Chapter 11 Op-Amp Applications - Webstaff.kmutt.ac.th

Chapter 11 

Op-Amp Applications

Op-Amp Applications 

Constant-gain multiplier 

Voltage summing 

Voltage buffer 

Controlled sources 

Instrumentation circuits 

Active filters 

Electronic Devices and Circuit Theory, 10/e 

Robert L. Boylestad and Louis Nashelsky 

2 

Copyright ©2009 by Pearson Education, Inc. 

Upper Saddle River, New Jersey 07458 • All rights reserved.

Constant-Gain Amplifier 

Inverting Version 

more… 



3 



Constant-Gain Amplifier 

Noninverting Version 



4 



Multiple-Stage Gains 

The total gain (3-stages) is given by: 

or 

A = A1A2A3 

⎛ R ⎞ ⎞ ⎞ 

⎜ 

⎛ ⎟⎜ 

⎛ = ⎜ + 

f 

R 

⎟ − 

f 

R − 

f 

A 1 

⎟ 

⎝ R ⎠⎝ 

R2 ⎠⎝ 

R3 

1 

⎠ 



5 



Voltage Summing 

The output is the sum 

of individual signals 

times the gain: 

V 

o 

R 

⎜⎛ 

f 

R 

f 

R 

f 

= −⎜⎜ 

V1 

+ 

V2 

+ 

V3 

⎝ R1 

R 2 R 3 

⎟⎞ 

⎟⎟ 

⎠ 

[Formula 14.3] 



6 



Voltage Buffer 

Any amplifier with no gain or loss is called a unity gain 

amplifier. 

The advantages of using a unity gain amplifier: 

• Very high input impedance 

• Very low output impedance 

Realistically these circuits 

are designed using equal 

resistors (R 1 = R f ) to avoid 

problems with offset 

voltages. 



7 



Controlled Sources 

Voltage-controlled voltage source 

Voltage-controlled current source 

Current-controlled voltage source 

Current-controlled current source 



8 



Voltage-Controlled Voltage Source 

The output voltage 

is the gain times the 

input voltage. What 

makes an op-amp 

different from other 

amplifiers is its 

impedance 

characteristics and 

gain calculations 

that depend solely 

on external 

resistors. 

Noninverting Amplifier Version 

more… 



9 



Voltage-Controlled Voltage Source 

The output voltage 

is the gain times the 

input voltage. What 

makes an op-amp 

different from other 

amplifiers is its 

impedance 

characteristics and 

gain calculations 

that depend solely 

on external 

resistors. 

Inverting Amplifier Version 



10 



Voltage-Controlled Current Source 

The output current 

is: 

V1 

I o 

= 

= 

R 

1 

kV 

1 



11 



Current-Controlled Controlled Voltage Source 

This is simply another way 

of applying the op-amp 

operation. Whether the 

input is a current 

determined by V in /R 1 or as 

I 1 : 

or 

V 

out 

= 

− 

R 

f 

R 

1 

V = −I 

out 

1 

V 

R 

in 

L 



12 



Current-Controlled Controlled Current Source 

This circuit may appear 

more complicated than 

the others but it is really 

the same thing. 

V 

V 

V 

out 

out 

R 

f 

out 

R 

f 

⎛ 

Rf 

⎞ 

= −⎜⎜ 

V 

R ⎟⎟ 

⎝ in ⎠ 

V 

= − 

in 

R || R 

V 

= − 

R 

1 

in 

in 

in 

2 

I 

I 

I 

I 

o 

o 

o 

o 

= 

= 

= 

= 

Vin 

− 

R || R 

−V 

in 

V 

− 

R 

1 

in 

1 

⎛ 

−I⎜⎜ 

1 + 

⎝ 

⎛ R 

⎜⎜ 

⎝ R 

1 

1 

R1 

R 

⎜⎜ 

⎛ + 

⎝ R 2 

R 

R 

2 

+ R 

× R 

1 

2 

2 

2 

⎞ 

⎟⎟ 

⎠ 

2 

⎞ 

⎟⎟ 

⎠ 

⎞ 

⎟⎟ = kI 

⎠ 



13 



Instrumentation Circuits 

Some examples of instrumentation circuits using opamps: 

• Display driver 

• Instrumentation amplifier 



14 



Display Driver 



15 



Instrumentation Amplifier 

For all Rs at the same value (except R p ): 

V 

o 

= 

⎛ 

⎜⎜ 1 + 

⎝ 

2R 

R 

P 

⎞ 

⎟⎟ 

⎠ 

( V − V ) = k( V − V ) 

1 

2 

1 

2 



16 



Active Filters 

Adding capacitors to op-amp circuits provides external control of the 

cutoff frequencies. The op-amp active filter provides controllable 

cutoff frequencies and controllable gain. 

• Low-pass filter 

• High-pass filter 

• Bandpass filter 



17 



Low-Pass Filter—First 

First-Order 

The upper cutoff frequency 

and voltage gain are given 

by: 

f = 

OH 

1 

2πR 

1 

C 

1 

A = 1 + 

v 

R 

R 

f 

1 



18 



Low-Pass Filter—Second 

Second-Order 

The roll-off can be made steeper by adding more RC networks. 



19 



High-Pass Filter 

The cutoff frequency is determined by: 

f = 

OL 

1 

2πR 

1 

C 

1 



20 



Bandpass Filter 

There are two cutoff 

frequencies: upper and 

lower. They can be 

calculated using the same 

low-pass cutoff and highpass 

cutoff frequency 

formulas in the 

appropriate sections. 



21

Chapter 11 Op-Amp Applications - Webstaff.kmutt.ac.th

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