The operational amplifier, cont.

Has a multitude of circuit applications

Linear applications
 Inverter, summer, integrator, etc.

Non-linear applications
 Comparator
ECE 194
S’01
Introduction to Engineering II
Arizona State University
1
Classwork #2
R
a
i
R
R
f
f
b
R
c
i
n
v
-V
n
cc
v
o
+
v
a
+
+
v
+
b
v
v
p
+V
cc
c
Find vo
ECE 194
S’01
Introduction to Engineering II
Arizona State University
2
Inverting summing amplifier
iA  iB  iC  iF  in  0
iA, B, C
vA, B, C  vn v A, B,C


RA, B,C
RA,B,C
vO  vn vo
iF 

RF
RF
RF
RF
RF
vO   v A  vB  vC
RA
RB
RC
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Op amp non-linear applications
The output voltage is a function of the input voltage difference
v
o
V
cc
v
_
_
p
v
n
V
cc
Op amp can be forced to work in the linear region with feedback
Without feedback, the operation is quite different
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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The comparator

A comparator is a circuit that monitors two input
voltages



One is called the reference voltage, and the other the
input voltage
The output voltage depends upon the relation
between the reference voltage and the input
voltage, i.e., whether Vin is larger or smaller than
Vref
Op amps operated without feedback can act as
comparators
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Comparator
Reference voltage to positive input
Inverting com parator
V
out
V
in
A
V
out
V
ref
+
B
V
ref
V
in
Reference voltage to negative input
V
ou t
Non-i nvertin g compa rator
A
V
ref
V
in
V
out
B
+
V
ref
ECE 194
S’01
Introduction to Engineering II
Arizona State University
V
in
6
Comparator circuits
V
ref
-
Electrical sign als
V
in
S’01
Action
+
Physical signals
ECE 194
V
out
Introduction to Engineering II
Physical outputs
Arizona State University
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Generic comparator circuit
Physical and electrical
input signals
R
R1
vin 
VCC
R1  RA
ECE 194
S’01
V
CC
Physical and electrical
outputs
R
d
R
A
LED
B
+
R
1
R
2
Introduction to Engineering II
vref
R2

VCC
R2  RB
Arizona State University
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Comparator circuits
What are RA and RB? Circuit components that convert
physical signals to electrical signals by having
electrical properties that change with external physical
changes.
Examples:
Thermistors - temperature sensitive resistors
Photoconductors - light sensitive resistors
Strain gauges - strain sensitive resistors
Microphones - sound sensitive components
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Comparator circuits
Output component examples
LEDs driven by op amp output
Power transistor stages to drive LEDs, buzzers, relays
Other amplifier stages
Voltage to frequency converters
Analog to digital converters
Multimeters
Motors
Timers, clocks, recorders
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Additional circuit components
Variable resistor
Diode
I
d
+
D
V
d
R
R
1
_
=
V
I
R
d
2
RV = R1 + R2
V
d
V
t
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Additional circuit components
Bipolar junction transistor
Transistor
Field effect transistor
FET
I
M
I
D
C
D
Q
IC ≈ I E
I
V
ID = I S
S
G +
B
_
I
E
I
S
Large current (VG > 0)
Large current (IB > 0)
IC
ECE 194
Zero (IB = 0)
S’01
Introduction to Engineering II
ID
Zero (VG = 0)
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Additional circuit components
Sensing resistor
Relay
R
R = R(Temperature)
R = R(Pressure)
R = R(Light)
ECE 194
S’01
Introduction to Engineering II
Electromechanical device a small current is used to control
a large current
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Example comparator circuit
Light activated switch
+ 9V
R
4
470
R
1
100 K
_
Q
1
+
R
741 OPAMP
3
4.7 K
2N222 2

LED
PC
1
LED is on when
input is below Vref
R
2
1M
Resistance of photoconductor
decreases when illuminated
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Classwork
Building upon ideas from previous circuit,
design a Dark Activated Switch
ECE 194
S’01
Introduction to Engineering II
Arizona State University
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Classwork
Build and test this circuit use a voltmeter to measure voltages in circuit
9V
1K
LED
10 0K
+
10 K to 100K
10 0K
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S’01
Introduction to Engineering II
Arizona State University
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