Comparator

Linear integrated circuit

1. Comparators
S.S.Chhatwani

2. Introduction:

6. Transfer characteristics

8. A comparator compares a signal voltage on one input of
an opamp with a known reference voltage on the other
input. We can say A comparator has two inputs one is
usually a constant reference voltage VR and other is a
time varying signal Vi and one output VO. We know
that in an op-amp with an open loop configuration with
a differential or single input signal has a value greater
than 0, the high gain which goes to infinity drives the
output of the op-amp into saturation. Thus, an op-amp
operating in open loop configuration will have an
output that goes to positive saturation or negative
saturation level or switch between positive and negative
saturation levels. This principle is used in a comparator
circuit with two inputs and an output.

10. Types of comparator
Depending on which input terminal
receives the signal input the
comparator are classified into two
categories:
Non inverting comparator
Inverting comparator

11. Applications:
ZCD(zero crossing detector)
Window comparator
Level detector
Phase detector
Schmitt trigger
Peak detector

12. Non-inverting comparator

13. It is called a non-inverting comparator
circuit as the sinusoidal input signal Vin is
applied to the non-inverting terminal. The
fixed reference voltage Vrefis connected to
the inverting terminal of the op-amp.
When the value of the input voltage Vin is
greater than the reference voltage Vrefthe
output voltage Vo goes to positive
saturation. This is because the voltage at the
non-inverting input is greater than the
voltage at the inverting input.

14. When the value of the input voltage Vin is
lesser than the reference voltage Vref, the
output voltage Vo goes to negative
saturation. This is because the voltage at the
non inverting input is smaller than the
voltage at the inverting input. Thus, output
voltage Vo changes from positive saturation
point to negative saturation point whenever
the difference between Vin and Vref
changes.

15. The comparator can be called a voltage level detector,
as for a fixed value of Vref, the voltage level of Vin
can be detected. The circuit diagram shows the diodes
D1and D2.
These two diodes are used to protect the op-amp from
damage due to increase in input voltage. These diodes
are called clamp diodes as they clamp the differential
input voltages to either 0.7V or -0.7V.
Most op-amps do not need clamp diodes as most of
them already have built in protection.
Resistance R1 is connected in series with input
voltage Vin and R is connected between the inverting
input and reference voltage Vref.
R1 limits the current through the clamp diodes and R
reduces the offset problem.

17. Inverting amplifier

18. It is called a inverting comparator circuit as
the sinusoidal input signal Vin is applied to
the inverting terminal.
The fixed reference voltage Vrefis given to
the non-inverting terminal (+) of the op-
amp. A potentiometer is used as a voltage
divider circuit to obtain the reference
voltage in the non-inverting input terminal.
Both ends of the Potentiometer are
connected to the dc supply voltage +VCC
and -VEE. The wiper is connected to the non
inverting input terminal.

19. When the wiper is moved to a value near +VCC,
Vrefbecomes more positive, and when the wiper is
moved towards -VEE, the value of Vrefbecomes
more negative. The waveforms are shown below.

20. Inverting Comparator

21. The transfer characteristics are basically a graph of output
voltage versus input voltage. From the above characteristics, it
is observed that the reference voltage (or reference point) is the
point at which the state change occurs i.e. the transition from
one state to other state. In other words, the circuit is triggered at
the reference point hence it is also called as triggering point. The
reference voltage can be changed externally and also can be
either positive or negative as discussed above. Thus the
reference point can have a trip on input axis anywhere, and
hence it is also referred as trip point or trip voltage. Also at the
reference point the state change occurs at the output when input
signal crosses the reference voltage. Thus reference voltage is
also called as threshold voltage at which the comparator is
changing its output state.

23. ZERO CROSSING
DETECTOR
The zero crossing detector circuit is an
important application of theop-amp
comparator circuit. It can also be called as the
sine to square wave converter. Anyone of the
inverting or non-inverting comparators can
be used as a zero-crossing detector. The only
change to be brought in is the reference
voltage with which the input voltage is to be
compared, must be made zero (Vref= 0V).
An input sine wave is given as Vin.

26. it detects the point where the input signal crosses
zero of the reference voltage level. For every crossing,
the saturation level of the output signal changes from
one to another.
the reference level is set at 0 and applied at the non-
inverting terminal of the op-amp. The sine wave
applied at the inverting terminal of the op-amp is
compared with the reference level each time the
phase of the wave changes either from positive to
negative or negative to positive.

27. Firstly, when positive half of the
sinusoidal signal appears at the input.
Then the op-amp comparator compares
the reference voltage level with the
peak level of the applied signal

28. in case of the negative half of the sinusoidal signal,
the op-amp comparator again compares the reference
voltage level with the peak of the applied signal.
As this time the circuit is dealing with negative half
of the signal, thus the peak will have a negative
polarity.

29. It can be seen in the above waveform that whenever
the sine wave crosses zero, the output of the Op-
amp will shift either from negative to positive or
from positive to negative.
It shifts negative to positive when sine wave
crosses positive to negative and vice versa. This is
how a Zero Crossing Detector detects when the
waveform is crossing zero every time. As you can
observe that the output waveform is a square wave,
so a Zero Crossing Detector is also called a Square
wave Generator Circuit.

30. Non inverting ZCD

31. Limitations:
If the input to a comparator contains noise, the output may
show error when Vin is near a trip point.
For instance, with a zero crossing, the output is low when vin
is positive and high when vinis negative. If the input
contains a noise voltage with a peak of 1mV or more, then the
comparator will detect the zero crossing produced by the
noise. Figure below, shows the output of zero crossing
detector if the input contains noise.

34. Operation:

Case I:Vin<VLT & Vin<VUT
With the above two conditions, output of op-amp A1 is negative (i.e. -
Vsat) which will make the diode D1 reverse biased. Similarly output of
op-amp A2 is positive (i.e. +Vsat) which will make the diode D2
forward biased. The positive voltage, +Vsatis now applied through
potential divider formed by resistance R1 and R2 to a base of transistor
Q. Due to the positive voltage at base the npntransistor Q moves into
the saturation. Thus the output voltage is zero.
Thus uptoVLT output voltage is zero.
Case II:Vin>VLT & Vin>VUT
With the above two conditions, output of op-amp A1 is positive (i.e.
+Vsat) which will make the diode D1 forward biased. Similarly output
of op-amp A2 is negative (i.e. -Vsat) which will make the diode D2
reverse biased.
The positive voltage, +Vsatis now applied through potential divider
formed by resistance R1 and R2 to a base of transistor Q. Due to the
positive voltage at base the npntransistor Q moves into the saturation.
Thus the output voltage is zero.
Thus above VUT output voltage is zero.

35. Case III:VLT<Vin<VUT
With this combined condition, both the op-amps A1
and A2 goes into negative saturation (i.e. -Vsat). Both
the diodes are reverse biased. So there is no voltage
applied for transistor Q. Thus transistor is in cut-off
and there no collector current and hence no drop
across resistance R. Thus the output voltage is
∴Vo=+5V
Thus between VLT and VUT (window of two
voltages) output voltage is high i.e. +5V and outside
this window it is zero. Therefore the circuit is called
as a window detector or comparator.
The transfer characteristic of such a window
comparator is shown below.

37. IC Comparators
OPAMP IC 741 has slew rate equal to 0.5 v/µs which
is too low for its use as a comparator.
Specially designed IC are LM311,LM339 etc.
Feature:
1.Fast switching speed due to higher slew rate
2.Output is compatible with any digital IC family
3.They have built in noise immunity .

38. LM 710 Voltage
comparator
General Description:
The LM710 series are high-speed voltage
comparators intended for use as an accurate, low-
level digital level sensor or as a replacement for
operational amplifiers in comparator applications
where speed is of prime importance. The circuit has
a differential input and a single-ended output, with
saturated output levels compatible with practically
all types of integrated logic. The device is built on a
single silicon chip which insures low offset and
thermal drift.

40. Features and applications
Features:
1.High accuracy
2.High speed
3.Low response time
4.Low cost
Applications:
1.Schmitt trigger
2.ADC
3.Level detector
4.Pulse width modulator

41. Schmitt trigger(Regenerative
comparator)
Comparator which use the positive feedback is
known as the Schmitt trigger or regenerative
comparators.
Types of Schmitt trigger:
1.Inverting Schmitt trigger
2.Non inverting Schmitt trigger

42. Inverting Schmitt trigger
Inverting Schmitt Trigger, the input is applied to the
inverting terminal of the Op-Amp. In this mode, the
output produced is of opposite polarity. This output
is applied to non-inverting terminal to ensure
positive feedback.


43. When V
INis slightly greater than V
REF, the output becomes -
V
SATand if V
INis slightly less that -V
REF(more negative than -
V
REF), then output becomes V
SAT. Hence, the output voltage
V
Ois either at V
SATor -V
SATand the input voltage at which these
state changes occur can be controlled using R
1and R
2.
The values of V
REFand -V
REFcan be formulated as follows:
V
REF= (V
O* R
2) / (R
1+ R
2)
ButV
O= V
SAT.
Hence,
V
REF= (V
SAT* R
2) / (R
1+ R
2)
-V
REF= (V
O* R
2) / (R
1+ R
2)
ButV
O= -V
SAT. Hence,
-V
REF= (-V
SAT* R
2) / (R
1+ R
2)

44. The reference voltages V
REFand -V
REFare called
Upper Threshold Voltage V
UTand Lower Threshold
Voltage V
LT. The following image shows the output
voltage versus input voltage graph. It is also known
as the Transfer Characteristic of Schmitt Trigger.

45. Non-Inverting Schmitt Trigger
Circuit
Coming to Non-Inverting Schmitt Trigger, the input
in this case is applied to the non-inverting input
terminal of the Op-Amp. The output voltage is fed
back to the non-inverting terminal through the
resistor R
1.


46. Let us assume that initially, the output voltage is at V
SAT. Until
V
INbecomes less than V
LT, the output stays at this saturation level.
Once the input voltage crosses the lower threshold voltage level, the
output changes state to -V
SAT.
The output remains at this state until the input rises beyond the upper
threshold voltage.
Following image shows the transfer characteristics of Non-Inverting
Schmitt Trigger circuit.

47. Applications
1.One important application of Schmitt
Trigger is to convert Sine waves into Square
waves.
2.They can be used to eliminate chatter in
Comparators (a phenomenon where
multiple output transitions are produced
due to swinging of input signal through the
threshold region).
3.They can also act as simple ON / OFF
Controllers (for example, temperature based
switches).

48. Hysteresis
When the input is below a different (lower) chosen
threshold the output is low, and when the input is
between the two levels the output retains its value.
This dual threshold action is calledhysteresis
Effects of hystresis:
1.It improves noise immunity
2.It reduces the response time and the operation becomes
faster.
3.It reduces the possibility of triggering produced by noise.

50. examples
R2=150Ω,R1=100KΩ,Vin=500mv sine
wave saturation
voltage=±15v.determine the threshold
voltages.

51. Peak detector using OPAMP
Peak detector circuits are used todeterminethepeak
(maximum)valueof aninput signal. It stores the
peak value of input voltages for infinite time
duration until it comes to reset condition.Usually,
the peak of non-sinusoidal waveforms is measured
using a peak detector. As traditional ac voltmeter
cannot measure the peak of such signals.

53. i) During the positive half cycle of Vin:
the o/p of the op-amp drives D
1on. (Forward biased)
Charging capacitor C to the positive peak value Vpof
the input volt Vin.

ii) During the negative half cycle of Vin:
D
1is reverse biased and voltage across C is retained.
The only discharge path for C is through R
Lsince the
input bias IB is negligible.

54. Applications of Peak detector
It is used in theanalysisofspectralandmass
spectrometer.
Peak detector finds its application in destructive
testing.
It is used forinstrumentation measurement, mostly
in amplitude modulated wave communication.
It widely finds applications insound measuring
instruments.

55. Negative peak detector

57. Peak to peak detector

58. Phase detector