13. Op amp Comparator
• Open loop gain of op amp is defined as:
AOL = Vo / VD
where VD = VNI – VINV
• Open loop gain of op amp is very high
(ideally infinite).
• Any small difference between VNI and VINV
results into saturation of output voltage ±VSAT
i.e. for VNI < VINV output is – VSAT and
for VNI > VINV output is + VSAT
• Value of VSAT is limited by the supply voltage
of op amp
14. Op amp Voltage Level Detector
VOUT
+
-
VREF
VIN
= +VSAT for VIN>VREF
= -VSAT for VIN<VREF
15. Important features of Op Amp
1. High open loop gain (ideally infinite)
which implies that even the smallest
difference between the two inputs
results into saturated output voltage
2. High input impedance (ideally infinite)
implies that there is no current
flowing into the input of an op amp
16. Virtual Ground
These two conditions give rise to VIRTUAL
GROUND, where the voltages at both the
inputs are maintained at exactly same level.
To achieve this condition, a feedback circuit
between the output and the inverting input
terminal of the op amp is necessary.
This results into many applications of op amp,
which qualify it to be OPERATIONAL:
adder, subtractor, multiplier, divider etc.
28. Common Mode Rejection Ratio
(CMRR)
• It is the ability of an op amp to reject the
signal which is present at its both inputs
simultaneously i.e. the common mode
signal
• CMRR = AOL / ACM, where ACM is common
mode voltage gain defined by Vout / VCM
• Ideally CMRR is infinite
• For IC 741 it is 90 dB
29. Slew Rate
• Maximum rate of change of output
voltage (when typically a step voltage is
given at the input terminal)
Step
Input
Vout
t
dVout / dt
30. Slew Rate (cont…)
• Slew rate is caused by the finite response
time of the circuit elements of an op amp
• It limits the highest possible frequency of
operation
Vin
Vout
Expected
Voltage
t
31. Open Loop Gain
• AOL is ideally infinite
• In case of typical practical op amp like
IC 741 it is of the order of 105 i.e.
about 100 dB
Input Resistance
• RI is ideally infinite
• In case of IC 741 it is 2 MΩ
Output Resistance
• RO is ideally zero
• In case of IC 741 it is 75 Ω
32. Input Offset Voltage
• Small voltage needed to be applied
between INV and NI terminals to get
zero output voltage
• Ideally it should be 0 V (CM operation)
Output Offset Voltage
• Under common mode operation, output
voltage should be zero, but due to
mismatch is devices it is non-zero
(Can be corrected by applying voltage
between Pins 1 and 5 of IC 741)
33. Summary
• Op amp can be used for various
(mathematical) operations like addition,
subtraction, multiplication, division,
differentiation, integration etc.
• Infinite open loop gain and infinite input
resistance give rise to VIRTUAL GROUND
• Op amp is available in a simple to use IC
form which require dual supply
• Many other applications can be
implemented using op amp
34. ACKNOWLEDGMENT
Some circuits in this presentation are
based on following materials:
• HANDBOOK OF OPERATIONAL
AMPLIFIER APPLICATIONS
by Texas Instruments
• www.williamson-labs.com
