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Chapter 11 Op-Amp Applications

OpOp-Amp Applications Constant-gain multiplier ConstantVoltage 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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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Constant--Gain Amplifier Constant Inverting Version

more…

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Constant--Gain Amplifier Constant Noninverting Version

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Multiple--Stage Gains Multiple 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] Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Voltage Buffer Any amplifier with no gain or loss is called a unity gain amplifier. 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.

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Controlled Sources Voltage-controlled voltage source VoltageVoltage--controlled current source Voltage Current--controlled voltage source Current Current--controlled current source Current

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Voltage--Controlled Voltage Source Voltage 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 Voltage 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 Voltage

The output current is:

Io =

V1 = kV1 R1

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Current--Controlled Voltage Source Current 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 = −I 1 R L

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Current--Controlled Current Source Current 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 R R ×  1 2  V  R + R2   I o = − in  1 R 1  R 2   R  I o = − I 1 + 1  = kI R2  

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Instrumentation Circuits Some examples of instrumentation circuits using opamps: • Display driver • Instrumentation amplifier

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Display Driver

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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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— Low Filter—First First--Order

The upper cutoff frequency and voltage gain are given by:

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

f OH =

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1 2 πR 1 C 1

Av = 1+

Rf R1

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Low--Pass Filter— Low Filter—Second Second--Order

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

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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High--Pass Filter High

The cutoff frequency is determined by:

f OL =

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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1 2 πR 1 C 1

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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.

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