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ABE425 Engineering Measurement
Systems
Operational Amplifiers(OpAmps)
Dr. Tony E. Grift
Dept. of Agricultural & Biological EngineeringUniversity of Illinois
Agenda
PackagingOpAmp as a near-ideal Voltage AmplifierNon-inverting OpAmp
Virtual ground derivation
Inverting OpAmpVirtual ground derivation
Differentiating OpAmpIntegrating OpAmpVoltage follower (buffer)
Dual-in-line (DIP) and Surface Mount Device (SMD) packaging
Typical OpAmp pin assignment (741)
OpAmp as a near-ideal Voltage amplifier
Open loop amplification A very large
Input impedance very highOutput impedance very lowOpAmp has a Voltage source output
VVAVO
Non-inverting OpAmp
Ofi
i
i
O
VRR
RV
VV
VVAV
i
fii
i
fiO
i
fi
i
O
i
fi
i
O
ifi
iO
Ofi
iiO
R
RVV
R
RRV
A
V
RR
R
A
V
V
RR
RA
AV
AVRR
RAV
VRR
RVAV
1lim
1
1
1
1
i
fiO R
RVV 1
Non-inverting OpAmp : Virtual ‘ground’ principle
i
fiO R
RVV 1
i
O
VV
VVVVAV
Since no current is flowing intothe OpAmp:
f
iO
i
i
R
VV
R
V
What is the input resistance for the source ?iV
Inverting OpAmp
if
iO
if
fi
O
RR
RV
RR
RVV
V
VVAV
0
ii
fO
fi
i
fi
fi
fi
i
fi
fi
O
fi
fi
fi
iO
fi
iO
fi
fiO
VR
RV
A
RR
R
A
RR
RV
RR
RA
RR
RAV
V
RR
RAV
RR
RAV
RR
RV
RR
RVAV
lim
11
1
0
ii
fO V
R
RV
Inverting OpAmp: Virtual ground principle
0
V
VVVVAVO
ii
fO V
R
RV
i
f
i
O
f
O
i
i
R
R
V
V
R
V
R
Vi
Since no current is flowing intothe OpAmp:
What is the input resistance for the source ?iV
Capacitor equation
Charge across Capacitor (Coulomb)Capacitance value (Farad)Voltage across Capacitor (Volt)Current through Capacitor (Ampere)
dt
dQti
tVCtQ
CC
cC
*
ti
tV
C
tQ
C
c
C
Integrating Inverting OpAmp : Virtual ground
dttVRC
tV iO
1
0
V
VVVVAVO
dttVRC
tVdt
tdVRCtV
R
tV
dt
tdVC
dt
tdQti
tVtVR
tVti
iOO
i
iOCC
OCi
C
1
,
Differentiating Inverting OpAmp: Virtual ground
0
V
VVVVAVO
dt
tdVRC
dt
tdVRCtV
R
tV
dt
tdVC
dt
tdQti
R
tVti
iCO
OCCC
OC
dt
tdVRCtV i
O
Integrating Inverting OpAmp : Virtual ground
dttVRC
tV iO
1
0
V
VVVVAVO
dttVRC
tVdt
tdVRCtV
R
tV
dt
tdVC
dt
tdQti
tVtVR
tVti
iOO
i
iOCC
OCi
C
1
,
Differentiating Inverting OpAmp: Virtual ground
0
V
VVVVAVO
dt
tdVRC
dt
tdVRCtV
R
tV
dt
tdVC
dt
tdQti
R
tVti
iCO
OCCC
OC
dt
tdVRCtV i
O
Unity gain OpAmp (Voltage follower, buffer)
O iV t V t
This circuit has no amplification, but provides a very high impedance input and low impedance outputUse it after any sensor configuration to prevent loading errorsYou will need a symmetric power supply
Comparator function
SATO
SATO
VVvvif
VVvvif
OpAmp as comparator I
What could this circuit be used for if Vref can be varied?
Slope is termed ‘slew rate’
Voltage range detector
When does the LED come on?Why do we need the diodes?
How would you limit the current through the LED?
When Vin > Vref_high OR Vin < Vref, low
Comparator with hysteresis (Schmitt Trigger)
TV
OVRR
Rv
21
2
SATSATOi
SATSATOi
VRR
RvVVvvVif
VRR
RvVVvvVif
21
2
21
2
Schmitt trigger as debouncer in noisy signals
Regular comparator Schmitt trigger with hysteresis
Subtracting OpAmp
Use superposition!!Use virtual ground principle!!
First consider
23
40
4
0
3
2
2
0
vR
Rv
R
v
R
v
v
v
2v
Subtracting OpAmp
Use superposition!!Use virtual ground principle!!
Next consider
121
2
3
430
121
2
043
3
vRR
R
R
RRv
vRR
Rv
vRR
Rv
1v
Subtracting OpAmp
Add influences of both sources
23
41
21
2
3
430 v
R
Rv
RR
R
R
RRv
Current to voltage converter
INO RiV
Application current to voltage converter
Active filters combine amplification and filtering in one circuit!
What is the input impedance the source ‘sees’?
Active Low-Pass filter analysis (1st order)
2
1 1
22
2 22
2
1*
1\ \
1 1
Z R
RRj C
Z Rj C j R CR
j C
20
1
O
i
V RG
V R 2
1
O
i
V ZG
V Z
2 2 20
1 1 2 1 2 2
1 1 1
1 1 1
Z R RG G
Z R j R j j
Without C In general with impedances
02
1
1dB dB
dB
G Gj
Active High-Pass filter with OpAmp and Capacitor / Resistor pair (1st order)
What is the input impedance the source ‘sees’?
Active High-Pass filter analysis
1 1
2 2
1Z R
j C
Z R
20
1
O
i
V RG
V R 2
1
O
i
V ZG
V Z
1
2 2 2 1 10
1 1 1 11
1 1 1
Z R R j R C jG G
Z R j R C jRj C
Without C
0 11
1
1dB dB dBdB
G G jj
In general with impedances
High-pass and low-pass section separated by OpAmp
ABE425 Engineering Measurement Systems
Operational Amplifiers(OpAmps)
The End
Dept. of Agricultural & Biological EngineeringUniversity of Illinois