ckt analysis with devices
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Chapter 31
Applications of Op-Amps
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Comparators Op-amp as a Comparator
No negative feedback
Output saturates with very small + or input
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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
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Comparators A comparator circuit: Sine wave in,
square wave out
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Comparators Input sine wave
Output square
waveV
out =V
SAT +VSAT (determined
by VCC) whensinusoid is +
VSAT (determinedby VEE) whensinusoid is
___-
___-
+
-vout
741
VCC
VEE
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Comparators Compare input waveform to reference
Reference can be ground or dc source
Can compare two waveforms
Specialized comparator ICs also available
Detects when waveform reaches givenlevel
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Comparators Zero-Crossing Detector
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Voltage Summing Amplifier Circuit
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Voltage Summing Amplifier Inverse sum
321F IIII
ffRIV out
i
F
R
R
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Voltage Summing Amplifier Multiplies each input by
3
3
2
2
1
1
VR
RV
R
RV
R
RV FFFout
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Integrators and Differentiators In general
Using resistors andcapacitors Integrators
Differentiators
-
+
___
-
vout
vin
ZF
Z1
in
1
Fout v
Z
Zv
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Integrators and Differentiators Voltage across capacitor
Current through capacitor
0
0
1( ) ( )
( )
t
C
CC
v t i t dt V C
dvi t C
dt
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Integrators and Differentiators Op-amp Integrator
0
1 0
1 0
1( ) ( )
1( ) ( )
t
C in
t
out in
v t v t dt V R C
v t v t dt
R C
-
+
___
-
vout
C
R1vin
0 Vi i= 0
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+ -
Integrators and Differentiators Op-amp
differentiator
Circuit inherentlyunstable
( ) inout F dvv t R C dt
-
+
___-
vout
CinRF
vin
i
i
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Integrators and Differentiators Stable op-amp differentiator
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Instrumentation Amplifiers Op-amp in differential amplifier
configuration
Noise suppression
High CMRR
Reasonable gain
IC instrumentation amps
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Instrumentation Amplifiers An op-amp instrumentation amp circuit
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Instrumentation Amplifiers Measurement of very small voltages
Transducer
Converts a physical change into an electrical
change
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Instrumentation Amplifiers Strain gage
Converts force into R
R is milliohms
Use bridge circuit
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Instrumentation Amplifiers Strain gage example
Thin metal foil (resistor) on plastic backing
Glued to metal bar
Bar subjected to tension and compression
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Instrumentation Amplifiers Strain gage example
Tension
Resistance of strain gage is R + R
Compression
Resistance of strain gage is RR
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Active Filters Basic filter types
Passive elements, gain < 1
Low-pass
High-pass
Bandpass
Band reject
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Active Filters With op-amps/active filters
Gain can be 1
Filter response closer to ideal
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Active Filters Low-pass (RF= R1)
Add resistor for gain > 1
11
1
1
1
1( )
1
C
out C in inC
Z j C
v v v vR ZR
j C
TF jj R C
-
+
___-
vout
C
RF = R1
I = 0
0 VR1
vin
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Active Filters High-pass (RF= R1)
Add resistor for gain > 1
-
+
___-
vout
RF = R1
I = 0
0 V
R1
vin
C
1 1
11
1
1
1
( )1
out R in inC
R Rv v v v
R Z Rj C
R CTF j
j R C
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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
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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
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Active Filters Narrowband BP
circuit
0
1
0.11251
0.1591
Rf
RC R
BWRC
-
+
___-
vout
2R
R1
C
C
___-
vin
R
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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
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Voltage Regulation Voltage regulator
Constant voltage to load
Specified current range
Specified input voltage range
Zener diode regulator
Inefficient Dissipates power
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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.
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Voltage Regulation Line Regulation
Small output
change with
change in input
RL
___-
Regulated
outputVoltageregulatorUnregulatedinput
% 100%out
in
vline regulation
v
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Voltage Regulation Load regulation
Small output voltage change with smallerRL
VNL = no-load voltage (open-circuit load)
VFL = full-load voltage (specified bymanufacturer)
% 100%NL FL
FL
V Vload regulation
V
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Voltage Regulation
Circuit toincrease
efficiency of
Zener regulator
with an op-amp
+
-
Q1Unregulated
inputRD
R1
R2
vout
+
-___
-
vin
+
-
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VoltageRegulation
Three-terminal IC
regulators 7800 series,
positive voltage
7900 series,negative voltage
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Voltage Regulation
5 V output, 7805
12 V output, 7812
5 V output, 7905
12 V output, 7912
RL
___-
Vout=12 V
A7812
Unregulated
input
+
-
+
-
___-
IN OUT
COM
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Voltage Regulation Ripple
Greatly reduced by IC regulator
Vr(in) = input ripple voltage
Vr(out) = output ripple voltage
( )
( )
20 logr in
dBr out
Vripple rejectionV
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