Dictionary of Engineering Terms

Transistor Multivibrator

Multivibrator Circuit. A multivibrator is a form of relaxation oscillator which comprises two stages that are coupled so that the input of one is derived from the output of the other. A multivibrator is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off.

The 3 types of multivibrators: ASTABLE, MONOSTABLE, BISTABLE.

Astable Multivibrator

Graphic of a transistor Astable multivibrator circuit
Dual Transistor Astable Multivibrator

The Astable multivibrator 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. So an Astable multivibrator could either be called a pulse generator or squarewave generator depending on the value of the resistor and capacitors used in the circuit. A related IC used is the 555 Timer configured as an 555 Astable multivibrator.

The common configuration is to make each side of the circuit identical; The collector resistors R1 and R4 are equal to each other. The Base resistors R2 and R3 are equal to each other. The feedback capacitors C1 and C2 are equal to each other. As soon as power is applied one transistor is assumed to be in saturation while the other transistor is in cutoff. The time constants are defined by R2C1 and R3C2 which in turn set the high and low periods of the output pulse.

Monostable Multivibrator

Graphic of a Transistor Monostable multivibrator circuit
Dual Transistor Monostable Multivibrator

The Monostable multivibrator circuit has one stable state; one transistor conducts while the other is cut off. The term One-Shot is also used to describe the circuit operation. An input signal [the Trigger] must be applied to change this condition. After a period of time, determined by the internal RC components [R2 and C1], a Monostable multivibrator circuit will return to its original condition where it remains until the next trigger signal arrives. This example uses two cross-coupled PNP transistors, with the pulse output derived from Q2. The length of the pulse is equal to 0.69 x R2 x C1.

Additional examples include an IC Multivibrator schematic. A related IC used is the 555 Timer configured as an 555 Monostable multivibrator. Another One-shot IC is the 4098 Monostable multivibrator. A Monostable multivibrator is basically a trigger activated pulse generator.

Another example of a monostable multivibrator is a transistor being used for a Pulse Duration Modulator, which also uses a slightly different configuration.

Bistable Multivibrator

Schematic of a transistor Bistable multivibrator circuit
Dual Transistor Bistable Multivibrator

The Bistable multivibrator has two stable states. A Bistable multivibrator remains in one of the stable states until a trigger is applied. The multivibrator 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. A type of Toggle flip flop in which the input [T] toggles the output from one state to the next.

Practical Astable Multivibrator

This circuit generates an FM modulated carrier wave. The audio frequency [AF] input is applied to transformer T1, which is in series with the base return. This input voltage causes the gate length and fundamental frequency to vary. The amount of change in the amplitude of the audio input determines the frequency variation of the signal. The circuit uses a series tuned circuit composed of C and L. No other values are provided, the circuit is only shown to illustrate a practical application for an astable multivibrator using transistors.

Schematic of a transistor Astable multivibrator circuit used as an FM modulator
Astable Multivibrator & Filter Circuit

The first part of the circuit is the Astable Multivibrator with the output taken from the same point as the circuit shown at the top of the page. The third transistor forms the amplifier circuit and tuned circuit.

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