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

This is repeated until the circuit drives Q1 to saturation and Q2 to cutoff. Although largely superseded by its equivalent op amp or timer IC versions in many applications, it is still a useful and flexible design for square wave and pulse generation. 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 mark to space ratio is maintained.

An astable multivibrator is a multivibrator that does not rest in an unstable state like other multivibrators, but continuously switches between two states. 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. What is an Astable Multivibrator? The circuits shown on this page will operate from a DC supply between 3.


The capacitor discharges through resistor R and charges again to 0. If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. Design and setup an astable multivibrator and calculate the time period and duty cycle of the output waveform.

Because a sudden voltage change on one plate of a capacitor causes the other plate to change by 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.

It is considered below for the transistor Q1. As this plate of the capacitor is also connected to the base of TR2, this transistor will begin to conduct heavily.

When the multivibratogs of C1 right-hand plate Q2 base voltage becomes positive and reaches 0.

BJT Astable Multivibrators

As a result, Q2 gets switched off. They are basically two-stage amplifiers with positive feedback from the output of one amplifier to the input of the other. One has high voltage while the other has low voltage, except during the brief transitions from one state to the other.

The circuit is usually drawn in a symmetric form as a cross-coupled pair. After multivubrators the time, it returns to its stable initial astble. As the charged capacitor discharges and transfers its energy to the second capacitor, the second capacitor recharges and prepares to discharge the energy back multivibratprs the input. Switching of state can be done via Set and Reset terminals connected to the bases. How long this takes is half our multivibrator switching time the other half comes from C1.

Q2 is quickly driven to saturation.


Multivibrator – Wikipedia

Annales de Physique in French. In the end, only R3 provides the needed input base current. Therefore full voltage Vcc appears across Rc1 and voltage across Rc2 will be zero.

This technique was used in early electronic organs, to keep notes of different octaves accurately in tune.

Module 4.1

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 are used in amateur radio equipment to receive and transmit radio signals.

Views Read Edit View history. Q2 begins conducting and this starts the avalanche-like positive feedback process as follows. An astable multivibrator consists of two amplifying stages connected in astale positive feedback loop by two capacitive-resistive coupling networks.


As its left-hand negative plate is connected to Q1 base, a maximum negative voltage – V is applied to Q1 base that keeps Q1 firmly off. 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 multivibrayors Fig 4. Q2 is on and connects the right-hand positive plate of C2 to ground. Now a negative trigger of magnitude V1 is applied to the non-inverting terminal so that the effective signal at this terminal is less than 0.

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