UNIT - 6.pdf

UNIT-6
MULTIVIBRATORS
Dr. Arun Kumar Singh
Assistant Professor, ECE
Electronics and Communication Engineering Department
PEC University of Technology, Sector-12, Chandigarh

Transistor as a Switch
• Transistor is a semiconductor device used for
switching and amplification of weak signals.
• At present, transistors are applied in most of the
electronic equipments for switching purposes. Digital
ICs, Microcontrollers, etc uses thousands of
embedded transistor for switching.
• The huge electrical networks are also switched by
simple transistor switching circuit.

Working principle of transistor switch
• Consider the first figure in which base terminal of the npn
transistor is closed then the transistor is said to be in ON state
(similar to a short circuit between Vcc
and Ground) so the
collector voltage is very low (0.02V approx)
• Whole current from Vcc
will flows through the transistor, no
current flow through the LED because current chooses low
resistance path.

Cont....
• Therefor LED connected at the Collector is in OFF state since
the voltage at the anode of LED is 0.02V.
• Consider the second figure, When the voltage at the base
terminal removed (open circuit), the transistor become OFF
(means an open circuit between Vcc and Ground) then its
collector voltage will be Vcc
(Supply voltage)
• Since the transistor is in OFF state, the whole current will
flows trough the LED, Then the LED glows

MOSFET AS A SWITCH
• Introduction:
• The MOSFET (Metal Oxide Semiconductor Field Effect
Transistor) transistor is a semiconductor device which is
widely used for switching and amplifying electronic signals in
the electronic devices.
• The MOSFET is a core of integrated circuit and it can be
designed and fabricated in a single chip because of these very
small sizes.
• The MOSFET is a four terminal device with source(S), gate
(G), drain (D) and body (B) terminals.

Cont....
• Types of MOSFET:
• N-Channel – For an N-Channel MOSFET, the source is
connected to ground. To turn the MOSFET on, we need
to raise the voltage on the gate. To turn it off we need to
connect the gate to ground.
• P-Channel – The source is connected to the power rail
(Vcc
). In order to allow current to flow the Gate needs to
be pulled to ground. To turn it off the gate needs to be
pulled to Vcc
.

Cont....
• The MOSFET can be function in two ways
1) Deflection Mode
2) Enhancement Mode

Cont....
• Deflection Mode:
• It requires the Gate-Source voltage ( Vgs
) applied to
switch the device “OFF”.

Cont....
• Enhancement Mode
• The transistor requires a Gate-Source voltage ( Vgs
)
applied to switch the device “ON”

Working of MOSFET
• The aim of the MOSTFET is to be able to control the voltage
and current flow between the source and drain.
• It works almost as a switch.
• The working of MOSFET depends upon the MOS capacitor.
The MOS capacitor is the main part of MOSFET.
• The semiconductor surface at the below oxide layer which is
located between source and drain terminal.
• It can be inverted from p-type to n-type by applying a positive
or negative gate voltages respectively.

Working of MOSFET
• When we apply the positive gate voltage the holes present
under the oxide layer with a repulsive force and holes are
pushed downward with the substrate.
• The deflection region populated by the bound negative
charges which are associated with the acceptor atoms.
• The electrons reach channel is formed. The positive voltage
also attracts electrons from the n+ source and drain regions
into the channel.
• Now, if a voltage is applied between the drain and source, the
current flows freely between the source and drain and the
gate voltage controls the electrons in the channel.
• Instead of positive voltage if we apply negative voltage , a
hole channel will be formed under the oxide layer.

Cont....
• Explanation:
• In this circuit arrangement an enhanced mode and N-channel
MOSFET is being used to switch a sample lamp ON and OFF.
• The positive gate voltage is applied to the base of the
transistor and the lamp is ON (VGS
=+v) or at zero voltage level
the device turns off (VGS
=0).
• If the resistive load of the lamp was to be replaced by an
inductive load and connected to the relay or diode which is
protect to the load. In the above circuit, it is a very simple
circuit for switching a resistive load such as lamp or LED.

Cont....
• But when using MOSFET to switch either inductive load or
capacitive load protection is required to contain the MOSFET
device. We are not giving the protection the MOSFET device is
damage.
• For the MOSFET to operate as an analog switching device, it
needs to be switched between its cutoff region where VGS
=0
and saturation region where VGS
=+v.

Unsymmetrical/symmetrical Triggering
• A pulse that initiates the action of another component is
called Triggering
• Triggering may be of two following types:
I. Asymmetrical triggering
II. Symmetrical triggering.

Asymmetrical triggering
• In asymmetrical triggering, there are two trigger inputs for the
transistors Q and Q .
• 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 Q .
• Thus it can be said that the asymmetrical triggering the trigger
pulses derived from two separate source and connected to
the two transistors Q and Q individually, sequentially change
the state of the bistable.

Symmetrical triggering
• There are various symmetrical triggering methods called
symmetrical collector triggering, symmetrical base triggering
and symmetrical hybrid triggering.
• When the positive differentiated pulse of amplitude greater ,
the diode D gets forward biased, and transistor Q enters the
active region and with subsequent regenerative feedback Q
gets ON, and transistor Q becomes OFF.
• On the arrival of the next trigger pulse now the diode D will
be forward biased and ultimately with regenerative feedback
it will be in the ON state.

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 name "multivibrator" was initially applied to the
free-running oscillator version of the circuit because its
output waveform was rich in harmonics.

Types of Multivibrators
•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

Contd.
Symmetrical collector-coupled AMV

Astable Multivibrator (AMV)
•In this circuit, neither of the two transistors reaches a
stable state.
• When one is ON, the other is OFF and they continuously
switch back and forth at a rate depending on the RC
time constant in the circuit.
•Hence, it oscillates and produces pulses of certain
mark-to-space ratio. Moreover, two outputs (180° out of
phase with each other) are available. It has two
energy-storing elements i.e. two capacitors.

Contd.
Circuit Operation
The circuit operation would be easy to understand if it
is remembered that due to feedback
(i) When Q1 is ON, Q2 is OFF; and
(ii) When Q2 is ON, Q1 is OFF.

Cont....
Voltage Levels at collectors Q1 and Q2

CIRCUIT OPERATION
• Since Q1
is in saturation, whole of VCC
drops across
RL1
. Hence, VC1
= 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 VCC
.
• Since A is at 0 V, C2
starts to charge through R2
towards VCC
.
• When voltage across C2
rises sufficiently (i.e. more
than 0.7 V), it biases Q2
in the forward direction so
that it starts conducting and is soon driven to
saturation.

Cont...
• VC2
decreases and becomes almost zero when Q2
gets
saturated. The potential of point B decreases from VCC
to
almost 0 V. This potential decrease 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 0 V, C1
starts charging through R1
towards the target voltage VCC
.
• When voltage of C1
increases sufficiently, Q1
becomes
forward-biased and starts conducting.
• In this way, the whole cycle is repeated. It is seen that the
circuit alternates between a state in which Q1
is ON and Q2
is
OFF and a state in which Q1
is OFF and Q2
is ON. The time in
each state depends on RC values. Since each transistor is
driven alternately into saturation and cut-off the voltage
wavefrom at either collector is essentially a square waveform
with a peak amplitude equal to VCC

Switching Times
• It can be proved that off-time for Q1
is T1
=
0.69 R1
C1
and that for Q2
is T2
= 0.69 R2
C2
.
• Hence, total time-period of the wave is T = T1
+ T2
= 0.69 (R1
C1
+ R2
C2
)
• If R1
= R2
= R and C1
= C2
= C i.e. the two stages
are symmetrical, then T = 1.38 RC

Frequency of Oscillation
• It is given by the reciprocal of time period,

Minimum Values of β
• To ensure oscillations, the transistors must
saturate for which minimum values of β are as
under:

Monostable Multivibrator (MMV)
• It is also called a single-shot or single swing or a
one-shot multivibrator.
•Monostable Multivibrator has
(i) One absolutely stable (stand-by) state; and
(ii) One quasi stable state.
•It can be switched to the quasi-stable state by an
external trigger pulse but it returns to the stable
condition after a time delay determined by the value
of circuit components.

A typical MMV (Monostable Multivibrator) circuit
Cont...

A typical MMV (Monostable Multivibrator) circuit

Initial Condition
In the absence of a triggering pulse at C2
and with S
closed,
1. VCC
provides reverse bias for C/B junctions of Q1
and Q2
but forward-bias for E/B junction of Q2
only. Hence, Q2
conducts at saturation.
2. VBB
and R3
reverse bias Q1
and keep it cut off.
3. C1
charges to nearly VCC
through RL1
to ground by the
low-resistance path provided by saturated Q2
.
As seen, the initial stable state is represented by
(i) Q2
conducting at saturation and (ii) Q1
cut-off

When Trigger Pulse is Applied
1. If positive trigger pulse is of sufficient amplitude, it will override
the reverse bias of the E/B junction of Q1
and give it a forward
bias. Hence, Q1
will start conducting.
2. As Q1
conducts, its collector voltage falls due to voltage drop
across RL1
. It means that potential of point A falls (negative-going
signal). This negative-going voltage is fed to Q2
.
3. As collector current of Q1
starts decreasing, potential of point B
increases (positive-going signal) due to lesser drop over RL2
. Soon,
Q2 comes out of conduction.
4. The positive-going signal at B is fed via R1
to the base of Q1
where
it increases its forward bias further. As Q1
conducts more,
potential of point A approaches 0 V.
5. This action is cumulative and ends with Q1
conducting at
saturation and Q2
cut-off.

Return to Initial Stable State
1. As point A is at almost 0 V, C1
starts to discharge through saturated Q1
to
ground.
2. As C1
discharges, the negative potential at the base of Q2
is decreased.
As C1
discharges further, Q2
is pulled out of cut-off.
3. As Q2 conducts further, a negative-going signal from point B via R1
drives Q1
into cut-off.
Hence, the circuit reverts to its original state with Q2
conducting at
saturation and Q1
cut-off. It remains in this state till another trigger
pulse comes along when the entire cycle repeats itself.
Since this MV 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 = 0.69 C1
R2
It is also known as the one-shot period.

Bistable Multivibrator (BMV)
• It is also called flip-flop multivibrator. It has two absolutely
stable states.
• It can remain in either of these two states unless an external
trigger pulse switches it from one state to the other.
Obviously, it does not oscillate.
• It has no energy storage element.
1. The base resistors are not joined to VCC
but to a common
source–VBB
,
2. The feedback is coupled through two resistors (not
capacitors).

Circuit Action
• If Q1
is conducting, then the fact that point A is at nearly 0 V
makes the base of Q2
negative (by the potential divider R2
–
R4
) and holds Q2
off.
• Similarly, with Q2
OFF, the potential divider from VCC
to –VBB
(RL2
, R1
, R3
) is designed to keep base of Q1
at about 0.7 V
ensuring that Q1 conducts. It is seen that Q1
holds Q2
OFF
and Q3
holds 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

Cont...
SATURATION CUT-OFF
1. IC
is maximum
2. Vc
=Ic
x(Rc
+RE
)
3. VCE
=0
4. IB
is greater than
zero
5. The transistor is
working like a
ON-Switch
1. IB
is roughly zero
2. IC
is also
proportional to IB
3. VCE
is maximum =
VCC
4. The transistor is
working like an
OFF-Switch

555 Timer
• The IC 555 is one of the most popular and most widely used
IC’s. It is a versatile and extremely robust integrated circuit
that is used in many applications like timers, wave generators
(pulse) and oscillators.
• The IC555, popularly known as the 555 Timer, was developed
by Hans Camenzind of Signetic Corporation in the year 1971.

Features
• The 555 timer can be operated at a wide range of
power supplies ranging from 5 V to 18 V.
• It is available in 3 different packages: 8-pin Metal Can
package, 8-pin DIP and 14-pin DIP.
• Timing can be anywhere from microseconds to hours.
• It can operate in both astable and monostable modes.
• High output current.
• It has an adjustable duty cycle.
• It is TTL compatible due to its high output current.
• It has a temperature stability of 0.005% per 0C

Cont...
• Pin 1. –Ground, The ground pin connects the 555 timer to the
negative (0v) supply rail.
• Pin 2. –Trigger, The negative input to comparator No 1. A
negative pulse on this pin “sets” the internal Flip-flop when
the voltage drops below 1/3 Vcc
causing the output to switch
from a “LOW” to a “HIGH” state.
• Pin 3. –Output, The output pin can drive any TTL circuit and is
capable of sourcing or sinking up to 200mA of current at an
output voltage equal to approximately Vcc
– 1.5V.
• Pin 4. –Reset, This pin is used to “reset” the internal Flip-flop
controlling the state of the output, pin 3. This is an active-low
input and is generally connected to a logic “1” level when not
used to prevent any unwanted resetting of the output.

Cont...
• Pin 5. –Control Voltage, This pin controls the timing of the 555
by overriding the 2/3Vcc
level of the voltage divider network.
By applying a voltage to this pin the width of the output signal
can be varied independently of the RC timing network.
• Pin 6. –Threshold, The positive input to comparator No 2. This
pin is used to reset the Flip-flop when the voltage applied to it
exceeds 2/3Vcc
causing the output to switch from “HIGH” to
“LOW” state. This pin connects directly to the RC timing circuit.
• Pin 7. –Discharge, The discharge pin is connected directly to the
Collector of an internal NPN transistor which is used to
“discharge” the timing capacitor to ground.
• Pin 8. –Supply +Vcc, This is the power supply pin and for
general purpose TTL 555 timers is between 4.5V and 15V.

555 Timer Internal Circuit Diagram

Working
• The three 5KΩ resistors form a voltage divider network. This
network provides two reference voltages to two comparators 2/3
VCC
to the inverting terminal of the upper comparator and 1/3 VCC
to the non-inverting terminal of the lower.
• When the threshold voltage is greater than 2/3 VCC
(i.e. the
control voltage), then the flip-flop is RESET and the output goes
LOW. This will turn the discharge transistor ON.
• When the trigger input is less than the reference voltage (1/3
VCC
), the lower comparator’s output is high.
• This is connected to the S input of the flip-flop and hence
the flip-flop is SET and the output goes HIGH and the timing
interval starts. As the output is high, the discharge transistor is
turned OFF and allows charging of any capacitor connected to it
externally.
• Hence, in order for the output to go HIGH, the trigger input should
be less than the reference voltage momentarily.

Different Modes Of Operation
• Generally, the 555 timer can be operated in
three modes:
1. Monostable (or one-shot)
2. Astable and
3. Bistable

Monostable Multivibrator
using 555 Timer
• This is the basic mode of operation of the IC 555.
• It requires only two extra components to make it work as a
monostable multivibrator: a resistor and a capacitor.
• As the name specifies, a monostable multivibrator has only
one stable state.
• When a trigger input is applied, a pulse is produced at the
output and returns back to the stable state after a time
interval.
• The duration of time for which the pulse is high will depend
on the timing circuit that comprises of a resistor (R) and a
capacitor (C)

Operation
• The sequence of events starts when a negative going trigger
pulse is applied to the trigger comparator.
• When this trigger comparator senses the short negative going
trigger pulse to be just below the reference voltage (1/3 VCC
),
the device triggers and the output goes HIGH.
• The discharge transistor is turned OFF and the capacitor C that
is externally connected to its collector will start charging to the
max value through the resistor R.
• The HIGH output pulse ends when the charge on the capacitor
reaches 2/3 VCC
.

Cont...
• Initially, the flip-flop is RESET.
• The capacitor C, which is connected to the open collector
(drain in case of CMOS) of the transistor, is provided with a
discharge path.
• The capacitor discharges completely.
• When a negative going trigger pulse input is applied to the
trigger comparator (comparator 2), it is compared with a
reference voltage of 1/3 VCC
.
• The moment trigger voltage goes below 1/3 VCC
, the output of
comparator goes high and this will SET the flip-flop.

Cont...
• At the same time, the discharge transistor is turned OFF and
the capacitor C will begin to charge and the voltage across it
rises exponentially. This is nothing but the threshold voltage at
pin 6.
• The output at pin 3 will remain HIGH until the voltage across
the capacitor reaches 2/3 VCC.
• The instance at which the threshold voltage, the output of the
comparator 1 goes high.
• This will RESET the flip-flop and hence the output at pin 3 will
fall to low (logic 0).
• As the output is low, the discharge transistor is driven to
saturation and the capacitor will completely discharge.

Cont...
• Hence it can be noted that the output at pin 3 is low at
start, when the trigger becomes less than 1/3 VCC.
• The output at pin 3 goes high and when the threshold
voltage is greater than 2/3 VCC
the output becomes low
until the occurrence of next trigger pulse.
• A rectangular pulse is produced at the output.

Pulse Width Derivation
• We know that the voltage across the capacitor C rises
exponentially. Hence the equation for the capacitor voltage VC
can
be written as
VC
= VCC
(1 – e-t/RC
)
• When the capacitor voltage is 2/3 VCC, then
2/3 VCC
= VCC
(1 – e-t/RC
)
2/3 = 1 – e-t/RC
e-t/RC
= 1/3
– t/RC = ln (1/3)
– t/RC = -1.098
t = 1.098 RC
∴ t ≈ 1.1 RC
• The pulse width of the output rectangular pulse is W = 1.1 RC.

Application:Frequency Divider
• When the IC 555 is used as a monostable multivibrator, a positive
going rectangular pulse is available at the output when a negative
going pulse of short duration is applied at the trigger input.
• By adjusting the time interval t of the charging or timing circuit the
device can be made to work as a Frequency Divider circuit.
• If the timing interval t is made slightly larger than the time period
of the input pulse (trigger pulse), the device can act as a Divide –
by – two circuit.
• The timing interval can be controlled by appropriately choosing the
values of the resistor R and the capacitor C in the timing circuit.

Application:Linear Ramp Generator
• The monostable multivibrator will act as a Linear Ramp
Generator with the addition of a constant current source.
• A current mirror, consisting of a diode and a PNP transistor, is
used as a Constant Current Source. This constant current
source is positioned in place of the timing resistor.

Cont...
• The current IC
from the constant current source will charge the
capacitor at a constant rate towards the peak voltage (VCC
)
resulting in a rising linear ramp.
• As the voltage across the capacitor reaches 2/3 VCC
, the
comparator 1 will drive the discharge transistor to saturation.
• As a result, the capacitor starts discharging. While
discharging, as the voltage across the capacitor falls to 1/3
VCC
, the comparator 2 will turn off the discharge capacitor.

Cont...
• Hence the capacitor will start charging again.
• The discharge time of the capacitor is very less when
compared to the charging time. As a result, the downward
ramp is very steep (almost an immediate discharge).
• Hence, the time period of the ramp output is practically equal
to the charging time of the capacitor. The time period of the
ramp output is approximately given by
T = (2/(3 ) Vcc
RE
(R1
+R2
)C)/(R1
Vcc
– Vbe
(R1
+R2
))

Astable Multivibrator Using 555 Timer
• Astable multivibrator is also called as Free Running
Multivibrator.
• It has no stable states and continuously switches between the
two states without application of any external trigger.
• The IC 555 can be made to work as an astable multivibrator
with the addition of three external components: two resistors
(R1
and R2
) and a capacitor (C).

Cont...
IC 555 as an astable multivibrator

Contd.
⚫ Operation for astable multivibrator :
i) Assume the initial is HIGH. Transistor Q1
OFF and capacitor
is charging through resistor RA
and RB
.
ii) When capacitor voltage reach 2/3 Vcc
, Comparator 1 will
trigger flip flop and output change from change from HIGH to
LOW. Resistor RB
and transistor Q1
.
iii) When the capacitor voltage reach 1/3 Vcc
, comparator
output 2 will trigger flip flop so the timer output is HIGH. The
cycle is repeated.
⚫ Period for capacitor charging from 1/3 Vcc
to 2/3 Vcc
same as
period for HIGH output at timer.
⚫ Period for capacitor discharging from 2/3Vcc
to 1/3 Vcc
same
as period for LOW output at timer.

DUTY CYCLE
• Duty cycle is the mathematical parameter that forms a
relation between the high output and the low output. Duty
Cycle is defined as the ratio of time of HIGH output i.e. the ON
time to the total time of a cycle.
• If TON
is the time for high output and T is the time period of
one cycle, then the duty cycle D is given by
D = TON
/ T
• Therefore, percentage Duty Cycle is given by
%D = (TON
/ T) * 100

Cont...
• T is sum of TON
(or TC
charge time) and TOFF
(or TD
discharge
time).
TC
= 0.693 * (R1
+ R2
) C
• The value of TOFF
or the discharge time (for low output) TD
is
given by
TD
= 0.693 * R2
C
• Therefore, the time period for one cycle T is given by
T = TON
+ TOFF
= TC
+ TD
T = 0.693 * (R1
+ R2
) C + 0.693 * R2
C
T = 0.693 * (R1
+ 2R2
) C

Cont...
• Therefore, %D = (TON
/ T) * 100
%D = (0.693 * (R1
+ R2
) C)/(0.693 * (R1
+ 2R2
) C) * 100
%D = ((R1
+ 2R2
))/((R1
+ 2R2
)) * 100
• If T = 0.693 * (R1
+ R2
) C, then the frequency f is given
by
f = 1 / T = 1 / 0.693 * (R1
+ 2R2
) C
f = 1.44/( (R1
+ 2R2
) C) Hz

FORMULA FOR TIMER
⚫ TH
= 0.693 (R1
+R2
) C
⚫ TL
= 0.693 (R2
) C
⚫ Period, T = TH
+ TL
= 0.693 (R1
+ 2R2
) C
⚫ Frequency, f = 1/T
= 1/ (TH
+ TL
)
= 1.44 / ((R1
+ 2R2
) C)
⚫ % Duty Cycle = [TH
/ (TH
+ TL
) ]x 100
= [(R1
+ R2
) / (R1
+ 2R2
)] x100

Applications of Astable Multivibrator
• Square Wave Generation
• Pulse Position Modulation
• Pulse Train
• Frequency Modulation
• And Many More......

Square Wave Generation
• The duty cycle of an astable multivibrator is always greater
than 50%.
• A square wave is obtained as the output of an astable
multivibrator when the duty cycle is 50% exactly.
• The modification is to add two diodes. one diode in parallel to
the resistor R2
with cathode facing the capacitor and another
diode in series with the resistor R2
with anode facing the
capacitor.

Contd.
• While charging, the capacitor charges through R1
and D1 by
passing R2
. While discharging, it discharges through D2 and R2
.
Therefore, the charging time constant is TON
= TC
and is given
by
TON
= 0.693 * R1
C and
• The discharging time constant TOFF
= TD
is given by
TOFF
= 0.693 * R2
C.
• Therefore, the duty cycle D is given by
D = R1
/(R1
+R2
)
• In order to get a square wave, the duty cycle can be made 50%
by making the values of R1
and R2
equal. The waveforms of the
square wave generator are shown below

Pulse Position Modulation
• In pulse position modulation, the position of the pulse varies
according to the modulating signal while the amplitude and
the width of the pulse are kept constant.
• The position of the each pulse changes according to the
instantaneous samples voltage of the modulating signal.
• In order to achieve Pulse Position Modulation, two 555 timer
IC’s are used.

Cont..
• The modulating signal is applied at the Pin 5 of the first IC 555
that is operating in astable mode. The output of this IC 555 is
a pulse width modulated wave.
• This PWM signal is applied as the trigger input to the second
IC 555 which operates in Monostable Mode .
• The position of the output pulses of the second IC 555
changes according to the PWM signal which is again
dependent on the modulating signal.

Bistable Multivibrator Using 555 Timer
• When an astable multivibrator has no stable states and a
monostable multivibrator has a single stable state, a device
with two absolute stable states is possible.
• A Bistable multivibrator is a type of circuit which has two
stable states (high and low). It stays in the same state until
and unless an external trigger input is applied.
• Generally, a bistable multivibrator stays low until a trigger
signal is applied and it stays high until a reset signal is applied.
• Bistable multi vibrators are also called as flip-flops or latches.
The term flip-flop is used because it ‘flips’ to one state and
stays there until a trigger is applied and once the trigger is
applied it ‘flops’ back to the original state

Cont.....
• A bistable multivibrator is one of the easiest circuits that can
be built using a 555 timer.
• The generation of high and low outputs is not dependent on
the charging and discharging of the capacitor in the RC unit
but rather it is controlled by the external trigger and reset
signals.
• The trigger and reset pins (pins 2 and 4 respectively) are
connected to the supply through two resistors R1
and R2
so
that they are always high.
• Two switches are connected between these pins and ground
in order to make them go low momentarily

Cont....
• When the switch S1
is pressed, the voltage from VCC
will
bypass the trigger terminal and is shorted to ground through
the resistor R1
.
• Hence, the trigger pulse will momentarily go low and the
output of the timer at pin 3 will become HIGH.
• he output stays HIGH because there is no input from the
threshold pin (pin 6 is left open or better if connected to
ground) and the output of the internal comparator
(comparator 1) will not go high.

Cont....
• When the switch S2
is pressed, the voltage from VCC
will
bypass the reset terminal and is shorted to ground through
the resistor R2
.
• When this signal goes low for a moment, the flip-flop receives
the reset signal and RESETs the flip-flop.
• Hence, the output will become LOW and stays there until the
trigger is applied.

555 Timer Flip-flop and
Memory Cell Circuit
Flip-flop
• It is not a great choice to use a 555 timer as a flip-flop in
computer applications.
• The output of the 555 timer is not responsive enough to
match the speed of the clock signals’ frequency. Readily
available flip-flop devices are preferable when used in
high speed operations.
• The 555 timer in bistable mode i.e. as a flip-flop can be
used in low speed, non-computer applications like
robotics. A simple application is a robot which moves
forward and backward every time it hits an object.

Cont....
Circuit of a flip-flop using a 555 timer

Cont..
• When the output at pin 3 is high, the capacitor C
charges through the resistor R1
to the peak value i.e.
VCC
.
• When the output at pin 3 is low, the capacitor
discharges through the same resistor to 0. In order to
switch the output from high to low or low to high, a
switch is used at the junction of trigger and threshold
pins.
• The voltage divider formed by the resistors R2
and R3
will provide a voltage of VCC
/ 2 at the pins 6 and 2.
• When the switch is pressed, this voltage is interrupted
and triggers the internal flip-flop. This will allow the
output to switch between the two states.

Cont..
A circuit that acts as a toggle flip-flop is shown . It is used to light an LED
and the LED switches between ON and OFF when the switch is pressed.

Schmitt Trigger
• The Schmitt trigger has found many applications in numerous
circuits, both analog and digital.
• The versatility of a TTL Schmitt is hampered by its narrow
supply range, limited interface capability, low input
impedance and unbalanced output characteristics.
• The Schmitt trigger could be built from discrete devices to
satisfy a particular parameter, but this is a careful and
sometimes time-consuming design.

Schmitt Trigger
• Analog signals are generally not perfect and might not have
clean edges all the times.
• If the edges are not fast enough, they tend to provide more
current and this might damage the device
• Schmitt Trigger is a special type of comparator that is used to
avoid such signals.

Cont...
• A comparator is a device that compares two voltages and
the outcome is the indication of whether one voltage is
higher than the other or not.
• Schmitt trigger, also called as Regenerative Comparator,
compares the input voltage to two reference voltages
and produces an equivalent output.
• The output of a Schmitt trigger is always a square or
rectangular wave irrespective of the shape of the input.

Cont...
• It is often used when we need to do the following:
• Convert sine wave to square wave
• To clean up the noisy signals
• To convert slow edges (like in a triangular wave) into
fast edges (like a square wave)

555 timer as Schmitt Trigger
• IC 555 has pins 4 and 8 are connected to the supply (VCC
).
The pins 2 and 6 are tied together and the input is given
to this common point through a capacitor C.
• This common point is supplied with an external bias
voltage of VCC
/ 2 with the help of the voltage divider
circuit formed by the resistors R1 and R2.
• The important characteristic of the Schmitt trigger is
Hysteresis.
• The output of the Schmitt trigger is high if the input
voltage is greater than the upper threshold value and the
output of the Schmitt trigger is low if the input voltage is
lower than the lower threshold value.

Cont...
• The output retains its value when the input is between the
two threshold values.
• The usage of two threshold values is called Hysteresis and the
Schmitt trigger acts as a memory element (a bistable
multivibrator or a flip-flop).
• The threshold values in this case are 2/3 VCC
and 1/3 VCC
i.e.
the upper comparator trips at 2/3 VCC
and the lower
comparator trips at 1/3 VCC
.
• The input voltage is compared to these threshold values by
the individual comparators and the flip-flop is SET or RESET
accordingly. Based on this the output becomes high or low.
• When a sine wave of amplitude greater than VCC
/ 6 is applied
at the input, the flip-flop is set and reset alternately for the
positive cycle and the negative cycle.

Cont...
Input and Output Waveform

Inverting Schmitt Trigger
• The normal operation of the 555 timer as a Schmitt trigger is
inverting in nature.
• When the trigger input, which is same as the external input,
falls below the threshold value of 1/3 VCC
, the output of the
lower comparator goes high and the flip-flop is SET and the
output at pin 3 goes high.
• Similarly, when the threshold input, which is same as the
external input, rises above the threshold value of 2/3 VCC
,
• The output of the upper comparator goes high and the
flip-flop is RESET and the output at pin 3 goes low.

Cont...
Waveform Inverting Schmitt Trigger

The Applications of Schmitt Trigger
• V
T+
• V
T
-
• V
I
• V
o
• t
• t
• Transform of Waveform

▪ Pulse Smoothening
VT+
VT-
VI
Vo
t

▪ Pulse Amplitude Detection
Vo
t
VT+
VT-
VI

QUESTIONS
• Q.1 A monostable multivibrator has R = 120kΩ and
the time delay T = 1000ms, calculate the value of C?
a) 0.9µF
b) 1.32µF
c) 7.5µF
d) 2.49µF

QUESTIONS
• Answer: c
Explanation: Time delay for a monostable
multivibrator,
T = 1.1RC
= > C = T/(1.1R) = 1000ms/(1.1×120kΩ) = 7.57
µF

QUESTIONS
• Q.2 A 555 timer in monostable application mode can
be used for
a) Pulse position modulation
b) Frequency shift keying
c) Speed control and measurement
d) Digital phase detector

QUESTIONS
• Answer: c
Explanation: In monostable operation mode, if input
trigger pulses are generated from a rotating wheel,
the circuit will determine the wheel speed whenever
it drops below a predetermined value. Therefore, it
can be used for speed control and measurement.

QUESTIONS
• Q.3How can a monostable multivibrator be modified
into a linear ramp generator?
a) Connect a constant current source to trigger input
b) Connect a constant current source to trigger
output
c) Replace resistor by constant current source
d) Replace capacitor by constant current source

QUESTIONS
• Answer: c
Explanation: The resistor R of the monostable circuit is
replaced by a constant current source. So, that the capacitor is
charged linearly and generates ramp signal.

QUESTIONS
• Q.4 Determine the time period of a monostable 555
multivibrator??
a) T= 0.33RC
b) T= 1.1RC
c) T= 3RC
d) T= RC

QUESTIONS
• Answer: b
Explanation: The time period of a monostable 555 timer is T =
RC×ln(1/3) = 1.1.RC.

QUESTIONS
• Q.5 Find the charging and discharging time of 0.5µF capacitor.
A)Charging time=2ms; Discharging
time=5ms.
b) Charging time=5ms; Discharging
time=2ms.
c) Charging time=3ms; Discharging
time=5ms.
d) Charging time=5ms; Discharging
time=3ms.

QUESTIONS
• Answer: b
Explanation: The time required to charge the
capacitor is tHigh
=0.69(RA
+RB
)C
=0.69(10kΩ+5kΩ)x0.5µF =5ms.
The time required to discharge the capacitor is
tLow
=0.69xRC
=0.69x5kΩx0.5µF=2ms.

QUESTIONS
Q.6 Astable multivibrator operating at 150Hz has a discharge
time of 2.5m. Find the duty cycle of the circuit.
a) 50%
b) 75%
c) 95.99%
d )37.5%

QUESTIONS
• Answer: d
Explanation: Given f=150Hz.Therefore,T=1/f =1/150 =6.67ms.
∴ Duty cycle,
D%=(tLow
/T)x100%
=(2.5ms/6.67ms)x100%
=37.5%.

QUESTIONS
• Q.7 What happens if the threshold voltages are made longer
than the noise voltages in schmitt trigger?
a) All the mentioned
b) Enhance the output signal
c) Reduce the transition effect
d) Eliminate false output transition

QUESTIONS
• Answer: d
Explanation: In schmitt trigger, if the threshold voltage
VUT
and VLT
are made larger than the input noise voltage. The
positive feedback will eliminate the false output transition.

QUESTIONS
• Q.8 Which circuit converts irregularly shaped waveform to
regular shaped waveforms?
a) Schmitt trigger
b) Voltage limiter
c) Comparator
d) None of the mentioned

QUESTIONS
• Answer: a
Explanation: Schmitt trigger are also called as squaring circuit
because, this type of circuit converts an irregularly shaped
wave to a square wave or pulse.

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UNIT - 6.pdf