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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Thus C1 restores its charge and prepares for the next State 1 when it will act again as a time-setting capacitor This new state does not last however. There are three basic types of multivibrators. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

Now the capacitor discharges towards -V sat. Please help improve this section by adding citations to reliable sources. When Vcc is applied, collector multivinrators start flowing in Q1 and Q2.

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. AF Sine Wave Oscillators 4.

Multivibrator – Wikipedia

Bistable multivibrator Astable multivibrator It is also called free-running relaxation oscillator. One has high voltage while the other has low voltage, except during the brief transitions from one state to the other. It is triggered by zero or negative input signal applied to Q2 base with the same success it can be triggered by applying a positive input signal through a resistor to Q1 base.


Astable multivibrators switch between two states without stopping by using their output signal to recharge the input signal. The rapidly increasing collector current through TR2 now causes a voltage drop across R4, and TR2 collector voltage falls, causing astabble right hand plate of C2 to fall rapidly in potential.


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.

Multivibrahors circuit keeps on changing state in this manner producing a square wave at each collector.

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. Operational Amplifiers, 2nd Ed. The forward-biased Q2 base-emitter junction multivibratkrs the voltage of C1 right-hand plate at 0.

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. The charges developed across C1 and C2 are sufficient to maintain the saturation and cutoff conditions at Q1 and Q2 respectively.

Thus collector loads of the two stages are equal i.

The diode D1 clamps the capacitor to 0. Therefore a 5V supply can be recommended for greater reliablity. Simultaneously, C1 that is fully discharged and even slightly charged to 0. The circuit has two astable unstable states that change alternatively with maximum transition rate because of the “accelerating” positive feedback. Thus, Set is used to “set” Q1 on, and Reset is used to “reset” it to multivivrators state. Although the amplifier restores the lost energy when it amplifies the signal, the signal will eventually be too small to be of multivibartors use.


This causes the collector current to start in Q2. Electronic oscillators Digital circuits Analog circuits.

The capacitor discharges through resistor R and charges again to 0. For this reason Abraham and Bloch called it a multivibrateur. The circuit is useful for generating single output pulse multlvibrators adjustable time duration in response to a triggering signal. During State 1Q2 base-emitter junction is reverse-biased and capacitor C1 is “unhooked” from ground.

Thus, the circuit remains stable in a single state continuously. The output voltage of the switched-on transistor Q1 changes rapidly astqble high to low since this low-resistive output is loaded by a high impedance load the series connected capacitor C1 and the high-resistive base resistor R2.

Ashable 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. The frequency of oscillation can be calculated, as the time for the relevant capacitor to charge sufficiently for a change of state to take place, will be approximately 0.

This multivibratoors due to resistance within the circuit, lack of a completely closed loop at the output terminals, and the tendency for one capacitor or transistor to absorb energy at a slightly different rate than the other.