UNIT 4

Operational amplifier
(OPAMP)
Prepared by
Mrs. M. A.
Maid
Ass.
 Syllabus
• Block Diagram
• Differential amplifier analysis for Dual Input
Balanced output mode
1. AC Analysis
2. DC Analysis
• Level Shifter
• OPAMP Parameter
• Current Mirror
• OPAMP Characteristics(AC & DC)
 Syllabus
• Voltage Series And Voltage Shunt Feedback
Amplifiers
• Effect on Ri, Ro, Gain and Bandwidth

CO4: Explain Internal Schematic of OPAMP and

Define its Performance Parameters.
 OPAMP
• Operational Amplifier
• Multi Terminal High Gain Amplifier which
amplifies the difference between both ac and
dc input signal
• It is used to perform mathematical operations
like addition, subtraction, log, antilog,
differentiation, integration, etc
OPAMP SYMBOL IC741
 Features of OPAMP
• High differential voltage gain
• Low common mode gain and hence high common
mode rejection ration (CMRR)
• HIGH Input Impedance
• LOW output Impedance
• Low offset Voltage and offset current
• Low Bias Current
• Large Bandwidth
• High slew rate
 Advantages
• Smaller size
• Less Power Consumption
• Reduced Cost due to mass production
• More reliable
• Easy to Replace
• Many application can be performed with the
same chip
 Applications
• Used in amplifiers
• Used in Oscillators
• Used in Filters
• Used in Multivibrator
• Used in Comparators
• Used in Voltage Regulator
Q) What will be the output if Vin is connected to Inverting
terminal and non inverting terminal is grounded?

Output will be
180 degree
Out of Phase
Q) What will be the output if Vin is connected to Non Inverting
terminal and inverting terminal is grounded?

Output will
be In phase
with Input
 Open Loop Concept
• Open Loop means no connection exists
between output and input terminal
• Output signal is not fed back to the input
signal
• When connected in open loop configuration
the OPAMP functions as high gain amplifier.
Types of Open Loop Configuration

1. Differential Configuration
2. Inverting Configuration
3. Non Inverting Mode
Differential Configuration

OPAMP
amplifies the
difference
between the
two input
signal
Vo=Av(V1-V2)
 Inverting Configuration
• Input is applied to inverting terminal and non
inverting terminal is grounded.
V1=0
V2=Vin

Vo= -A Vin

-ve sign indicates that

Vo is out of phase with
Vin
 Non Inverting Mode
• Input is applied to non inverting terminal and
inverting terminal is grounded.

V1= Vin ; V2= 0

Vo= A Vin
Transfer Characteristics of OPAMP
• It is t characteristics plotted in between
output voltage Vo and input voltage Vid
• Output voltage depend on input voltage so
dependent parameter Vo is on Y axis
• Independent parameter is on X axis
• Vid is the difference between the two input
signal
Ideal Transfer Characteristics of OPAMP
 Characteristics of an Ideal OPAMP
Characteristics of OPAMP Ideal Value

Open Loop Voltage Gain ∞

Input Impedance ∞
Output Impedance
0
Bandwidth ∞
 Equivalent Circuit of OPAMP
• The circuit which represent OPAMP parameter
in terms of physical component for the
purpose of analysis is called equivalent circuit
of OPAMP
• The circuit shows the OPAMP parameter like
1. Input Resistance
2. Output resistance
3. Equivalent Thevenin voltage source AoL*Vid
Equivalent Circuit of OPAMP
• V1 is non inverting input voltage w.r.t ground
• V2 is inverting voltage w.r.t ground
• +Vcc and +Vee are DC power supplies
• OPAMP amplifies the difference between two
input signal
• Vo is given as Vo= Aol *Vid =Aol(V1-V2)
Where Aol is large signal open loop gain
Vid is Difference input signal
• Output equation indicates that output voltage
is directly proportional to the difference
between the two input voltage
Block Diagram of OPAMP

Dual Input Dual Input Emitter Complimentary

Balanced Unbalanced Follower Symmetry Push
Output Output using Pull amplifier
Differential Differential constant
amplifier amplifier current
source
Internal Schematic
Input Stage: Dual Input Balanced Output
Differential amplifier
 Input Stage: Dual Input Balanced Output
Differential amplifier

• The two inputs are inverting and non-inverting

input terminals. This stage provides most of
the voltage gain of the OP-AMP and decides
the value of Input Resistance Ri.
• It provides high voltage gain.
• It provides high input resistance.
 Intermediate Stage: Dual Input
Unbalanced Output Differential amplifier
 Intermediate Stage: Dual Input
Unbalanced Output Differential amplifier

• It is driven from output of input stage

• It provides high voltage gain
• It provides high input resistance
Level shifting stage
 Level shifting stage
• Due to the direct coupling used between the
first two stages, the input level shifting stage is
an amplified signal with some non-zero dc
level.
• Level shifting stage is used to bring this dc
level to zero volts with respect to ground.
Output Stage: Complimentary Symmetry
Push Pull amplifier
 Output Stage: Complimentary Symmetry
Push Pull amplifier

• This stage e is normally a complementary

output stage. It increases the magnitude of
voltage and raises the current supplying
capability of the OP-AMP.
• It also ensures that the output resistance of
OP-AMP is low.
• Output stage provides high output power and
low output resistance, to avoid loading of
OPAMP.
 Differential Amplifier
Configurations
• Depending upon the input applied to base terminal
and output taken across the collector terminal of
transistor
1. Dual Input Balanced Output Differential Amplifier
(DIBO)
2. Dual Input Unbalanced Output Differential Amplifier
(DIUO)
3. Single Input Balanced Output Differential Amplifier
(SIBO)
4. Single Input Unbalanced Output Differential
Amplifier (SIUO)
Dual Input Balanced Output Differential Amplifier (DIBO)

Dual Input
means
separate
inputs are
applied to
base of each
transistor
Dual Input Unbalanced Output Differential Amplifier (DIUO)

Single Input means

only one inputs are
applied to base of
any one transistor,
while other
transistor’s base is
grounded
Single Input Balanced Output Differential Amplifier (SIBO)

Balance
output means
output is
taken across
collectors of
two transistor
Single Input Unbalanced Output Differential
Amplifier (SIUO)

Unbalance output
means output is
taken across
collectors of any
one transistor
with respect to
Ground.
 Analysis of DIBO
• To determine the operating point values
(VCEQ and lcQ) for the differential amplifier,
we need to use the DC analysis whereas
• The AC analysis is useful in knowing its
behavior as an amplifier and calculating
various parameters of the amplifier.
DC Analysis of DIBO

Both Ac inputs are

Grounded
i.e. Vin1=Vin2=0
The transistor Q1 and Q2 Apply KVL to base emitter loop
are matched transistor of Q1
RC1=RC2=RC and
RE1=RE2=RE -Rin *IB-VBE-2IE*RE-(-VEE)=0
|Vcc|=|Vee|
VBE1=VBE2=VBE We know β= IC/IB for common
emitter configuration
For ICQ
β1=β2=β IC= β*IB and IC ≈ IE
IC1=IC2=IC
IB1=IB2=IB IB= IE / β
IE1=IE2=IE
Put IB in KVL Equation

-Rin* IE/ β – VBE -2IE*RE + VEE=0

IE=(VEE- VBE)/ (Rin / β)+ 2RE

Where, VBE =0.7V for Silicon

and VBE= 0.2V for Germanium transistors
Rin/ β << 2RE therefore neglect Rin / β
IE=(VEE- VBE)/ 2RE
Since IE=IC, at operating point IE=ICQ

ICQ=(VEE- VBE)/ 2RE

For VCEQ
Neglecting drop across and applying KVL to the collector
base loop

VC=VCC-IC*RC
VCE= VC-VE
Therefore VCE= VCC-IC*RC-VE
But VE=-VBE

VCE= VCC-IC*RC+VBE

At operating point VCE=VCEQ

VCEQ= VCC-IC*RC+VBE
AC Analysis using r parameter
AC Analysis data
 Level Shifter
• Need:
1. OPAMP uses direct coupling , no capacitor is used
2. Capacitor Block DC
3. So in OPAMP DC level Increases and it tends to shift
the operating point of next stage
4. This limit the output voltage swing and there will be
distortion in output
5. To avoid this Level shifter is used
6. Level shifter shifts the DC level of output of
intermediate stage downward to zero w.r.t ground
 Level Shifter
1. Level shifter circuit using basic emitter
follower
Applying KVL to input loop,
Vin-VBE-Vo=0

VBE=Vin-Vo

As VBE= 0.7V

Vin-Vo=0.7 V

So, This circuit adjust only 0.7 V

shift between input and output
 Level Shifter
2. Level shifter circuit using modified emitter follower circuit

Applying KVL to input

loop,
Vin-VBE-I(R1+R2)=0

I= (Vin-VBE)/(R1+R2)

Vo= IR2

Vo=R2(Vin-VBE)/(R1+R2)
 Level Shifter
3. Level shifter circuit using current Mirror Circuit

Instead of R2 this circuit

uses current mirror circuit
consisting of Q2 and Q3
Transistor

Current flowing through the

current mirror circuit is
equal and opposite to the
emitter current of Q1
transistor
I1=I
Applying KVL To input loop base emitter
of Q1
Vin- VBE- I1R1-Vo=0

Vo-Vin= -(VBE+ I1R1)

By choosing value of R1 and R required

output voltage shift can be obtained
 Current Mirror
• The output current is a mirror current of input
current.
• A current mirror is a circuit designed to copy a
current through one active device by
controlling the current in another active
device of a circuit keeping output current
constant regardless of loading.
Current Mirror circuit
As both transistor Q3 and Q4 And as β ∞
are identical IC/ β  0
VBE3=VBE4 Therefore IB= 0
IC3=IC4 I2= IC3
IB3=IB4
From fig., IC3=Io I2= Iref This indicates that once the
current I2 is setup, IC3 is
Applying KCL at node VB3, we automatically equal to I2
get
I2= I + IC4 I2= I + IC3 Applying KVL to Base Emitter
of Q3
Applying KCL at node A , we get -I2R2-VBE3-(-VEE)=0
I= IB3 + IB4
Since IB3=IB4 I2= VEE-VBE3
I= 2IB3 = 2IB4 R2

I2= 2IB3 + IC3= 2IB4 + IC4

Drawbacks of basic Current Mirror Circuit

• It is not suitable for low value of current

• for low value of current I2 , R2 should be high.
If R2is increased excess amount of heat will
generate which may increase IE3 and circuit
becomes unstable
DC Characteristics of OPAMP
1. Input Bias Current
2. Input Offset Current
3. Input offset Voltage
4. CMRR
5. Output offset voltage
6. Input Impedance
7. Output Impedance
AC Characteristics of OPAMP

1. Frequency Response
2. Stability
3. Slew Rate
 1. Input Offset Voltage
• The voltage which is applied between two
input terminals of an OPAMP so as to get zero
output voltage is known as Input Bias Current
• It is denoted by Vio
• The smaller the value of Vio better the input
terminals are matched.
• Ideal value is 0
• Practical value for IC741 is 2mV
 2. Output Offset Voltage
• It is the voltage appearing at the output of
OPAMP whenever both the input terminals
are connected to ground
• It is denoted by Voo
• Smaller the value of Voo, better is the
performance
• Ideal value is 0
• Practical value for IC741 is 20mV
 3. Input Offset Current
• The algebraic difference between the currents
entering into inverting and non inverting input
terminals is called as input offset current
• It is denoted as Iio and given as
Iio= |IB1-IB2|
• Smaller is the value of Iio better the
performance of OPAMP.
• Ideal value is 0
• Practical value for IC 741 is 6nA.
 4. Input Bias Current
• It is the average of the currents flowing into
inverting and non inverting input terminals of
an OPAMP.
• It is denoted as IB and expressed as
IB= (IB1+IB2)/2
• Smaller the Value, better is the performance of
OPAMP.
• Ideal Value is 0.
• Practical value for IC741 is 50nA.
 5. Input Impedance
• It is resistance observed at any of the input
terminal of an OPAMP when another terminal
is connected to ground.
• It is denoted by Ri
• The input resistance should be as high as
possible
• Ideal value is ∞
• Practical value for IC741 is 2MΩ
 6. Output Impedance
• It is equivalent resistance that can be
measured between the output terminal of an
OPAMP and ground.
• It is denoted by Ro.
• The output resistance should be as small as
possible.
• Ideal value is 0.
• Practical value for IC741 is 75Ω.
 7. Voltage Gain
• It is the ratio of amplified output of an OPAMP to
the differential input voltage applied to the input of
an OPAMP.
• Since the amplitude of the output signal is much
larger than the input signal the voltage gain is
commonly referred as large signal voltage gain
• It is denoted by Av.
• It is expressed as Av= Vo / Vid
• The voltage gain should be as high as possible.
• Ideal value is ∞
• Practical value for IC741 is 2*10^5.
8. Common Mode Rejection Ratio (CMRR)

• It is the ability of an OPAMP to reject common

mode signals such as noise applied at both the
input terminal.
• It is defined as the ratio of differential voltage gain
to common mode gain.
• It is given by CMRR= Ad / Acm.
• Ideally Acm is 0 and hence CMRR is ∞.
• CMRR expressed in decibels dB.
• The CMRR should be as high as possible.
• Ideal value is ∞.
 9. Power Supply Rejection Ratio(PSRR)
• Also known as supply voltage rejection ratio
(SVRR).
• It is defined as the change in input offset voltage of
an OPAMP due to change in supply voltage.
• It is expressed in µV/V.
• PSRR= Δ Vio / Δ V
• The PSRR should be as low as possible
• Ideal value is 0
• Practical value for IC741 is 150 µV/V.
 10. Slew Rate
• It is maximum rate of change of output voltage per unit time
• It is expressed in v/ ms
• Slew rate = d Vo / d t
• Slew rate should be as high as possible
• It also given by expression
SR= 2πFmVm
where Fm is max permitted frequency
Vm is peak AC input voltage
• Ideal value is ∞
• Practical value for IC741 is 0.5V/µs
 11. Bandwidth
• It is the rate of change of frequencies over
which all the input signal frequencies are
amplified almost equally
• The bandwidth should be as large as possible.
• Ideal value is ∞
• Practical value for IC741 is 1 MHz.
 12. Gain Bandwidth Product
• It is the bandwidth of an OPAMP when the
voltage gain is 1.
• It is also known as closed loop bandwidth ,
Unity gain Bandwidth and small signal
bandwidth
• Ideal value is ∞
• Practical value for IC741 is approx. 1 MHz.
 Ideal and Practical Value of OPAMP
Parameters
Sr no. Parameter Ideal Value Practical value for
IC741
1 Input Offset Voltage 0 2 mV
2 Output Offset Voltage 0 20 mV
3 Input Offset Current 0 6 nA
4 Input Bias Current 0 50 nA
5 Input Impedance ∞ 2MΩ
6 Output Impedance 0 75 Ω
7 Voltage Gain ∞ 2*10^5

8 CMRR ∞ 90dB
9 PSRR 0 150 µV/V
10 Slew Rate ∞ 0.5V/µs
11 Bandwidth ∞ 1 MHz
12 Gain Bandwidth Product ∞ Approx. 1 MHz
 Feedback Amplifiers
• The feedback means fraction or some portion
of output signal is feedback to the input
• Types of feedback-
1. Positive Feedback
2. Negative Feedback
Positive Feedback Negative Feedback
1. Positive Feedback- in phase with the input
signal. It is also called as regenerative
feedback.
2. Negative Feedback – 180 degree out of phase
with the input signal. It is also called as
Degenerative feedback.
 Comparison of Positive and Negative
Feedback
Sr No. Parameter Positive Feedback Negative Feedback
1 Gain Gain Increases Stabilizes gain i.e. gain
Decreases
2 Bandwidth Decreases Increases
3 Distortion Increases Decreases
4 Operating Point Not Stable Stable
5 Noise Increases Decreases
6 Application Oscillators Amplifiers
7 Phase shift between input In phase Out of Phase
and output
 Configuration of Negative Feedback
Amplifier

1. Voltage Series Feedback Amplifier

2. Voltage Shunt Feedback Amplifier
3. Current Series Feedback Amplifier
4. Current Shunt Feedback Amplifier
Comparison Between Voltage Series and
Voltage Shunt
Thank
You!!