Operational Amplifier Configurations

1
Operational Amplifier Configurations
CSE598A/EE597G Spring 2006

• Jaehyun Lim, Kyusun Choi
• Department of Computer Science and Engineering
• The Pennsylvania State University

2
Ideal Op-Amp

• infinite voltage gain (A 8, V-V for finite
  VO)
• infinite input impedance (i- i 0)
• zero output impedance
• infinite bandwidth
• zero input offset

i-
V-
VO
V
i
VO A(V - V-)
3
Ideal Op-Amp

• Inverting Amplifier Configuration
• current through R1 current through Rf (? i-i
  0) (Vin-V1) / R1 (V--VO) / Rf
• and V- V 0
• VO -(Rf / R1)Vin

Rf
R1
V-
Vin
VO
4
Ideal Op-Amp

• Non-Inverting Amplifier Configuration
• current through R1 current through Rf (? i-i
  0) (VO-V-) / Rf V- / R1
• and V- V Vin
• VO (1 Rf / R1)Vin

Vin
VO
V-
Rf
R1
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Ideal Op-Amp

• Voltage Follower (Unit Gain Buffer)
• unit-gain non-inverting amplifier
• VO Vin

VO
Vin
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Non-Ideal Op-Amp

• Real World
• gain is not infinite
• ? V- ? V
• gate current ? i ? 0, i- ? 0 (but still
  negligible)
• finite bandwidth ? requires frequency
  compensation

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Non-Ideal Op-Amp

• Inverting Amplifier Configuration

Rf
R1
V-
Vin
VO
-Rf/R1
VO A(V - V-) ? V- -VO / A (Vin - V-) / R1
(V- - VO) / Rf
VO
Vin
1(1Rf/R1)/A
note that if A ? 8, VO ? -(Rf / R1)Vin
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Non-Ideal Op-Amp

• Non-Inverting Amplifier Configuration

Vin
VO
V-
A
VO
Vin
R1
Rf
1
A
R1
R1Rf
note that if A ? 8, VO ? (1Rf / R1)Vin
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Op-Amp with Single Supply

• Inverting Amplifier Configuration

Rf
R1
Vin
VO
R
Vdd / 2
R
10
Op-Amp with Single Supply

• Non-Inverting Amplifier Configuration

Vin
VO
R
Vdd / 2
Rf
R
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Op-Amp with Single Supply

• Amplifier Design
• well-designed amplifier should have a transition
  point centered at Vdd / 2

Vdd
V
VO
Vdd/2
V
VO
Vdd / 2
V
V
Vdd / 2
12
Op-Amp with Single Supply

• Examples from Last Class
• voltage comparator
• V 2.5 V ? VO ?
• V 2.501 V ? VO ?
• V 2.499 V ? VO ?

5 V
2.5 V
VO
V
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Op-Amp with Single Supply

• Examples from Last Class
• ideal case (gain 8)
• V 2.5 V ? VO 2.5 V(V V- for finite VO
  and VO centered at 2.5 V)
• V 2.501 V ? VO 2.5 A(V-V-) 8 ? VO
  5 V (limited to supply)
• V 2.449 V ? VO 2.5 A(V-V-) -8 ? VO
  0 V (limited to supply)

5 V
2.5 V
VO
8
V
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Op-Amp with Single Supply

• Examples from Last Class
• gain A 1000
• V 2.5 V ? VO 2.5 V (VO centered at 2.5
  V)
• V 2.501 V ? VO 2.5 1000(V-V-) 3.5 V
• V 2.449 V ? VO 2.5 1000(V-V-) 1.5 V

5 V
2.5 V
VO
A
V
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Op-Amp with Single Supply

• HSpice Simulation

• .DC analysis
• centered at 2.5 V
• gain 1,175
• V 2.501 V ? VO 3.421 V
• V 2.449 V ? VO 1.324 V

V
VO