The word astable means “unstable,” and, like other unstable devices, an astable rnultivibrator is a (square-wave) oscillator. (A bistable multivibrator, also called a flip-flop, is a digital device with two stable stages; a monostable multivibrator h:> one stable state, and an astable mu1tivibrator has zero stable states.) An ,W;’ multivibrator can be constructed by using an operational amplifier as a ‘-“”age comparator in a circuit. This circuit is an example of a relaxation oscillator, one whose operation depends on the repetetive chargi I! and discharging of a capacitor
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For analysis purposes, let us assume that .the output voltages of the comparator .are equal in magnitude and opposite in polarity VII/a.,” Figure 15-10 shows the voltage across capacitor C and the output waveform produced by the comparst We begin by assuming that the output is at +VII/ •.e- Then, the voltage fed ‘,} the noninverting input.
Notice that o: equals the voltage across the capacitor. The capacitor begins to charge through R toward a final voltage (If+ Vlllllt” However, as soon as the capacitor .voltage reaches a voltage equal to v+, the comparator switches slate. In other words, switching occurs at the point in time where v- = v f = +,BV max’After the comparator switches state, its output is , and the voltage fed back to the noninverting input becomes.
Since the comparator output is now negative, the capacitor begins to discharge through R toward – VmlJX’ But, when that voltage falls to -,BV max, we once again have v+ = v-, and the comparator switches back to +VmlJX’ This cycle repeats continuously, as shown in Figure 15-10, with the result that the output is a square wave that alternates between ±VmlJX volts. It can be shown that the period of the rnultivibrator oscillation.
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