Digital Electronics: Principles, Devices and Applications

362 Digital ElectronicsTrigger PulsesOutput PulseT tTFigure 10.5Retriggerable monostable multivibrator output for repetitive trigger pulses.10.1.4 Astable MultivibratorIn the case of an astable multivibrator, neither of the two states is stable. Both output states are quasistable.The output switches from one state to the other and the circuit functions like a free-runningsquare-wave oscillator. Figure 10.6 shows the basic astable multivibrator circuit. It can be proved that,in this type of circuit, neither of the output states is stable. Both states, LOW as well as HIGH, arequasi-stable. The time periods for which the output remains LOW and HIGH depends upon R 2 C 2 andR 1 C 1 time constants respectively. For R 1 C 1 = R 2 C 2 , the output is a symmetrical square waveform.The circuit functions as follows. Let us assume that transistor Q 2 is initially conducting, that is, theoutput is LOW. Capacitor C 2 in this case charges through R 2 and the conducting transistor from V CC ,and, the moment the Q 1 base potential exceeds its cut-in voltage, it is turned ON. A fall in Q 1 collectorVCCR c1Q 2R1R2R c2C1C2VoQ1Figure 10.6Astable multivibrator.