After Studying this section, you should be able to: • Recognise BJT astable multivibrator circuits. • Understand the operation of astable multivibrators. • Calculate. An Astable Multivibrator or a Free Running Multivibrator is the multivibrator which has no stable states,oscillates continuously between two unstable states. A multivibrator is a device that switches between two states. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider. It.

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Define your site main menu. Cancel reply Your email address will not be published. Chaos 22 A single pair of active devices can be used to divide a reference by a large ratio, however, the stability of the technique is poor owing to the variability of the power supply and the circuit elements.

Multivibrator – Wikipedia

By varying VR1 the voltage at the top of both R3 and R4 is varied so that whatever mark to space ratio is used, only the frequency alters, whilst the multivibrattors to space ratio is maintained. A diode D1 clamps the capacitor voltage to 0. Therefore full voltage Vcc appears across Rc1 and voltage across Rc2 will be zero. It is useful to be able to vary the frequency of operation, and multivibdators may be done as shown in Fig.

Module 4.1

A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. Asstable All rights reserved. For this reason Abraham and Bloch called it a multivibrateur. After Studying this section, you should be able to: However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on.


An astable multivibrator can be synchronized to an external chain of pulses.


When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state.

The resistors R1 and R2 must be able to provide base current enough to keep the transistors in saturation. This switching action produces the collector and base waveforms shown in Fig. In the end, only R3 provides the needed input base current. Each time TR2 collector voltage goes high as the transistor turns off, D2 becomes reverse biased, isolating TR2 from the effect of C2 charging. In the bistable multivibrator, both resistive-capacitive networks C 1 -R 2 and C 2 -R 3 in Figure 1 are replaced by resistive networks just resistors or direct coupling.

Time bases scanning generators: Got Something To Say: A Butterworth filter is a signal processing filter that has an extremely flat frequency response Q2 collector voltage is the output of the circuit in contrast to the astable circuitit has a perfect square waveform since the output is not loaded by the capacitor.

AF Sine Wave Oscillators 4. The rapidly increasing collector current through TR2 now causes a voltage drop across R4, and TR2 collector voltage falls, causing the right hand plate of C2 to fall rapidly in potential.

Thus, Set is used to “set” Q1 on, and Reset is used to “reset” it to off multivibratlrs. For example, if Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes Q1 on.


For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers.

How long this takes is half our multivibrator switching time the other half comes from C1. During State 2Q2 base-emitter junction is forward-biased and capacitor C1 is “hooked” to ground. mulhivibrators

A problem with the basic astable circuit is that the capacitor action described above astxble down the rise in voltage as each transistor turns off, producing the curved rising edges to the square wave as can be seen in Fig.

In the monostable multivibrator, one resistive-capacitive network C 2 -R 3 in Figure 1 is replaced by a resistive network just a resistor.

A division ratio of 10, for example, is easy to obtain but not dependable. This allows the astable multivibrator to switch between a high state and a low state on a continuous cycle. It has no stable state but only two quasi-stable states between which it keeps oscillating continuously of its own accord without any external excitation.

The charging current for C2 is now supplied by R5 instead of R6. The voltage at inverting terminal will be greater than the voltage at the non-inverting terminal of the op-amp. Other astabe included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal. This feedback is supplied in such a manner that one transistor is driven to saturation ON state and other to cut-off OFF state.

The pulse width T of a monostable multivibrator is calculated as follows: