Astable and Bistable Multivibrators

Multivibrators

Introduction

The type of circuit most often used to generate square or rectangular waves is the multivibrator. A multivibrator, is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off When an input signal to one amplifier is large enough, the transistor can be driven into cutoff, and its collector voltage will be almost V CC. However, when the transistor is driven into saturation, its collector voltage will be about 0 volts.

A circuit that is designed to go quickly from cutoff to saturation will produce a square or rectangular wave at its output. This principle is used in multivibrators. Multivibrators are classified according to the number of steady (stable) states of the circuit. A steady state exists when circuit operation is essentially constant; that is, one transistor remains in conduction and the other remains cut off until an external signal is applied.

The three types of multivibrators :

ü   ASTABLE

ü   MONOSTABLE

ü   BISTABLE.

The astable circuit has no stable state. With no external signal applied, the transistors alternately switch from cutoff to saturation at a frequency determined by the RC time constants of the coupling circuits.

The monostable circuit has one stable state; one transistor conducts while the other is cut off. A signal must be applied to change this condition. After a period of time, determined by the internal RC components, the circuit will return to its original condition where it remains until the next signal arrives.

The bistable multivibrator has two stable states. It remains in one of the stable states until a trigger is applied. It then FLIPS to the other stable condition and remains there until another trigger is applied. The multivibrator then changes back (FLOPS) to its first stable state.

1. Astable Multivibrator

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-stables (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.

Here we like to describe.

ü   Collector - coupled Astabe multivibrator

ü   Emitter - coupled Astable multivibrator

Figure (a) shows the circuit of a collector coupled astable multivibrator using two identical NPN transistors Q₁ and 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 symmetrical astable multivibrator. The transistor Q₁ is forward biased by the V_cc supply through resistor R₂. Similarly the transistor Q₂ is forward biased by the V_cc 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 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 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 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

When Q₁is ON, Q₂ is OFF and

When Q₂ is ON, Q₁ is 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 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.

Since Q₁ is in saturation whole of V_CC drops across R_L1. Hence V_C1 = 0 and point A is at zero or ground potential. Since Q₂ is in cut-off i.e. it conducts no current, there is no drop across R _L2. Hence point B is at V_CC. Since A is at 0V C₂ starts to charge through R₂ towards V_CC.

When voltage across C₂ rises 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.

V_CC decreases and becomes almost zero when Q₂ gets saturated. The potential of point B decreases from V_CC to almost 0V. This potential decrease (negative swing) is applied to the base of Q₁ through C₁. Consequently, Q₁ is pulled out of saturation and is soon driven to cut-off.

Since, now point B is at 0V, C₁ starts charging through R₁ towards the target voltage V_CC.

When voltage of C1 increases sufficiently. Q₁ becomes 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 Q₁ is ON and Q₂ is OFF and the state in which Q₁ is OFF and Q₂ is 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 peak amplitude equal to V_CC.

Calculation of switching times and frequency of oscillations:

The frequency of oscillations can be calculated by charging and discharging capacitances and its base resistance R_B.

The voltage across the capacitor can be written as

V_i= intial voltage = V_B =-V_CC thus the transistors enters from ON to OFF state

V_f = final voltage = V_B = -V_CC then the resistor enters from OFF to ON state

T₁ is ON & T₂ is OFF  the above equation can be written as

substitute  at t=T₁, V_B1=0 hence this equation becomes

T₁=.69R_B2C₂

The total time period T=.694(R_B1C₁+R_B2C₂)

When R_B!=R_B2=R & C₁=C₂=C

T=1.39RC

Frequency of free running multivibrator is given by

the frequency stability of the circuit is not good as only the function of the product of RC but also depends on load resistances, supply voltages and circuit parameters. In order to stabilize the frequency, synchronizing signals are injected which terminate the unstable periods earlier than would occur naturally.

2. Bistable multivibrator

The bistable multivibrator has two absolutely stable states. It will remain in whichever state it happens to be until a trigger pulse causes it to switch to the other state. For instance, suppose at any particular instant, transistor Q₁ is conducting and transistor Q ₂ is at cut-off. If left to itself, the bistable multivibrator will stay in this position for ever. However, if an external pulse is applied to the circuit in such a way that Q₁ is cut-off and Q₂ is turned on, the circuit will stay in the new position. Another trigger pulse is then required to switch the circuit back to its original state.

In other words a multivibrator which has both the state stable is called a bistable multivibrator. It is also called flip-flop, trigger circuit or binary. The output pulse is obtained when, and why a driving (triggering) pulse is applied to the input. A full cycle of output is produced for every two triggering pulses of correct polarity and amplitude.

Figure (a) shows the circuit of a bistable multivibrator using two NPN transistors. Here the output of a transistor Q₂ is coupled put of a transistor Q₁ through a resistor R₂. Similarly, the output of a transistor Q₁ is coupled to the base of transistor Q₂ through a resistor R₁. The capacitors C ₂ and C₁ are known as speed up capacitors. Their function is to increase the speed of the circuit in making abrupt transition from one stable state to another stable state. The base resistors (R₃ and R₄) of both the transistors are connected to a common source (-V_BB). The output of a bistable multivibrator is available at the collector terminal of the both the transistor Q₁ and Q. However, the two outputs are the complements of each other.

Let us suppose, if Q₁ is conducting, then the fact that point A is at nearly ON makes the base of Q₂ negative (by the potential divider R₂ - R₄) and holds Q₂ off.

Similarly with Q₂ OFF, the potential divider from V_CC to -V_BB (R_L2, R₁, R₃) is designed to keep base of Q₁ at about 0.7V ensuring that Q₁ conducts. It is seen that Q₁ holds Q₂ OFF and Q₂ hold Q₁ ON. Suppose, now a positive pulse is applied momentarily to R. It will cause Q₂ to conduct. As collector of Q₂ falls to zero, it cuts Q₁ OFF and consequently, the BMV switches over to its other state.

Similarly, a positive trigger pulse applied to S will switch the BMV back to its original state.

Uses:

ü   In timing circuits as frequency divider

ü   In counting circuits

ü   In computer memory circuits

Bistable Multivibrator Triggering

To change the stable state of the binary it is necessary to apply an appropriate pulse in the circuit, which will try to bring both the transistors to active region and the resulting regenerative feedback will result on the change of state.

Triggering may be of two following types:

Asymmetrical triggering

Symmetrical triggering

(I) Asymmetrical triggering

In asymmetrical triggering, there are two trigger inputs for the transistors Q₁ and Q₂. Each trigger input is derived from a separate triggering source. To induce transition among the stable states, let us say that initially the trigger is applied to the bistable. For the next transition, now the identical trigger must appear at the transistor Q₂. Thus it can be said that the asymmetrical triggering the trigger pulses derived from two separate source and connected to the two transistors Q₁ and Q₂ individually, sequentially change the state of the bistable.

Figure (b) shows the circuit diagram of an asymmetrically triggered bistable multivibrator.

Initially Q₁ is OFF and transistor Q₂ is ON. The first pulse derived from the trigger source A, applied to the terminal turn it OFF by bringing it from saturation region to active transistor Q₁ is ON and transistor Q₂ is OFF. Any further pulse next time then the trigger pulse is applied at the terminal B, the change of stable state will result with transistor Q₂ On and transistor Q₁ OFF.

Asymmetrical triggering finds its application in the generation of a gate waveform, the duration of which is controlled by any two independent events occurring at different time instants. Thus measurement of time interval is facilitated.

(II) symmetrical triggering

There are various symmetrical triggering methods called symmetrical collector triggering, symmetrical base triggering and symmetrical hybrid triggering. Here we would liked to explain only symmetrical base triggering (positive pulse) only as given under symmetrical Base Triggering.

Figure (c) shows the circuit diagram of a binary with symmetrical base triggering applying a positive trigger pulses.

Diodes D₁ and D₂ are steering diodes. Here the positive pulses, try to turn ON and OFF transistor. Thus when transistor Q₁ is OFF and transistor Q₂ is ON, the respective base voltages and V_{B1N, OFF} and V _{B2N, ON}. It will be seen that V_{B1N, OFF} > V_{B1N, ON}. Thus diode D₂ is more reverse-biased compared to diode D₁.

When the positive differentiated pulse of amplitude greater than (V_{B1N, OFF} + Vɣ) appears, the diode D₁ gets forward biased, and transistor Q₁ enters the active region and with subsequent regenerative feedback Q₁ gets ON, and transistor Q₂ becomes OFF. On the arrival of the next trigger pulse now the diode D₂ will be forward biased and ultimately with regenerative feedback it will be in the ON state.