3-6
Figure 3-3.—Astable Multivibrator.
When an input signal to one amplifier is large enough, the transistor can be driven into cutoff, and its
collector voltage will be almost V
CC. However, when the transistor is driven into saturation, its collector
voltage will be about 0 volts. A circuit that is designed to go quickly from cutoff to saturation will
produce a square or rectangular wave at its output. This principle is used in multivibrators.
Multivibrators are classified according to the number of steady (stable) states of the circuit. A steady
state exists when circuit operation is essentially constant; that is, one transistor remains in conduction and
the other remains cut off until an external signal is applied. The three types of multivibrators are the
ASTABLE, MONOSTABLE, and BISTABLE.
The astable circuit has no stable state. With no external signal applied, the transistors alternately
switch from cutoff to saturation at a frequency determined by the RC time constants of the coupling
circuits.
The monostable circuit has one stable state; one transistor conducts while the other is cut off. A
signal must be applied to change this condition. After a period of time, determined by the internal RC
components, the circuit will return to its original condition where it remains until the next signal arrives.
The bistable multivibrator has two stable states. It remains in one of the stable states until a trigger is
applied. It then FLIPS to the other stable condition and remains there until another trigger is applied. The
multivibrator then changes back (FLOPS) to its first stable state.
Q1. What type circuit is used to produce square or rectangular waves?
Q2. What type of multivibrator does not have a stable state?
Q3. What type of multvibrator has one stable state?
Q4. What type of multivibrator has two stable states?
Astable Multivibrator
An astable multivibrator is also known as a FREE-RUNNING MULTIVIBRATOR. It is called free-
running because it alternates between two different output voltage levels during the time it is on. The
