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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 )(