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Analogue Electronics Circuit II
EKT 214/4
Chapter 1
Operational Amplifier
1
Semester 2 2010/11
By: Norizan Binti Mohamed Nawawi
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1.0 Operational Amplifier
1.1 Introduction 1.2 Ideal Op-Amp 1.3 Op-amp Input Modes 1.4 Op-amp Parameters 1.5 Operation
Single-mode Differential-mode Common-mode operation
1.6 Op-Amps Basics 1.7 Practical Op Amp Circuits 1.8 Op Amp Datasheet
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1.1 Introduction
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Typical IC packages
IC packages placed on circuit board
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1.1 Introduction
The operational amplifier or op-amp is a circuit ofcomponents integrated into one chip.
A typical op-amp is powered by two dc voltages and has oneinverting(-) input, one non-inverting input (+) and oneoutput.
Op-amps are used to model the basic mathematical
operations ; addition, subtraction, integration anddifferentiation in electronic analog computers.
Other operations include buffering and amplification of DCand AC signals.
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Definition
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1.1 Introduction
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Op-amp schematic symbol One Output Terminal
Two Input Terminals
Inverting input Non-inverting input
Two Power Supply (PS)
+V : Positive PS
-V : Negative PS
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1.1 Introduction
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Applications of Op-Amp
To provide voltage amplitude changes
(amplitude and polarity)
Comparators
Oscillators
Filters Sensors
Instrumentation amplifiers
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1.1 Introduction Stages of an op-amp
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INPUT
STAGE
OUTPUT
STAGE
GAIN
STAGE
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1.1 Introduction Typical op-amp packages
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1.1 Introduction The 741 op-amp
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Literally a black boxReal op-amp : 741
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1.2 Ideal Op-Amp
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Ideal Op-AmpPractical Op-Amp
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1.2 Ideal Op-Amp
Infinite input impedance
Zero output impedance
Infinite open-loop gain
Infinite bandwidth
Zero noise contribution
Zero DC output offset
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Ideal Op-Amp Practical Op-Amp
Input impedance 500k-2M
Output impedance 20-100
Open-loop gain (20k to 200k)
Bandwidth limited (a few kHz)
Has noise contribution
Non-zero DC output offset
Properties
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i
i
i
I
VZ
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Infinite Input Impedance
Input impedance is measured across the input
terminals.
It is the Thevenin resistance of the internal connection
between the two input terminals. Input impedance is the ratio of input voltage to input
current.
When Zi is infinite, the input current is zero.
The op amp will neither supply current to a circuit nor
will it accept current from any external circuit.
In real op-amp, the impedance is 500k
to 2M
1.2 Ideal Op-Amp
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Zero Output Impedance
Looking back into the output terminal, we see it as avoltage source with an internal resistance.
The internal resistance of the op-amp is the outputimpedance of op-amp
This internal resistance is in series with the load, reducingthe output voltage available to the load
Real op-amps have output impedance in the range of 20-100 .
1.2 Ideal Op-Amp
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Infinite Open-Loop Gain
Open-Loop Gain, A is the gain of the op-ampwithout feedback.
In the ideal op-amp, A is infinite
In real op-amp, A is 20k to 200k
invout VAV
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1.2 Ideal Op-Amp
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Infinite Bandwidth
The ideal op-amp will amplify all signals from DC to the
highest AC frequencies
In real op-amps, the bandwidth is rather limited This limitation is specified by the Gain-Bandwidth product,
which is equal to the frequency where the amplifier gainbecomes unity
Some op-amps, such as 741 family, have very limitedbandwidth, up to a few kHz only
1.2 Ideal Op-Amp
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Zero Noise Contribution
in an ideal op amp, all noise voltages produced
are external to the op amp. Thus any noise in theoutput signal must have been in the input signal aswell.
the ideal op amp contributes nothing extra to theoutput noise.
In real op-amp, there is noise due to the internalcircuitry of the op-amp that contributes to theoutput noise
1.2 Ideal Op-Amp
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Zero Output Offset
The output offset voltage of any amplifier is the outputvoltage that exists when it should be zero.
The voltage amplifier sees zero input voltage when bothinputs are grounded. This connection should produce azero output voltage.
If the output is not zero then there is said to be anoutput voltage present.
In the ideal op amp this offset voltage is zero volts, butin practical op amps the output offset voltage is nonzero(a few miliVolts).
1.2 Ideal Op-Amp
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Both Differential Inputs Stick Together
this means that a voltage applied to one inverting
inputs also appears at the other non-inverting inputs.
If we apply a voltage to the inverting input and thenconnect a voltmeter between the non-inverting inputand the power supply common, then the voltmeter willread the same potential on non-inverting as on theinverting input.
1.2 Ideal Op-Amp
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1.3 Op-Amp Input Modes Single-Ended Input Mode
Input signal is connected to ONE input and the other input isgrounded.
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Non- Inverting Mode
input signal at +ve terminal
output same polarity as
the applied input signal
Inverting Mode
input signal atve terminal
output opposite in phase
to the applied input signal
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1.3 Op-Amp Input Modes Differential Input Mode
TWO out-of-phase signals are applied with the difference ofthe two amplified is produced at the output.
21 inind
ddout
VVV
VAV
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1.3 Op-Amp Input Modes Common Mode Input
Two signals of same phase, frequency, and amplitude are applied tothe inputs which results in no output (signals cancel). But, inpractical, a small output signal will result.
This is called common-mode rejection. This type of mode is usedfor removal of unwanted noise signals.
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1.4 Op-Amp Parameters COMMON-MODE REJECTION (CMRR) COMMON-MODE INPUT VOLTAGE
INPUT OFFSET VOLTAGE
INPUT BIAS CURRENT
INPUT IMPEDANCE
INPUT OFFSET CURRENT
OUTPUT IMPEDANCE
SLEW RATE 22
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1.4 Op-Amp Parameters Common-Mode Rejection Ratio (CMRR)
The ability of amplifier to reject the common-mode signals(unwanted signals) while amplifying the differential signal(desired signal)
Ratio of open-loop gain,Aolto common-mode gain,Acm The open-loop gain is a datasheet value
cm
ol
A
ACMRR
cm
ol
A
ACMRR log20
23
The higher the CMRR, the better, in which the open-loop gainis high and common-mode gain is low.
CMRR is usually expressed in dB & decreases withfrequency
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1.4 Op-Amp Parameters Common-Mode Input Voltage
The range of input voltages which, when applied to both inputs,will not cause clipping or other output distortion.
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Input Offset Voltage
Ideally, output of an op-amp is 0 Volt if the input is 0 Volt.
Realistically, a small dc voltage will appear at the output whenno input voltage is applied.
Thus, differential dc voltage is required between the inputs toforce the output to zero volts.
This is called the Input Offset Voltage, Vos. Range between 2mV or less.
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1.4 Op-Amp Parameters Input Bias Current
Ideally should be zero
The dc current required by the inputs of the amplifier to
properly operate the first stage. Is the average of both input currents
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1.4 Op-Amp Parameters Input Impedance
Is the total resistance between the inverting and non-invertinginputs.
Differential input impedance : total resistance between theinverting and non-inverting inputs
Common-mode input impedance: total resistance betweeneach input and ground
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1.4 Op-Amp Parameters Input Offset Current
Is the difference of input bias currents
21 IIIos
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inininos RIIRIRIV 2121
inosos RIV
inosverrorout RIAV )(
Input offset current Offset voltage
Thus, error
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1.4 Op-Amp Parameters Output Impedance
Ideally should be zero
Is the resistance viewed from the output terminal of the op-amp. In reality, it is non-zero.
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1.4 Op-Amp Parameters Slew Rate
Is the maximum rate of change of the output voltage in responseto a step input voltage.
t
V
SlewRateout
29)(where maxmax VVVout
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1.4 Op-Amp Parameters Slew Rate
Its a measure of how fast the output can follow the input signal.
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1.4 Op-Amp Parameters Example
Determine the slew rate:
t
VSlewRate out
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sVs
VVSlewRate
/181
)9(9
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1.5 Operation
Types of Op-amp Operation
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Differential Amplifier Circuit
If an input signal is applied to either input with the other input is
connected to ground, the operation is referred to as single-
ended.
If two opposite-polarity input signals are applied, the operation is
referred to as double-ended.
If the same input is applied to both inputs, the operation is called
common-mode.
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1.5 Operation
Basic amplifier circuit 33
Differential Amplifier Circuit
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1.5 OperationDC bias of differential amplifier circuit
0)( EEEEBE VRIV
E
BEEE
E R
VV
I
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Differential Amplifier Circuit
DC ANALYSIS
0since2
21 BE
CC VI
II
C
E
CCCCCCCCRIVRIVVV
221
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1.5 OperationExample : Differential Amplifier Circuits
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Calculate the dc voltages and currents
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1.5 OperationExample
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E
BEEEE
R
VVI
Differential Amplifier Circuit
Solution
mAmI
II ECC 25.12
5.2
221
CCCCCRIVV
mAkVVIE 5.2
3.37.09
VkmVVC 1.4)9.3)(25.1(9
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1.5 Operation
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Differential Amplifier Circuit
Connection to calculate : Av1 = Vo1 / Vi1
AC ANALYSIS Single-Ended
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1.5 Operation
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Differential Amplifier Circuit
AC ANALYSIS Single-Ended
AC equivalent of differential amplifier circuit
B
E
C
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1.5 Operation
Scan figure 10.11 & 10.15
ibibi rIrIV 1
21
39
Differential Amplifier Circuit
AC Analysis - Single ended
i
i
b r
V
I 2
1
bbb III 21iii rrr
21
KVL
i
i
bcr
VII
2
1
1
1
22i
e
c
i
ci
cco Vr
R
r
RVRIV
e
c
i
ov
r
R
V
VA
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Partial circuit for calculating Ib
Hence
:Note
ie
CQ
Ti
rr
I
Vrr
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1.5 Operation
7521
AI
I E
C
5.962
VkAV 5.4)47)(5.96(9 2690965.0
26
CQ
TeI
Vr
40
Differential Amplifier Circuit
Example
krrii 2021
Ak
VV
R
VVI
E
EEE 193
43
7.097.0
ccccc RIVV
4.87)269(2
47
2
k
r
RA
e
cv
Calculate the single-ended output
voltage Vo1
Solution
VkVc 5.4)47)(5.96(9
VmVAV ivo 175.0)2)(4.87(1
mV26TV
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1.5 Operation
i
c
d
od
r
R
V
VA
2
21where iid VVV
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Differential Amplifier Circuit
AC Analysis - Double ended
A similar analysis can be used to show that for the condition of
signals applied to both inputs, the differential voltage gain
magnitude is
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1.5 Operation
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Differential Amplifier Circuit
AC Analysis - Common-mode
Common-mode connection
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1.5 Operation
iVI
g,Rearrangin
co
v
RVA
Differential Amplifier Circuit
AC Analysis - Common-mode
i
Ebib
r
RIVI
)1(2
Ei
ci
cbccoRr
RVRIRIV
)1(2
1