Chapter 18Chapter 18Operational

Amplifiers (Op-Amps)

Introduction to Operational

Amplifiers

• The standard Operational amplifier has two input

terminals, the inverting (-) and noninverting (+)

FIGURE 18-3 Practical op-amp representation. The ideal op-amp has:

•Infinite voltage gain

•An infinite input impedance

•Does not load the driving

source

• Low (Zero) output impedance

Thomas L. Floyd

Electronics Fundamentals, 6e

Electric Circuit Fundamentals, 6e

Copyright ©2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

High

Low

High Gain

The Operational Amplifier

• A typical op-amp is made up of three types

of amplifier circuits:

– Differential amplifier

• Input stage for the op-amp; it has two inputs and

provides amplification of the “difference voltage”

– Voltage amplifier

• Usually a class A amplifier that provides gain

– Push-pull amplifier

• Class B amplifier is used for the output stage

Basic internal arrangement of an op-amp

Thomas L. Floyd

Electronics Fundamentals, 6e

Electric Circuit Fundamentals, 6e

Copyright ©2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

The Differential Amplifier

• The heart of the Op-Amp is the Differential Amplifier (Diff-Amp)

• It is the first stage of the Op-Amp and is the basis of their inverting and non-inverting

• It is the first stage of the Op-Amp and is the basis of their inverting and non-inverting capability

• They also have a positive and negative power supply so they can take ac signal inputs directly to an ac output

The Differential Amplifier

• A basic differential amplifier is shown below

– There are two outputs

• It requires a negative and positive supply voltage

Not Used

Signal on Input 1 is Inverted on Output 1 and Non-Inverted on Output 2

Non-Inverting Input (+)

OutputNot Used

Signal on Input 1 is Inverted on Output 1 and Non-Inverted on Output 2

Signal on Input 2 is Inverted on Output 2 and Non-Inverted on Output 1

Inverting Input (-)

OutputNot Used

Negative Feedback Required

• Since the inherent open-loop gain of a typical op-

amp is very high, usually > 100,000, an extremely

small difference in the two input voltages drives

the op-amp into its saturated output statesthe op-amp into its saturated output states

– With negative feedback, the overall closed-loop gain

(Acl) can be reduced and controlled so that the op-amp

can function as a linear amplifier

Without negative feedback, an extremely small difference in the two input voltages

drives the op-amp to its output limits and it becomes nonlinear.

Thomas L. Floyd

Electronics Fundamentals, 6e

Electric Circuit Fundamentals, 6e

Copyright ©2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

•Vout is limited by the positive

and negative voltage supply

•For example, if the supply

was +/-15 Volts, the output

signal would clip at +/- 15

Volts

(+15V)

(-15V)

Negative Feedback

• A portion of the output is fed back to the Input

• The Feedback Voltage effectively subtracts effectively subtracts from the input voltage

• A Higher Negative Feedback produces a Lower Amplifier Output

Generic Block

Diagram

Op-amp Configurations with

Negative Feedback

• Closed-loop voltage gain

– Closed-loop voltage gain is the voltage gain of an op-

amp with negative feedback

– An external feedback network connects the output to – An external feedback network connects the output to

the inverting input

– The closed-loop voltage gain is determined by the

component values in the feedback network

– There are Non-Inverting and Inverting configurations

Non-Inverting Op-amp Configuration

• An op-amp connected as a

non-inverting amplifier

has the input signal

applied to the non-

inverting inputinverting input

• A portion of the output

applied back to the

inverting input through the

feedback network

• It is internally inverted

which effectively subtracts

from the input voltage

Non-Inverting Op-amp Configuration

Thomas L. Floyd

Electronics Fundamentals, 6e

Electric Circuit Fundamentals, 6e

Copyright ©2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

Differential input, Vin – Vf.

Thomas L. Floyd

Electronics Fundamentals, 6e

Electric Circuit Fundamentals, 6e

Copyright ©2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

1)( +=

i

f

NIclR

RA

• The feedback Voltage (Vf) is determined by the voltage

divider equation as:

Non-Inverting Op-amp

Configuration

out

fi

if V

RR

RV

+

=

• The closed-loop voltage gain Acl(NI) of the non-inverting

(NI) amplifier can be calculated with the following formula:

1)( +=

i

f

NIclR

RA

fi RR

+

Non-Inverting Op-amp Voltage

Follower• The voltage-follower is a

special non-inverting amplifier were 100% of the output voltage is fed back to the inverting inputback to the inverting input

• The gain of a Voltage Follower is 1

• Voltage Followers are used to buffer between a circuit with a high output impedance and a circuit with a low input impedance.

1)( =VFclA

Inverting Op-amp Configurations

• An op-amp connected an

inverting amplifier

• A portion of the inverted output

is applied directly back to the

inverting input through the

feedback network which feedback network which

effectively subtracts from the

input voltage

– Closed-loop gain is:

– The minus symbol means the

signal has been inverted

−=

i

f

IclR

RA )(