OPERATIONAL AMPLIFIER
BASICS
INTRODUCTION OF OP-AMP
An “Operational amplifier” is a direct coupled high-gain amplifier usually
consisting of one or more differential amplifiers and usually followed by a
level translator and output stage.
The operational amplifier is a versatile device that can be used to amplify
dc as well as ac input signals and was originally designed for computing
such mathematical functions as addition, subtraction, multiplication and
integration.
2
Characteristics of Operational
Amplifier
o Very high differential gain
o High input impedance
o Low output impedance
o Provide voltage changes (amplitude and polarity)
o Used in oscillator, filter and instrumentation
o Accumulate a very high gain by multiple stages
3
OP-AMP SYMBOL
Positive power supply
(+ve supply)
Non-inverting
Input terminal
Output terminal
Inverting input
terminal
Negative power supply
(-ve supply)
Basic Information of Op-Amp
Op-amps have five basic terminals:
1. Two input terminals:
•
Inverting terminal
•
Non-Inverting Terminal
2. One output terminal
3. Two power supply terminals. (+VE and –VE Supply)
Block Diagram of Op-Amp
Dual input
unbalanced
output
Pushpull amplifier
Non
inverting
terminal
INPUT STAGE
INTERMEDIATE
STAGE
LEVEL
SHIFTING
STAGE
Inverting
terminal
Dual input
balanced
output
Emitter Follower
OUTPUT
STAGE
output
Differential Amplifier
A Circuit which amplifies the difference between two signals is
called a differential amplifier
V0 =Ad (V1 – V2 )
Where, Ad = RF/R1
Ad= differential gain
Rf = feedback resistance
R1= input resistance
Characteristics and performance
parameters of Op-amp
• Input offset Voltage
• Input offset current
• Input bias current
• input resistance
• Output resistance
• Open loop voltage gain
• CMRR
• Output voltage swing
Input Offset Voltage
The differential voltage that must be
applied between the two input terminals of
an op-amp, to make the output voltage
zero
It is denoted by Vio For op-amp 741C the
input offset voltage is 6mV.
It can be +ve or –ve. The smaller the value,
the better the input terminals are matched.
Input offset current
The algebraic difference between the currents flowing
into the two input terminals of the op-amp
It is denoted as Iios = | Ib1 – Ib2|
It is of the Order Of 200nA
Input bias current
The average value of the two currents flowing into the
op-amp input terminals
It is expressed as
I b1  I b 2
2
Open loop Voltage gain
It is the ratio of output voltage to the differential input
voltage, when op-amp is in open loop configuration,
without any feedback. It is also called as large signal
voltage gain.
It is denoted as AOL
It is of the order of 200,000
𝑉𝑜
𝐴=
𝑉𝑖𝑑
Output Resistance
It is the equivalent resistance measured between the
output terminal of the op-amp and ground
It is denoted as Ro
Its value is 75Ω
Common Mode Rejection Ratio
The average value of the two currents flowing into the
op-amp input terminals
It is expressed as
CMRR = Ad / Ac
Its value is 90 db
Input Resistance
It is the equivalent resistance measured at either the
inverting or non-inverting input terminal with the
other input terminal grounded
It is denoted as Ri.
Its value is of the order of MΩ
Output Voltage swing
The op-amp output voltage gets saturated at +Vcc
and –VEE and it cannot produce output voltage
more than +Vcc and –VEE. Practically voltages +Vsat
and –Vsat are slightly less than +Vcc and –VEE .
For op-amp 741C the saturation voltages are
+ 13V for supply voltages + 15V
Slew Rate
-
It is defined as the maximum rate of change of output voltage per unit
time and is expressed in volts per microseconds.
-
𝑆𝑅 = 𝑜
𝑑𝑡
It indicates how rapidly the output of op-amp can change in response
to changes in the input frequency.
The slew rate changes with a change in voltage gain and is normally
specified at unity gain.
The o/p of an op amp does not change instantaneously.
The rate of change of o/p of an op amp is limited (about 0.5 V/ µ sec)
So, if we want to change the o/p voltage from 0 to 10 V, it would take
20 µs
-
-
𝑑𝑉
Equivalent circuit of an opamp
The Ideal Opamp
1.Infinite voltage gain
2.Infinite input resistance
3.Zero output resistance
4.Zero output voltage when input voltage is
zero.
5.Infinite bandwidth
Simple Op Amp Comparator
Here an op amp is being supplied by ±15 V, and is driving a 10 kΩ
load. As seen in our model of figure 1, Vout should equal the
differential input voltage times the op amp's gain,
Avol. Vout =Avol (Vin+ −Vin )
Vout =200,000×(0.1V−0V)
Vout =20,000V
The op amp cannot produce 20,000 V.
The data sheet lists a maximum output swing of only ±13.5 V
when using ±15 V supplies.
The output will be truncated at 13.5 V.
If the input signal is reduced to only 1 mV, the output will still be
clipped at 13.5 V.
This holds true even if we apply a signal to the inverting input,
as in Figure 2.
Vout =Avol (Vin+ −Vin )
Vout =200,000×(0.5V−0.3V)
Vout =40,000V
Vout =13.5V, due to clipping
The value of gain A depends on the
frequency of the i/p signal and is very high at
low frequencies.
At DC, (f = 0 Hz), gain A is about 105 .
But the gain decreases with frequency.
Open loop configuration of op-amp
•
The voltage transfer curve indicates the inability of op-amp to
work as a linear small signal amplifier in the open loop mode
•
Such an open loop behaviour of the op-amp finds some rare
applications like voltage comparator, zero crossing detector
etc.
Open loop op-amp configurations
•
The configuration in which output depends on input, but output has no
effect on the input is called open loop configuration.
•
No feed back from output to input is used in such configuration.
•
The opamp works as high gain amplifier
•
The op-amp can be used in three modes in open loop configuration
they are
1.
Differential amplifier
2.
Inverting amplifier
3.
Non inverting amplifier
Closed loop operation of op-amp
The utility of the op-amp can be increased considerably by
operating in closed loop mode. The closed loop operation is
possible with the help of feedback. The feedback allows to
feed some part of the output back to the input terminals. In
the linear applications, the op-amp is always used with
negative feedback. The negative feedback helps in controlling
gain, which otherwise drives the op-amp out of its linear
range, even for a small noise voltage at the input terminals
Differential Amplifier
 The opamp amplifies the difference between the two
input signals hence this configuration is called the
differential amplifier.
 𝑉𝑜 = 𝐴(𝑉𝑖𝑛1 − 𝑉𝑖𝑛2 )
Inverting Amplifier
The amplifier in which the output is inverted i.e. having 180o
phase shift with respect to the input is called an inverting
amplifier
Vo = -AOL Vin2
Non-inverting Amplifier
The amplifier in which the output is amplified without any
phase shift in between input and output is called non
inverting amplifier
Vo = AOL Vin1
Feedback amplifier

Consists of : opamp and feedback circuit

Based on application of amplifier the feedback circuit can take any form.

There are four ways to connect the feedback circuit.
- Voltage series feedback
- Voltage shunt feedback
- Current series feedback
- Current shunt feedback