A multivibrator which generates square waves of its own (i.e without any external trigger pulse) is known as astable multivibrator. It is also called free ramming multivibrator. It has no stable state but only two quasi-stable (half-stable) makes oscillating continuously between these states. Thus it is just an oscillator since it requires no external pulse for its operation of course it does require D.C power.
In such circuit neither of the two transistors reaches a stable state. It switches back and forth from one state to the other, remaining in each state for a time determined by circuit constants. In other words, at first one transistor conducts (i.e. ON state) and the other stays in the OFF state for some time. After this period of time , the second transistor is automatically turned ON and the first transistor turned OFF. Thus the multivibrator will generate a square wave of its own. The width of the square wave and it frequency will depend upon the circuit constants.
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Figure (a) shows the circuit of a collector coupled astable multivibrator using two identical NPN transistors Q1 and Q2. It is possible to have RL1 = RL2 = RL = R1 = R2 = R and C1 = C2 = C. In that case , the circuit is known as symmetrical astable multivibrator. The transistor Q1 is forward biased by the Vcc supply through resistor R2. Similarly the transistor Q2 is forward biased by the Vcc supply through resistor R1. The output of transistor Q1 is coupled to the input of transistor Q2 through the capacitor C2. Similarly the output of transistor Q2 is coupled to the input of transistor Q1 through the capacitor C1.
It consists of two common emitter amplifying stages. Each stage provides a feedback through a capacitor at the input of the other . Since the amplifying stage introduces a 180o phase shift and another 180o phase shift is introduced by a capacitor , therefore the feedback signal and the circuit works as an oscillator. In other words because of capacitive coupling none of the transistor can remain permanently out-off or saturated, instead of circuit has two quasi-stable states (ON and OFF) and it makes periodic transition between these two states.
The output of an astable multivibrator is available at the collector terminal of the either transistors as shown in figure (a). However, the two outputs are 180o out of phase with each other. Therefore one of the output is said to be the complement of the other.
Let us suppose that
When the D.C power supply is switched ON by closing S, one of the transistors will start conducting before the other (or slightly faster then the other). it is so because characteristics of no two similar transistors can be exactly alike suppose that Q1 starts conducting before Q2 does. The feedback system is such that Q1 will be very rapidly driven ton saturation and Q2 to cut-off. The circuit operation may be explained as follows.
It is observed that the circuit alternates between a state in which Q1 is ON and Q2 is OFF and the state in which Q1 is OFF and Q2 is ON. This time in each states depends on RC values. Since each transistor is driven alternately into saturation and cut-off. The voltage waveform at either collector (points A and B in figure (b)) is essentially a square waveform with a peak amplitude equal to VCC.
Figure (c) shows the circuit diagram of an emitter coupled asable multivibrator.
In a collector coupled symmetrical astable multivibrator if it is desired to vary the frequency., then it is necessary
(i) to vary both the timing capacitor simultaneously
(ii) to vary both the timing resistor subject ot the conduction that the values are enough to keep the transistors in saturation
(iii) to vary VBB which also can not be varied over a long range. Thus it is difficult to achieve frequency control in collector coupled astable multivibrator, not an emitter coupled multivibrator, to be described here, has a single timing capacitor connected across the emitter. This capacitor can be varied easily.
In order to explain the operation of the circuit, ti is necessary that the following conditions must be satisfied.
Figure (d) shows differential input operational amplifier acting as a free running symmetrical multivibrator.
The two states of the circuit between which it switches are those in which the amplifier output is at positive and negative saturation. R1 and R2 provide a fixed level of positive feedback and R and C provide a frequency dependent level of negative feedback. At high frequencies the negative feedback is reduced and the circuit becomes unstable. The circuit can not hang up in either output stage and is self starting.
As terminal B is positive with respect to terminal A and its potential is decreasing as C charges down through R. When the potential difference between the two input terminals approached zero the amplifier comes out of saturation. The positive feedback from the output to terminal A causes a regenerative switching which drives the amplifier to positive saturation. The voltage across a capacitor in series with a resistor can not change instantaneously, the potential at the terminal B, therefore, remain substantially constant during this rapid transition. Capacitor C now charges up through R and the potential at C rises exponentially, when it reaches βV01 (sat) the circuit switches back to the state in which the amplifier is in negative saturation.
Figure (d) shows the circuit diagram of a astable multivibrator using a 555 timer.
Figure shows an astable mode of operation for 555 timer. Here, the capacitor C will charge through RA and RS then discharge through RS only. The duty cycle may be controlled, therefore, by the values of RA and RS.
In order to understand the operation of both astable and monostable multivibrator circuit using 555 timer, functional diagram of 555 timer is shown in figure (e).