Chapter 31
Applications of Op-Amps
Comparators
• Op-amp as a Comparator
– No negative feedback
– Output saturates with very small + or – input
2
Comparators
• Comparator
– Non-linear device
– vout has two discrete values, ±VSAT
– vout = +VSAT if + input is greater than – input
– vout = –VSAT if – input is greater than + input
3
Comparators
• A comparator circuit: Sine wave in,
square wave out
4
Comparators
• Input sine wave
• Output square
wave Vout = ±VSAT
• +VSAT (determined
by VCC) when
sinusoid is +
• –VSAT (determined
by VEE) when
sinusoid is –
VCC
+
741
__
_
-
__
_
-
-
vout
VEE
5
Comparators
•
•
•
•
•
Compare input waveform to reference
Reference can be ground or dc source
Can compare two waveforms
Specialized comparator IC’s also available
Detects when waveform reaches given
level
6
Comparators
• Zero-Crossing Detector
7
Voltage Summing Amplifier
• Circuit
8
Voltage Summing Amplifier
• Inverse sum
I F  I1  I 2  I 3
Vout  I f R f
RF
Ri
9
Voltage Summing Amplifier
• Multiplies each input by
Vout
 RF
RF
RF 
 
V1 
V2 
V3 
R2
R3 
 R1
10
Integrators and Differentiators
• In general
ZF
vout  
vin
Z1
• Using resistors and
capacitors
– Integrators
– Differentiators
ZF
vin
-
Z1
+
vout
__
_
-
11
Integrators and Differentiators
• Voltage across capacitor
t
1
vC (t )   i (t )dt  V0
C0
• Current through capacitor
dvC
iC (t )  C
dt
12
Integrators and Differentiators
• Op-amp Integrator
t
1
vC (t ) 
vin (t )dt  V0

R1C 0
t
1
vout (t ) 
vin (t )dt

R1C 0
C
vin
R1
i
i=0 0V
+
vout
__
_
-
13
Integrators and Differentiators
• Op-amp
differentiator
dvin
vout (t )   RF C
dt
+ RF Cin
-
vin
i
• Circuit inherently
unstable
+
i
vout
__
_
-
14
Integrators and Differentiators
• Stable op-amp differentiator
15
Instrumentation Amplifiers
• Op-amp in differential amplifier
configuration
• Noise suppression
• High CMRR
• Reasonable gain
• IC instrumentation amps
16
Instrumentation Amplifiers
• An op-amp instrumentation amp circuit
17
Instrumentation Amplifiers
• Measurement of very small voltages
• Transducer
– Converts a physical change into an electrical
change
18
Instrumentation Amplifiers
• Strain gage
– Converts force into ∆R
– ∆R is milliohms
– Use bridge circuit
19
Instrumentation Amplifiers
• Strain gage example
– Thin metal foil (resistor) on plastic backing
– Glued to metal bar
– Bar subjected to tension and compression
20
Instrumentation Amplifiers
• Strain gage example
– Tension
• Resistance of strain gage is R + ∆R
– Compression
• Resistance of strain gage is R – ∆R
21
Active Filters
• Basic filter types
– Passive elements, gain < 1
– Low-pass
– High-pass
– Bandpass
– Band reject
22
Active Filters
• With op-amps/active filters
– Gain can be ≥ 1
– Filter response closer to ideal
23
Active Filters
• Low-pass (RF = R1)
1
ZC
jC
vout  vC 
vin 
vin
1
R1  Z C
R1 
jC
1
TF ( j ) 
1  j R1C
• Add resistor for gain > 1
RF = R1
I=0
0V
vin
R1
+
vout
C
__
_
24
Active Filters
• High-pass (RF = R1)
R1
R1
vout  vR 
vin 
vin
1
R1  ZC
R1 
jC
R1C
TF ( j ) 
1  j R1C
• Add resistor for gain > 1
RF = R1
I=0
C
0V
+
vin
vout
R1
__
_
25
Active Filters
• dc gain
– Easily achieved
– Not used much due to gain-bandwidth product
• Example
– GBWP = 106, Gain = 10
– Cutoff for filter (HP or LP) only 105
26
Active Filters
• Bandpass
• Wideband
– Cascade HP and LP active filters
– LP must have higher cutoff frequency
– HP and LP cutoff frequencies far apart
• Narrowband
– Can use single op-amp
27
Active Filters
• Narrowband BP
circuit
0.1125
R
f0 
1
RC
R1
0.1591
BW 
RC
C
C
vin
-
2R
R
R1
+
vout
_
__
_ __
- 28
Active Filters
• Active notch filter
– Cascade narrowband BP filter
– Adder circuit
– Result is 1 – (frequency response of BP filter)
– Frequency at resonant frequency of BP filter
will be eliminated
29
Voltage Regulation
• Voltage regulator
– Constant voltage to load
– Specified current range
– Specified input voltage range
– Zener diode regulator
• Inefficient
• Dissipates power
30
Voltage Regulation
• Types of regulators
– Fixed voltage regulator
– Variable voltage regulator
– Switching regulator
• Specialized IC regulators
– For different voltages, e.g. +5 V, –5 V, +12 V,
–12 V, +15 V, –15 V, etc.
31
Voltage Regulation
• Line Regulation
– Small output
change with
change in input
Unregulated
input
Regulated
output
Voltage
regulator
vout
% line regulation 
100%
vin
RL
__
_
32
Voltage Regulation
• Load regulation
– Small output voltage change with smaller RL
VNL  VFL
% load regulation 
100%
VFL
– VNL = no-load voltage (open-circuit load)
– VFL = full-load voltage (specified by
manufacturer)
33
Voltage Regulation
Unregulated
input
+
• Circuit to
increase
efficiency of
Zener regulator
with an op-amp
vin
Q1
+
RD
R1
+
vout
-
-
__
_
-
R2
34
Voltage
Regulation
• Three-terminal IC
regulators
– 7800 series,
positive voltage
– 7900 series,
negative voltage
35
Voltage Regulation
•
•
•
•
5 V output, 7805
12 V output, 7812
–5 V output, 7905
–12 V output, 7912
IN
+
μA7812
Unregulated
input
-
OUT
+
__
_COM
RL
Vout=12 V
__
_
-
-
36
Voltage Regulation
• Ripple
• Greatly reduced by IC regulator
ripple rejectiondB  20 log
Vr (in)
Vr (out )
Vr(in) = input ripple voltage
Vr(out) = output ripple voltage
37