ECE211: Electronic Circuits and Network

Analysis

Lecture 11
Dr. Ananda Y R
Assistant Professor

Department of Electronics and Communication Engineering

IIIT Bangalore

9th September 2024

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

An operational amplifier (often op-amp or opamp) is a

DC-coupled electronic voltage amplifier with a differential
input, a (usually) single-ended output, and an extremely high
gain. Its name comes from its original use of performing
mathematical operations in analog computers.
Op-amps are used in various applications like signal
conditioning, filtering, and mathematical operations such as
addition, subtraction, integration, and differentiation.
They are essential in analog electronics due to their high gain
and versatility. Understanding op-amps is crucial for designing
and troubleshooting analog circuits, making them a
fundamental component in electronics.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Brief History
In 1947, the first operational amplifier developed from vaccum
tubes by John R. Ragazzini of Columbia University.
With the development of silicon-based transistor, the concept
of ICs became a reality. In the early 1960s, Robert J. Wildar
of Fairchild Semiconductor fabricated opamp, the µA702.
In 1968, the µA741 was released, leading it to wide
production.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics
Op-amp symbol:
An Operational Amplifier is basically a three-terminal device which consists of
two high impedance inputs. One of the inputs is called the Inverting Input,
marked with a negative or “minus” sign, (–). The other input is called the
Non-inverting Input, marked with a positive or “plus” sign (+).
The V+ and V- power supply terminals are connected to the positive and
negative terminals of a DC voltage source respectively. The common terminal of
the V+ and V- is connected to a reference point or ground, else twice the
supply voltage may damage the op-amp.

Figure: Opamp symbol

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Types of Op-Amps:
An op-amp has countless applications and forms the basic building block of linear and
non-linear analogue systems. Some of the types of op-amp include:
A differential amplifier, which is a circuit that amplifies the difference between
two signals.
The instrumentation amplifier, which is usually built from three op-amps and
helps amplify the output of a transducer (consisting of measured physical
quantities).
The isolation amplifier, which is like an instrumentation amplifier, but having
tolerance to common-mode voltages (that destroy an ordinary op-amp).
A negative-feedback amplifier, which is usually built from one or more op-amps
and a resistive feedback network.
Power amplifiers to amplify small signals received from an input source such as
microphone or antenna.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Operational Amplifier Basics of Classification:

Voltage – Voltage “in” and Voltage “out”
Current – Current “in” and Current “out”
Transconductance – Voltage “in” and Current “out”
Transresistance – Current “in” and Voltage “out”
Since most of the circuits dealing with operational amplifiers are
voltage amplifiers, we will limit to voltage amplifiers only, (Vin and
Vout).
The output voltage signal from an Operational Amplifier is the
difference between the signals being applied to its two individual
inputs. In other words, an op-amps output signal is the difference
between the two input signals as the input stage of an Operational
Amplifier is in fact a differential amplifier.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics
Block diagram of Op-amp:

OPAMP is basically a differential amp i.e., it will amplify the voltage which is
differentially present between its i/p terminals.
Input stage: The i/p stage is a dual i/p, balanced o/p differential amp. The 2
i/p are inverting and non- inverting i/p terminals. This stage provides most of
the voltage gain of the OP-AMP and decides the value of i/p resistance Ri .
Intermediate stage: This stage is dual input, unbalanced output differential
amplifier, which is driven by the output of first stage.
Level- shifting stage: Due to direct coupling used between the 1st 2 stages, the
i/p of level shifting stage is an amplified signal with some non-zero dc level.
Level shifting stage is used to bring dc level to zero volts with respect to gnd.
Output stage: This stage is normally a complementary o/p stage. It increases
the magnitude of voltage and raises the current supplying capability of
OP-AMP. It also ensures that the o/p resistance of OPAMP is low.
Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Block diagram of Op-amp:

Input stage:
1 Dual i/p balanced differential amplifier
2 Reject noise
3 Provide high input impedance
4 Provide high voltage gain
Intermediate stage:
1 Dual i/p unbalanced o/p differential amplifier
2 Provide very high voltage gain
Level- shifting stage:
1 Emitter follower
2 High i/p resistance
3 Low o/p resistance
4 Set DC level to zero

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Direct coupling or DC coupling:

In electronics, direct coupling or DC coupling (also called conductive coupling
and galvanic coupling) is the transfer of electrical energy by means of physical
contact via a conductive medium, in contrast to inductive coupling and
capacitive coupling.
It is a way of interconnecting two circuits such that, in addition to transferring
the AC signal (or information), the first circuit also provides DC bias to the
second. Thus, DC blocking capacitors are not used or needed to interconnect
the circuits.
Conductive coupling passes the full spectrum of frequencies including direct
current.
Such coupling may be achieved by a wire, resistor, or common terminal, such as
a binding post or metallic bonding.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

What is Direct Coupled Amplifier?

A DC amplifier is the device where the output one stage is directly coupled to
the input od the next stage so that even zero range of frequency signals are
permitted. For the amplification purpose of a minimal range of frequencies, like
thermocouple or photoelectric currents, these DC amplifiers are used.
As the name itself specifies that, there is no need of any kind of coupling device
to couple input and output stages.
The only means of DC amplifiers is not for direct current amplification, it can
be even employed for both alternating and direct current amplification.
DC amplifiers got more prominence as because these are the only suitable
devices for the amplification of zero frequency signals.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

DC Amplifier Characteristics:
The characteristics of dc amplifier include the following.
The frequency response range of DC amplifiers is the best
The constructional equipment and cost is not expensive
The design of DC amplifiers is so streamlined
Not much weight and space are necessary
When compared with another coupling method, the direct coupling method
offers good impedance matching

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Advantages of Direct Coupled Amplifier:

There are extensive benefits of employing a direct-coupled amplifier and those are
stated as
As because a minimal number of components are there in the constructional
design of DC amplifiers, the arrangement and size of the circuit are less.
Because of less circuitry arrangement, the cost is also less
It is employed to boost up the levels of either minimal or zero level frequency
signals
Also, there are a smaller number of resistors usage in the circuit.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics
Comparison of Coupling Methods:
When various coupling methods like a transformer, direct and RC couplings are

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Applications of DC Amplifier:
Employed in TV receivers
Computers
Electronic devices like speakers, Audio amplifiers and others
Regulation devices
Differential, pulse, and operational amplifiers

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Operational Amplifiers (or) Op-amps Basics

Schematic Symbol of Op-Amp:

Operational amplifiers have three main terminals: two high input
impedance inputs and one low impedance output. The
non-inverting input terminal is denoted by positive sign and the
inverting input terminal is denoted by a negative sign. Other than
input-output terminals there are two Op-Amp power supply
terminal denoted by (+VCC) and (-VCC).

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Working Principle of Operational Amplifier

Open Loop Operation:

In the open loop operation there is no feedback provided in the amplifier circuit.
Therefore, two signals one at inverting terminal and the other at the
non-inverting terminal applied then, the Op-Amp amplifies the difference of the
two applied signals. This difference of the two input signals is called as
differential input voltage.
The output of Op-Amp is given by
Vout = Aopen loop (V1 − V2 )
Where,
Vout =output voltage
Aopen loop =Open Loop gain of Op-Amp
V1 =Voltage at the non-inverting terminal
V2 =Voltage at the inverting terminal
(V1 − V2 )=Differential input voltage
The output of the Op-Amp is non-zero only when the differential input voltage
is non-zero i.e. V1 and V2 are not equal. The open loop gain (A) of Op-Amp is
very high. Thus an open loop Op-Amp can amplify a small differential input
signal to a high value. The Op-Amp can amplify the input signal to a very high
value but cannot exceed the supply voltage of operational amplifier.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Working Principle of Operational Amplifier
Closed Loop Operation:

When the Op-Amp provided with a feedback signal is known as closed loop
operational amplifier. The feedback path feeds the output signal to the input.
The output equation for the closed loop operation is given by
Vout = Aclose loop (V1 − V2 )
When the feedback is connected to non-inverting terminal then feedback is
called as the positive feedback. The positive feedback is used in oscillator
applications.
When the feedback is connected to inverting terminal then it is called as
negative feedback. The negative feedback is used the amplification applications.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Working Principle of Operational Amplifier

Common-mode Operation:

−
Vo
+
Vi

In this mode, the same signal is applied to both the terminals as shown in Fig.
The two inputs will cause outputs with equal, but opposite polarity. Hence,
ideally the output should be zero. Practically, a small output signal will exit.
It is important to note that signals which have opposite polarity at inputs, are
amplified, while those inputs which have same polarity and which are equal are
rejected. This feature is called common-mode rejection.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Working Principle of Operational Amplifier
Basic differential amplifier:

+VCC

Vo1 Vo2
Vi1 Vi2
Vo= Vo1- Vo2

RE

-VEE

The basic differential amplifier is shown in Fig. The circuit has two inputs and
two outputs.
The different modes in which it can be operated are as follows
Input is applied to one of the terminals and other one is grounded. This is
single-ended operation.
Signals of opposite polarity are applied to the two input signals. This is
double-ended differential mode.
Same input is applied to both terminals resulting in common-mode.
Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Working Principle of Operational Amplifier
Basic differential amplifier:
+VCC

Vo1 Vo2
Vi1 Vi2
Vo= Vo1- Vo2

RE

-VEE

The main feature of the differential amplifier is the large gain when signals of
opposite polarity are applied to the inputs and the very small gain when
common input is applied. The ratio of the differential gain to common gain is
called common-mode rejection ration (CMRR).
Ad
CMRR =
Ac
where Ad is differential gain and Ac is common-mode gain. Note that the
differential amplifier of Fig. needs two input power supplies: +VCC and −V EE
for biasing.
Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Working Principle of Operational Amplifier

AC equivalent of Op-Amp:

Ro
AdVd
Vd Ri Ad Vd Vo Vd Vo

(a) Practical (b) Ideal

Fig. (a) is the equivalent circuit of a practical op-amp. The input signal applied
between the input terminals sees an input impedance Ri , which is very high.
The voltage is amplified to Ad Vd and taken through the output impedance Ro ,
which is typically low, to the output terminal.
The ideal op-amp has infinite input impedance and zero output impedance, and
infinite gain is shown in Fig. (b).

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Characteristic of op-amp

1. Differential gain Ad . Ad is the gain with which the op-amp

amplifies the difference between two input signals.

Vd = V1 − V2

where V1 is input to non-inverting terminal and V2 is input to

inverting terminal.
Vo = Ad Vd
Vo
Ad =
Vd
In decibels, Ad = 20 log10 (Ad ) dB

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Characteristic of op-amp

2. Common mode gain, Ac . When we apply equal voltages,

V1 = V2 , ideally Vo = 0. But in a practical op-amp, Vo = 0
depends not only on the difference of the two input signals,
but also on the average of the common signal.
V1 + V2
Vc =
2
The gain with which it amplifies the common mode signal is
called the common mode gain, Ac .

Vo = Ac Vc

The total output is given by

Vo = Ad Vd + Ac Vc

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Characteristic of op-amp

3. Common Mode Rejection Ratio (CMRR). It is defined as

Ad
CMRR =
Ac

In decibels, CMRR = 20 log10 ( AAdc ) dB

It indicates the ability of the op-amp to reject common mode
signals. Since noise is generally common to both inputs, the
op-amp tends to attenuate noise.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Characteristic of op-amp
4. Open loop voltage gain AOL . It is the voltage gain of the
op-amp with no feedback. In practical op-amps it is around
105 to 106 .
5. Input impedance. It is the impedance looking from the input
terminals of the op-amp. It is around 1 MΩ.
6. Output impedance. It is the impedance looking from the
output terminals of the op-am. It is around 50 Ω - 75 Ω.
7. Bandwidth BW. The bandwidth of a practical op-amp is
around 1 MHz.
8. Input offset voltage vios . When both the input terminals of
the op-amp are grounded, ideally the output voltage should be
zero. But in a practical op-amp a small voltage exists at the
output. The input voltage required to be applied to any one
of the inputs, to make the small output voltage zero, is called
the input offset voltage. Practically, it is around 2 mV.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Characteristic of op-amp

9. Slew rate S. Slew rate is defined as the maximum rate of

change of output voltage with time.
dVo
S= |max
dt
It is expressed in V/µs. Typical value is around 0.5 V/µs.
10. Power supply rejection ratio (PSRR). It is defined as the
ratio of the change in output voltage corresponding to change
in one of the power supply voltages, with all other voltages
held constant. It is also called power supply sensitivity. It is
expressed in µV/V or in decibels. Typical values are around
150 µV/V which means the output changes by 150 µV for a 1
V change in the supply voltage.

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Ideal op-amp characteristics

An ideal op amp is usually considered to have the following

characteristics:
1 Infinite open-loop gain G = vout /vin
2 Infinite input impedance Rin , and so zero input current
3 Zero input offset voltage
4 Infinite output voltage range
5 Infinite bandwidth with zero phase shift and infinite slew rate
6 Zero output impedance Rout , and so infinite output current
range
7 Zero noise
8 Infinite common-mode rejection ratio (CMRR)
9 Zero power supply rejection ratio (PSRR).

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis
Ideal and practical op-amp characteristics

Lecture 11 Dr. Ananda Y R Assistant Professor ECE211: Electronic Circuits and Network Analysis