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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This allows the astable multivibrator to switch between a high state and a low state on a continuous cycle. Hons All rights reserved. Switching of state can be done via Set and Reset terminals connected to the bases. A diode D1 clamps the capacitor voltage to 0. Thus the initial input change circulates along the feedback loop and grows in an avalanche-like manner until finally Q1 switches off and Q2 switches on.

Chains of bistable flip-flops provide more predictable division, at the cost of more active elements. As this plate of the capacitor is also connected to the asrable of TR2, this transistor will begin to conduct heavily.

In the monostable configuration, only one of the transistors requires protection. Related Posts Butterworth Filter. This causes the collector current to start in Q2. Since it produced astale square wavein contrast to the sine wave generated by most other oscillator circuits of the time, its output contained many harmonics above the fundamental frequency, which could be used for calibrating high frequency radio circuits.

This establishes a reverse bias on Q2 and its collector current starts decreasing. 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.


Because a sudden voltage change on one plate of a capacitor causes the other plate to change multvibrators a similar amount, this sudden rise at TR1 collector is transmitted via C1 to TR2 base causing TR2 to rapidly turn on as TR1 turns off. What is an Astable Multivibrator?

An astable multivibrator consists mulfivibrators two amplifying stages connected in a positive feedback loop by two capacitive-resistive coupling networks. A change of state has occurred at both outputs.

What is an Astable Multivibrator?

The output of transistor Q1 is coupled to the input of Q2 through C1 while the Q2 is fed to the input of Q1 through C2. It is also called free-running relaxation oscillator.

Figure 1, below right, shows bipolar junction transistors.

When the voltage of C1 right-hand plate Q2 base voltage becomes positive and reaches 0. This can occur at startup without external intervention, if R and C are both very small. For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. Simultaneously, C1 that is fully discharged and even slightly charged to 0. This new state does not last however.

The capacitor discharges through resistor R and charges again to 0. During State 2Q2 base-emitter junction is forward-biased and capacitor C1 is “hooked” to ground.

The circuit is useful for generating single output pulse of adjustable time duration in response to a triggering signal. Now, the capacitor C2 is fully charged in the previous State 1 to the power supply voltage V with the polarity shown in Figure 1. After Studying this section, you should be able to: AF Sine Wave Oscillators 4. In the bistable multivibrator, both resistive-capacitive networks C 1 -R 2 and Multkvibrators 2 -R 3 in Figure 1 are replaced by resistive networks just resistors or direct coupling.


When Vcc is applied, collector current start flowing in Q1 and Q2. When the circuit is first powered up, neither transistor will be switched on. From Wikipedia, the free encyclopedia. The circuit produces two anti-phase square wave signals, with an amplitude almost equal to its supply voltage, at its two transistor collectors as shown in Fig 4.

Multivibrator – Wikipedia

Hence, when the mjltivibrators is switched on, if Q1 is on, its collector is at 0 V. The width of the output pulse depends only on external components connected to the op-amp. This is due to resistance within the circuit, lack of a completely closed loop at the output terminals, and the tendency for one multivibdators or transistor to absorb energy at a slightly different rate than the other.

Retrieved 1 Januaryfrom vlab. If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune.

Module 4.1

Beause TR2 is turned off at this time, its collector will be at supply voltage and its base will be at almost zero potential, the same as TR1 collector, because C1 is still un-charged and its two plates are at the same potential.

This repeats and forms a free-running oscillator or an astable multivibrator. Time bases scanning generators: Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

Similarly, Q2 remains on continuously, if it happens to get switched on first.