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 feedback is supplied in such a manner that one transistor is driven to saturation ON state and other to cut-off OFF state.

Astable multivibrators switch between two states without stopping by using their output signal to recharge the input signal.

This rapidly turns off TR1 causing a rapid rise in its collector voltage. January Learn how and when to remove this template message. Related Posts Butterworth Filter. As the collector of Q2 is connected to the base of Q1 through C2, therefore the base of Q1 becomes more negative i. Both will not be in same state at the same time. The circuit operation is based on the fact that the forward-biased base-emitter junction of the switched-on bipolar transistor can provide a path for the capacitor restoration.

Leave this field empty. Got Something To Say: While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the V eb breakdown voltage, typically around volts for general purpose silicon transistors. This works via a transistor or amplifier that amplifies the output signal and forwards the charge to the input.


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. Thus, the circuit remains stable in a single state continuously.

Therefore a 5V supply can be recommended for greater reliablity. One has high voltage while the other has low voltage, except during the brief transitions from one state to the other. Your email address will not be published. As the charged capacitor discharges and transfers its energy to the second capacitor, the second capacitor recharges and prepares to discharge the energy back to the input.

This latch circuit is similar to an astable multivibrator, except that there is no charge or discharge time, due to the absence of capacitors. A problem with the basic astable circuit is that the capacitor mulhivibrators described above slows 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.

Because they do not need to be the same, an asymmetric duty cycle is easily achieved. The circuit essentially consists of two symmetrical CE amplifier multjvibrators, each providing a feedback to another.

They are basically two-stage amplifiers with positive feedback from the output of one amplifier to the input of the other.


So, the time period of the square wave generated at the output is:. Learn about electronics Oscillators. This new state does not last however. The output voltage of the switched-off transistor Q1 changes exponentially from low to high since this relatively high resistive output multivibratogs loaded by a low impedance load capacitor C1.

Multivibrator – Wikipedia

Mail this article Print this article. The action is reversed after a certain time, depending upon the circuit conditions i.

The two output terminals can be defined at the active devices and have complementary states. Thus C1 restores its charge and prepares for the next State 1 when it will act again as a time-setting capacitor As Q2 base-emitter junction is reverse-biased, it does not conduct, so all the current from R2 goes into C1.

The voltage at inverting terminal will be greater than the voltage at the non-inverting terminal of the op-amp.


This establishes a reverse bias on Q2 and its collector current starts aastable. 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. In each state, one transistor is switched on and the other is switched off.

The period of time during which Q2 remains at saturation and Q1 at cutoff is determined by C2 and R2.