Transcript 1.2 Ideal Op-Amp

Analogue Electronics Circuit II
EKT 214/4
Chapter 1
Operational Amplifier
Semester 2 2015/16
By: Dr.Hilal, Dr. Naseer,
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1.0 Operational Amplifier
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1.1 Introduction
1.2 Ideal Op-Amp
1.3 Op-amp Input Modes
1.4 Op-amp Parameters
1.5 Operation
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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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1.1Introduction
Typical IC packages
IC packages placed on circuit board
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1.1Introduction
Definition
 The operational amplifier or op-amp is a circuit of
components integrated into one chip.
 A typical op-amp is powered by two dc voltages and has one
inverting(-) input, one non-inverting input (+) and one
output.
 Op-amps are used to model the basic mathematical
operations ; addition, subtraction, integration and
differentiation in electronic analog computers.
 Other operations include buffering and amplification of DC
and AC signals.
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1.1Introduction
 Two Power Supply (PS)
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Op-amp schematic symbol
+V : Positive PS
-V : Negative PS
 One Output Terminal
 Two Input Terminals
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Inverting input
Non-inverting input
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1.1Introduction
Applications of Op-Amp
 To provide voltage amplitude changes
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(amplitude and polarity)
Comparators
Oscillators
Filters
Sensors
Instrumentation amplifiers
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1.1Introduction
 Stages of an op-amp
INPUT
STAGE
OUTPUT
STAGE
GAIN
STAGE
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1.1Introduction
 Typical op-amp packages
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1.1Introduction
 The 741 op-amp
Real op-amp : 741
Literally a black box
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1.2 Ideal Op-Amp
Practical Op-Amp
Ideal Op-Amp
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1.2 Ideal Op-Amp
Properties
Ideal Op-Amp
Practical Op-Amp
 Infinite input impedance
 Input impedance 500k-2M
 Zero output impedance
 Output impedance 20-100 
 Infinite open-loop gain
 Open-loop gain (20k to 200k)
 Infinite bandwidth
 Bandwidth limited (a few kHz)
 Zero noise contribution
 Has noise contribution
 Zero DC output offset
 Non-zero DC output offset
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1.2 Ideal Op-Amp
 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.
Vi
Zi 
Ii
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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
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1.2 Ideal Op-Amp
 Zero Output Impedance
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Looking back into the output terminal, we see it as a
voltage source with an internal resistance.
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The internal resistance of the op-amp is the output
impedance of op-amp
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This internal resistance is in series with the load, reducing
the output voltage available to the load
Real op-amps have output impedance in the range of 20100  .
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1.2 Ideal Op-Amp
Vout  AvVin
 Infinite Open-Loop Gain
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Open-Loop Gain, A is the gain of the op-amp
without feedback.
In the ideal op-amp, A is infinite
In real op-amp, A is 20k to 200k
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1.2 Ideal Op-Amp
 Infinite Bandwidth
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The ideal op-amp will amplify all signals from DC to the
highest AC frequencies
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In real op-amps, the bandwidth is rather limited
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This limitation is specified by the Gain-Bandwidth product,
which is equal to the frequency where the amplifier gain
becomes unity
Some op-amps, such as 741 family, have very limited
bandwidth, up to a few kHz only
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1.2 Ideal Op-Amp
 Zero Noise Contribution
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in an ideal op amp, all noise voltages produced
are external to the op amp. Thus any noise in the
output signal must have been in the input signal as
well.
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the ideal op amp contributes nothing extra to the
output noise.
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In real op-amp, there is noise due to the internal
circuitry of the op-amp that contributes to the
output noise
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1.2 Ideal Op-Amp
 Zero Output Offset
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The output offset voltage of any amplifier is the output
voltage that exists when it should be zero.
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The voltage amplifier sees zero input voltage when both
inputs are grounded. This connection should produce a
zero output voltage.
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If the output is not zero then there is said to be an
output voltage present.
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In the ideal op amp this offset voltage is zero volts, but
in practical op amps the output offset voltage is nonzero
(a few miliVolts).
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1.2 Ideal Op-Amp
 Both Differential Inputs Stick Together
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this means that a voltage applied to one inverting
inputs also appears at the other non-inverting inputs.
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If we apply a voltage to the inverting input and then
connect a voltmeter between the non-inverting input
and the power supply common, then the voltmeter will
read the same potential on non-inverting as on the
inverting input.
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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 is
grounded.
 Non- Inverting Mode
 Inverting Mode
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 input signal at –ve terminal
input signal at +ve terminal
 output same polarity as
the applied input signal
 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 of
the two amplified is produced at the output.
Vout  Ad Vd
Vd  Vin1  Vin2
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1.3 Op-Amp Input Modes
 Common Mode Input
Two signals of same phase, frequency, and amplitude are applied to
the inputs which results in no output (signals cancel). But, in
practical, a small output signal will result.
 This is called common-mode rejection. This type of mode is used
for 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
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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, Aol to common-mode gain, Acm
 The open-loop gain is a datasheet value
CMRR 
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Aol
Acm
A 
CMRR  20 log  ol 
 Acm 
The higher the CMRR, the better, in which the open-loop gain
is high and common-mode gain is low.
CMRR is usually expressed in dB & decreases with
frequency
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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.
 Input Offset Voltage
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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 when
no input voltage is applied.
Thus, differential dc voltage is required between the inputs to
force the output to zero volts.
This is called the Input Offset Voltage, Vos. Range between 2
mV 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-inverting
inputs.
 Differential input impedance : total resistance between the
inverting and non-inverting inputs
 Common-mode input impedance: total resistance between
each input and ground
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1.4 Op-Amp Parameters
 Input Offset Current
 Is the difference of input bias currents
Input offset current
Offset voltage
I os  I1  I 2
Vos  I1Rin  I 2 Rin  I1  I 2 Rin
Thus, error
Vos  I os Rin
Vout( error)  Av I os Rin
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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 response
to a step input voltage.
SlewRate 
Vout
t
where Vout  Vmax  (Vmax )
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1.4 Op-Amp Parameters
 Slew Rate
 It’s 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:
SlewRate 
Vout
t
SlewRate 
 9V  (9V )
 18V / s
1s
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1.5 Operation Differential Amplifier Circuit
 Types of Op-amp Operation
 If an input signal is applied to either input with the other input is
connected to ground, the operation is referred to as ‘singleended.’
 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
Differential Amplifier Circuit
Basic amplifier circuit
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1.5 Operation Differential Amplifier Circuit
DC bias of differential amplifier circuit
DC ANALYSIS
VBE  I E RE  (VEE )  0
IE 
VEE  VBE
RE
I C1  I C 2
IE

2
since VB  0
VC1  VC 2  VCC  I C RC  VCC 
IE
RC
2
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1.5 Operation
Example : Differential Amplifier Circuits
• Calculate the dc voltages and currents
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1.5Example
Operation
Differential Amplifier Circuit
Solution
IE 
VEE  VBE
RE
IE 
9V  0.7V
 2.5mA
3.3k
I C1  I C 2
I E 2.5m


 1.25mA
2
2
VC  VCC  I C RC
VC  9V  (1.25m)(3.9k )  4.1V
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1.5 Operation Differential Amplifier Circuit
AC ANALYSIS
 Single-Ended
Connection to calculate :
Av1 = Vo1 / Vi1
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1.5 Operation
Differential Amplifier Circuit
 Single-Ended
AC ANALYSIS
C
B
E
AC equivalent of differential amplifier circuit
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1.5 Operation Differential Amplifier Circuit
AC Analysis - Single ended
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Scan figure 10.11 & 10.15
 KVL
Vi1  I b ri  I b ri
Ib 
Vi1
2ri
Note : ri  r 
Hence re 
I c  I b 
Partial circuit for calculating Ib
1   2  
ri1  ri2  ri
I b1  I b2  I b
I CQ
ri

Vi
Vo  I c Rc 
Av 
 VT
1
2ri
Vi Rc
1
2ri
Rc

Vi
2re
1
Vo Rc

Vi1 2re
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1.5 Operation Differential Amplifier Circuit
Example
Solution
IE 
VEE  0.7V 9V  0.7V

 193A
RE
43k
IC 
IE
 96.5A
2
1  2  75
Vc  Vcc  I c Rc
Vc  9  (96.5 )( 47k )  4.5V
 9V  (96.5A)( 47k)  4.5V
Calculate the single-ended output
voltage Vo1 V  26 mV
T
ri1  ri2  20k
re 
VT
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
 269
I CQ 0.0965
Av 
Rc
47 k

 87.4
2re 2(269)
Vo1  AvVi  (87.4)( 2m)  0.175V
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1.5 Operation
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
Vo  Rc
Ad 

Vd
2ri
where Vd  Vi1  Vi 2
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1.5 Operation Differential Amplifier Circuit
AC Analysis - Common-mode
Common-mode connection
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1.5 Operation Differential Amplifier Circuit
AC Analysis - Common-mode
Vi  2(   1) I b RE
Ib 
ri
Rearrangin g,
Vi
Ib 
ri  2(   1) R E
Vi Rc
Vo  I c Rc  I b Rc 
ri  2(   1) RE
1
Vo
Rc
Av 

Vi ri  2(   1) RE
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