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Multivibrators (MV)

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems

such as oscillators, timers and flip-flops. It is characterized by two amplifying devices

(transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The namemultivibrator was initially applied to the free-running oscillator version of the circuit because

its output waveform was rich in harmonics. There are three types of multivibrator circuits

depending on the circuit operation!

Astable! in which the circuit is not stable in either state "it continually switches from

one state to the other#

Monostable! in which one of the states is stable, but the other state is unstable#

Bistable! in which the circuit is stable in either state

1. Astable Multivibrator (AMV)

It is also called free-running relaxation oscillator. It has no stable state but only two $uasi

stable (half-stable) states between which it %eeps oscillating continuously of its own accord

without any e&ternal e&citation.

In this circuit, neither of the two transistors reaches a stable state. 'hen one is , the other is

** and they continuously switch bac% and forth at a rate depending on the RC time constant in

the circuit. +ence, it oscillates and produces pulses of certain mar%-to-space ratio. oreover, two

outputs (/0 out of phase with each other) are available. It has two energy-storing elements i.e.two capacitors.

The figure below shows the circuit of a symmetrical collector-coupled A1 using two similar

transistors.

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It, in fact, consists of two CE amplifier stages, each providing a feedbac% to the other. The

feedbac% ratio is unity and positive because of /0 phase shift in each stage. +ence, the circuit

oscillates. 2ecause of the very strong feedbac% signal, the transistors are driven either to

saturation or to cut-off (they do not wor% on the linear region of their characteristics).

The transistor Q is forward-biased by VCC andR whereas Q3 is forward-biased by VCCandR3. The collector-emitter voltages of Q and Q3 are determined respectively byRL andRL3

together with VCC. The output of Q is coupled to the input of Q3 by C3 whereas output of Q3

is coupled to Q by C.

The output can be ta%en either from point A or B though these would be phase-reversed with

respect to each.

Circuit Operation

The circuit operation would be easy to understand if it is remembered that due to feedbac%

(i) 'hen Q is , Q3 is **# and(ii) 'hen Q3 is , Q is **.

'hen the power is switched on by closing S, one of the transistors will start conducting before

the other does (or slightly faster than the other). It is so because characteristics of no two

seemingly similar transistors can be e&actly ali%e. 4uppose that Q starts conducting before Q3

does. The feedbac% system is such that Q will be very rapidly driven to saturation and 53 to

cut-off.

The following se$uence of events will occur!

1. 4ince 5 is in saturation, whole of V66 drops acrossRL . +ence, V6 7 / and pointA is atzero or ground potential.

2. 4ince Q3 is in cut-off i.e. it conducts no current, there is no drop acrossR L3. +ence, pointB

is at V66.

3. 4inceA is at / 1, 63 starts to charge throughR3 towards VCC.

4. 'hen voltage across C3 rises sufficiently (i.e. more than /.8 1), it biases Q3 in the forward

direction so that it starts conducting and is soon driven to saturation.

5. VC3 decreases and becomes almost zero when Q3 gets saturated. The potential of point B

decreases from VCC to almost / 1. This potential decrease (negative swing) is applied to thebase of Q through C. 6onse$uently, Q is pulled out of saturation and is soon driven to cut-

off.

6. 4ince, now, pointB is at / 1, C starts charging throughR towards the target voltage VCC.

7. 'hen voltage of C1 increases sufficiently, Q1becomes forward-biased and starts conducting.

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In this way, the whole cycle is repeated. It is seen that the circuit alternates between a state in

which Q is and Q3 is ** and a state in which Q is ** and Q3 is . The time in each

state depends onRC values. 4ince each transistor is driven alternately into saturation and cut-off

the voltage wavefrom at either collector (pointsA andB in *ig. above) is essentially a s$uare

waveform with a pea% amplitude e$ual to VCC(figure below).

Switcin! "i#es

It can be proved that off-time for Q is T 7 /.9:RC and that for Q3 is T3 7 /.9:R3C3.

+ence, total time-period of the wave is T = T ; T37 /.9: (RC ;R3 C3)

IfR 7R3 7R and C 7 C3 7 C i.e. the two stages are symmetrical, then T 7 .


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2. Monostable Multivibrator (MMV)

It is also called a sin!lesot or sin!le swin! or a onesot multivibrator.

It has (i) ne absolutely stable (stand-by) state# and

(ii) ne $uasi stable state.

It can be switched to the $uasi-stable state by an e&ternal trigger pulse but it returns to the stable

condition after a time delay determined by the value of circuit components. It supplies a single

output pulse of a desired duration for every input trigger pulse. It has one energy-storing element

i.e. onecapacitor

A typical MMV circuit is shown in *ig. below. +ere, Q is coupled to Q3 base. In this

multivibrator, a single narrow input trigger pulse produces single rectangular pulse whose

amplitude, pulse width and wave shape depend upon the values of circuit components rather than

upon the trigger pulse.

*nitial Con+ition

In the absence of a triggering pulse at C3 and with S closed,

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1. VCCprovides reverse bias for C/B>unctions of Q and Q3 but forward-bias forE/B>unction

of Q3 only. +ence, Q3 conducts at saturation.

2. VBB andR< reverse bias Q and %eep it cut off.

3. C charges to nearly VCC through RL to ground by the low-resistance path provided by

saturated Q3.

As seen, the initial stable state is represented by

(i) Q3 conducting at saturation and (ii) Q cut-off

,en "ri!!er -ulse is Applie+

'hen a trigger pulse is applied to Q through C3,MMV will switch to its opposite unstable state

where Q3 is cut-off and Q conducts at saturation. The chain of circuit actions is as under!

1. If positive trigger pulse is of sufficient amplitude, it will override the reverse bias of the E/B

>unction of Q and give it a forward bias. +ence, Q will start conducting.

2. As Q conducts, its collector voltage falls due to voltage drop across R?. It means that

potential of pointA falls (negative-going signal). This negative-going voltage is fed to Q3 via C

where it decreases its forward bias.

3. As collector current of Q starts decreasing, potential of point B increases (positive-going

signal) due to lesser drop over @L3. 4oon, Q3 comes out of conduction.

4. The positive-going signal at B is fed via R to the base of Q where it increases its forward

bias further. As Q conducts more, potential of pointA approaches / 1.

5. This action is cumulative and ends with Q conducting at saturation and Q3 cut-off.

eturn to *nitial Stable State

1. As point A is at almost / 1, C starts to discharge through saturated Q to ground.

2. As C discharges, the negative potential at the base of Q3 is decreased. As C discharges

further, Q3 is pulled out of cut-off.

3. As Q3 conducts further, a negative-going signal from pointB via R drives Q into cut-off.

+ence, the circuit reverts to its original state with Q3 conducting at saturation and Q cut-off. It

remains in this state till another trigger pulse comes along when the entire cycle repeats itself.

As shown in *ig. 9.


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this 1 produces one output pulse for every input trigger pulse it receives, it is called mono or

one-shot multivibrator.

The width or duration of the pulse is given by T 7 /.9: CR3

It is also %nown as the one-shot period.

3. Bistable Multivibrator (BMV)

It is also called flip-flop multivibrator. It has two absolutel& stable states. It can remain in either

of these two states unless an e&ternal trigger pulse switches it from one state to the other.

bviously, it does not oscillate. It has no ener!& stora!e ele#ent.

1. The base resistors are not >oined to VCCbut to a common sourceBVBB,

2. The feedbac% is coupled through two resistors (not capacitors).

Circuit Action

If Q is conducting, then the fact that pointA is at nearly / 1 ma%es the base of Q3 negative (by

the potential dividerR3 BRC) and holds Q3 off.

4imilarly, with Q3 **, the potential divider from VCC to BVBB (R?3,R, R


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4uppose, now, a positive pulse is applied momentarily to R, it will cause Q3 to conduct. As

collector of Q3 falls to zero, it cuts Q ** and, conse$uently, theBMV switches over to its

other state.

4imilarly, a positive trigger pulse applied to S will switch theBMVbac% to its original state.

SA"/A"*O0 C/"O$$

I6is ma&imum

1c7Ic&(@c ;@D)

16D7/

I2is greater than zero

The transistor is wor%ing li%e a -4witch

I2is roughly zero

I6is also proportional to I2

16Dis ma&imum 7 166

The transistor is wor%ing li%e an **-4witch

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