oscillation. The output of a common-base or common-gate amplifier is in phase with the input. But these circuits have limited gain. It s hard to make them oscillate, even with positive feedback. Common-collector and common-drain circuits don t have enough gain to make oscillators. But take heart: There are lots of ways to make circuits oscillate. Obtaining oscillation has never been a problem in electronics. Public-address systems do it willingly enough!
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Concept of the oscillator
For a circuit to oscillate, the gain must be high, the feedback must be positive, and the coupling from output to input must be good. The feedback path must be easy for a signal to follow. The phase of a fed-back signal can be reversed without any trouble, so that common-emitter or common-source amplifiers can be made to oscillate.
Feedback at a single frequency
Recalling the public-address fiasco, some variation of which you ve doubtless heard many times, could you know in advance whether the feedback would have a low pitch, a midrange pitch, or a high pitch No. The oscillation was not intended, and might have started at any audio frequency. The frequency of an oscillator is controlled by means of tuned, or resonant, circuits. These are usually inductance-capacitance (LC) or resistance-capacitance (RC) combinations. The LC scheme is common at RF; the RC method is more often used for audio oscillators. The tuned circuit makes the feedback path easy for a signal to follow at one frequency, but hard to follow at all other frequencies (Fig. 25-1). The result is that the oscillation takes place at a predictable and stable frequency, determined by the inductance and capacitance or by the resistance and capacitance.
25-1 Basic concept of the oscillator.
The Hartley circuit 459
The Armstrong oscillator
A common-emitter or common-source amplifier can be made to oscillate by coupling the output back to the input through a transformer that reverses the phase of the fed-back signal. The phase at a transformer output can be inverted by reversing the secondary terminals. The schematic diagram of Fig. 25-2 shows a common-source amplifier whose drain circuit is coupled to the gate circuit via a transformer. In practice, getting oscillation is easy. If the circuit won t oscillate with the transformer secondary hooked up one way, you can just switch the wires.
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