Introduction: We have already studied the working of several sinusoidal oscillators. For certain purposes, we need non-sinusoidal generators, also called relaxation oscillators. A relaxation oscillator generates voltage or current which varies abruptly one or more times in a cycle of operation. Typical examples of non-sinusoidal generators are: multivibrators, Schmitt trigger, UJT oscillators, blocking oscillators, saw-tooth (sweep) voltage generators, saw tooth (sweep) current generators etc.
Multivibrators are used for various purposes such as generation of square waveforms, counting, frequency division, generation of time delays and storage of binary bit of information.
Types of Multivibrators
Multivibrators are of the following three types
- Astable or Free-running Multivibrator: It generates a square wave of known periodic time. It does not have any permanent stable state but has two quasi-stable or temporary states. The circuit changes state continuously from one quasi-stable state to another after a predetermined period of time without the help of any external trigger. The periodic time is determined by circuit time constants and parameters.
- Monostable (or one-shot) Multivibrator: It also generates a square wave but it has only one stable state and another quasi-stable or unstable state. Normally the circuit stays in the stable state. When an external trigger signal is applied, the circuit abruptly changes to quasi-stable and remains in the quasi-stable state for a predetermined period of time and then reverts back to the stable state automatically. The time duration of quasi-stable state is determined by the circuit time constant and parameters and is independent of triggering pulse duration.
- Bistable Multivibrator or Flip-flop: It also generates square wave but it has two stable states. The circuit stays in any stable state say state one indefinitely. Only when a trigger pulse is applied, the circuit changes to the other stable state say state two. Now again the circuit will continue to stay in stable state 2 indefinitely unless a suitable trigger pulse is applied causing it to change over to stable state No.1. The property of the circuit information.
Block Diagram of a Multivibrator:
A multivibrator consists of two amplifiers coupled to each other in such a way as to produce positive feedback as shown in Fig.1. The coupling network may provide a.c. coupling such as through a coupling capacitor or provide dc coupling (direct coupling). In astable multivibrator, both coupling networks CN1 and CN2 provide accoupling using coupling capacitors. Each amplifier provides phase shift of 180° in the midband so that overall phase-shift in the complete loop is 360° or 00, thus providing positive feedback. The circuit, therefore, oscillates provides that the total gain in the loop exceeds unity (one). In the case of monostable multivibrator, one coupling network provides a.c. coupling while the other provides dc coupling. In the case of bistable multivibrator, both coupling network provides dc coupling.
Amplifying device may be bipolar junction transistor (BJT) or FET or an operational amplifier. Two states of a Multivibrator: In a multivibrator at a time only one device is ON (conducting) and the other is OFF (nonconducting). Thus, we have the following two sets of states:
- Device A1 OFF and device A2 ON
- Device A1 ON and device A2 OFF