Download Chapter 11 Op-Amp Applications

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
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Constant-Gain Amplifier
Inverting Version
more…
Electronic Devices and Circuit Theory, 10/e
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Constant-Gain Amplifier
Noninverting Version
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Multiple-Stage Gains
The total gain (3-stages) is given by:
A  A1 A 2 A 3
or
 R f  R f  R f 
 
A   1 
 

 R 1  R2  R3 
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Voltage Summing
The output is the sum
of individual signals
times the gain:
R

R
R
Vo   f V1  f V2  f V3 
R2
R3
 R1

[Formula 14.3]
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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 (R1 = Rf) to avoid
problems with offset
voltages.
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Controlled Sources
Voltage-controlled voltage source
Voltage-controlled current source
Current-controlled voltage source
Current-controlled current source
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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…
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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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.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Inverting Amplifier Version
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Voltage-Controlled Current Source
The output current
is:
Io 
V1
 kV1
R1
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Current-Controlled Voltage Source
This is simply another way
of applying the op-amp
operation. Whether the
input is a current
determined by Vin/R1 or as
I1 :
Vout 
 Rf
Vin
R1
or
Vout  I1R L
Electronic Devices and Circuit Theory, 10/e
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Current-Controlled Current Source
This circuit may appear
more complicated than
the others but it is really
the same thing.
 R 
Vout    f  Vin
 R in 
Vout
Vin

Rf
R 1 || R 2
Vout
V
  in
Rf
R in
Io  
Vin
R 1 || R 2
 R  R2 

I o   Vin  1
 R1  R 2 
V  R  R2 

I o   in  1
R 1  R 2 

R 
I o   I 1  1   kI
R2 

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Instrumentation Circuits
Some examples of instrumentation circuits using opamps:
• Display driver
• Instrumentation amplifier
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Display Driver
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Instrumentation Amplifier
For all Rs at the same value (except Rp):

2R 
V1  V2   k V1  V2 
Vo   1 
RP 

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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
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Low-Pass Filter—First-Order
The upper cutoff frequency
and voltage gain are given
by:
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f OH 
18
1
2πR 1C1
Av  1 
Rf
R1
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Low-Pass Filter—Second-Order
The roll-off can be made steeper by adding more RC networks.
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High-Pass Filter
The cutoff frequency is determined by:
f OL 
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1
2πR 1C1
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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.
Electronic Devices and Circuit Theory, 10/e
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