Astable Multivibrator

A multivibrator that generates square waves of its own (i.e. without any external trigger pulse) is known as an 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 DC power.

In such a 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 is 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.

Here we like to describe.

• Collector-coupled Astable multivibrator
• Emitter-coupled Astable multivibrator
• Astable multivibrator using OP-AMP
• 555 timer IC as Astable multivibraotor

Collector - Coupled Astable Multivibrator

Figure (a) shows the circuit of a collector-coupled astable multivibrator using two identical NPN transistors Q₁and Q₂. It is possible to have R_L1 = R_L2 = R_L = R₁ = R₂ = R and C₁ = C₂ = C. In that case, the circuit is known as a symmetrical astable multivibrator. The transistor Q₁ is forward-biased by the V_cc supply through resistor R₂. Similarly the transistor Q₂is forward biased by the V_cc supply through resistor R₁. The output of transistor Q₁is coupled to the input of transistor Q₂through the capacitor C₂. Similarly, the output of transistor Q₂is coupled to the input of transistor Q₁ through the capacitor C₁.

It consists of two common emitter amplifying stages. Each stage provides feedback through a capacitor at the input of the other. Since the amplifying stage introduces a 180^o phase shift and another 180^o 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 transistors can remain permanently out-off or saturated, instead of the circuit has two quasi-stable states (ON and OFF) and it makes periodic transitions between these two states.

The output of an astable multivibrator is available at the collector terminal of either transistor as shown in Figure (a). However, the two outputs are 180^o out of phase with each other. Therefore one of the outputs is said to be the complement of the other.

Let us suppose that

1. When Q₁ is ON, Q₂is OFF
2. When Q₂is ON, Q₁is OFF

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 than the other). it is so because characteristics of no two similar transistors can be exactly alike suppose that Q₁starts conducting before Q₂does. The feedback system is such that Q₁will be very rapidly driven to saturation and Q₂to cut-off. The circuit operation may be explained as follows.

1. Since Q₁is in saturation whole of V_CCdrops across R_L1. Hence V_C1= 0 and point A is at zero or ground potential.
2. Since Q₂is in cut-off i.e. it conducts no current, there is no drop across R_L2. Hence point B is at V_CC.
3. Since A is at 0V C₂starts to charge through R₂towards V_CC.
4. When the voltage across C₂rises sufficiently (i.e. more than 0.7V), it biases Q₂ in the forward direction so that it starts conducting and is soon driven to saturation.
5. V_CC decreases and becomes almost zero when Q₂gets saturated. The potential of point B decreases from V_CC to almost 0V. This potential decrease (negative swing) is applied to the base of Q₁through C₁. Consequently, Q₁is pulled out of saturation and is soon driven to cut-off.
6. Since now point B is at 0V, C₁starts charging through R₁towards the target voltage V_CC.
7. When the voltage of C₁increases sufficiently. Q₁becomes forward-biased and starts conducting. In this way, the whole cycle is repeated.

It is observed that the circuit alternates between a state in which Q₁is ON and Q₂is OFF and the state in which Q₁is OFF and Q₂is ON. This time in each state 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 V_CC.

Emitter - Coupled Astable Multivibrator

Figure (b) 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

1. to vary both the timing capacitor simultaneously
2. to vary the timing resistor subject to the conduction that the values are enough to keep the transistors in saturation
3. to vary V_BBwhich 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, which has a single timing capacitor connected across the emitter. This capacitor can be varied easily.

In order to explain the operation of the circuit, it is necessary that the following conditions must be satisfied.

1. In D.C. conduction i.e. with timing capacitor C removed bias should be so adjusted that both the transistors are in the active region.
2. Under the D.C. conditions, the D.C. loop gain should be less than unity to void the bistable operation.
3. In the active region, the loop gain must be greater than unity at some non-zero frequency.
4. Bias conditions are so adjusted that with capacitor C concerned, during normal operation, transistor C₁operates between cut-off and saturation while transistor C₂operates at the same time between its active region and the off region. This transistor Q₁operates in saturated mode and transistor Q₁operates in saturated mode and transistor Q₂operates in unsaturated mode.

Astable multivibrator using OP-AMP

Figure (c) shows a 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. R₁and R₂provide 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, remains 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.

555 Timer Circuit as a Astable Multivibrator

Figure (d) shows the circuit diagram of an astable multivibrator using a 555 timer.

Astable Operation

The figure shows an astable mode of operation for the 555 timer. Here, the capacitor C will charge through R_A and R_S then discharge through R_S only. The duty cycle may be controlled, therefore, by the values of R_A and R_S.

In order to understand the operation of both astable and monostable multivibrator circuits using a 555 timer, a functional diagram of a 555 timer is shown in Figure (e).