Analogue Electronics IIAnalogue Electronics IIEMT 212/4EMT 212/4

Chapter 1Operational Amplifier

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Semester 2 2010/2011

1.0 Operational Amplifier1.1 Introduction1.2 Ideal Op-Amp1.3 Op-amp Input Modes1.4 Op-amp Parameters1.5 Operation

Single-modeDifferential-modeCommon-mode operation

1.6 Op-Amps Basics1.7 Practical Op Amp Circuits1.8 Op Amp Datasheet

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1.1Introduction

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Typical IC packages

IC packages placed on circuit board

1.1Introduction

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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Definition

1.1Introduction

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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

1.1Introduction

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Applications of Op-Amp

To provide voltage amplitude changes (amplitude and polarity)

Comparators Oscillators Filters Sensors Instrumentation amplifiers

1.1IntroductionStages of an op-amp

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INPUT STAGE

OUTPUT STAGE

GAIN STAGE

1.1IntroductionTypical op-amp packages

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1.1IntroductionThe 741 op-amp

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Literally a black box

Real op-amp : 741

1.2 Ideal Op-Amp

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Ideal Op-AmpPractical Op-Amp

1.2 Ideal Op-Amp

Infinite input impedanceZero output impedanceInfinite open-loop gainInfinite bandwidthZero noise contributionZero 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

i

ii 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 a voltage source with an internal resistance.

The internal resistance of the op-amp is the output

impedance of op-amp 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 20-100 .

1.2 Ideal Op-Amp

Infinite Open-Loop Gain

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

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 gain becomes unity

Some op-amps, such as 741 family, have very limited bandwidth, 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 the output signal must have been in the input signal as well.

the ideal op amp contributes nothing extra to the output noise.

In real op-amp, there is noise due to the internal circuitry of the op-amp that contributes to the output noise

1.2 Ideal Op-Amp

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Zero Output Offset

The output offset voltage of any amplifier is the output voltage that exists when it should be zero.

The voltage amplifier sees zero input voltage when both inputs are grounded. This connection should produce a zero output voltage.

If the output is not zero then there is said to be an output voltage present.

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).

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 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.

1.2 Ideal Op-Amp

1.3 Op-Amp Input Modes Single-Ended Input Mode Input signal is connected to ONE input and the other

input is grounded.

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Non- Inverting Mode input signal at +ve terminal

output same polarity as the applied input signal

Inverting Mode input signal at –ve terminal

output opposite in phase to the applied input signal

1.3 Op-Amp Input ModesDifferential Input Mode

TWO out-of-phase signals are applied with the difference of the two amplified is produced at the output.

21 inind

ddout

VVV

VAV

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1.3 Op-Amp Input ModesCommon 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 ParametersCOMMON-MODE REJECTION (CMRR)

COMMON-MODE INPUT VOLTAGE

INPUT OFFSET VOLTAGE

INPUT BIAS CURRENT

INPUT IMPEDANCE

INPUT OFFSET CURRENT

OUTPUT IMPEDANCE

SLEW RATE22

1.4 Op-Amp ParametersCommon-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

cm

ol

A

ACMRR

cm

ol

A

ACMRR log20

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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

1.4 Op-Amp ParametersCommon-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 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.

1.4 Op-Amp ParametersInput Bias Current

Ideally should be zeroThe 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 ParametersInput 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 ParametersInput 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

1.4 Op-Amp ParametersOutput Impedance

Ideally should be zeroIs 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 ParametersSlew Rate

Is the maximum rate of change of the output voltage in response to a step input voltage.

t

VSlewRate out

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)( where maxmax VVVout

1.4 Op-Amp ParametersSlew Rate

It’s a measure of how fast the output can “follow” the input signal.

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1.4 Op-Amp ParametersExample

Determine the slew rate:

t

VSlewRate out

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sVs

VVSlewRate

/18

1

)9(9

1.5 OperationTypes 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.’

1.5 Operation

Basic amplifier circuit33

Differential Amplifier Circuit

1.5 OperationDC bias of differential amplifier circuit

0)( EEEEBE VRIV

E

BEEEE R

VVI

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Differential Amplifier Circuit

DC ANALYSIS

0 since 221 BE

CC VI

II

CE

CCCCCCCC RI

VRIVVV221

1.5 OperationExample : Differential Amplifier Circuits

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• Calculate the dc voltages and currents

1.5 OperationExample

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E

BEEEE R

VVI

Differential Amplifier Circuit

Solution

mAmI

II ECC 25.1

2

5.2

221

CCCCC RIVV

mAk

VVI E 5.2

3.3

7.09

VkmVVC 1.4)9.3)(25.1(9

1.5 Operation

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Differential Amplifier Circuit

Connection to calculate : Av1 = Vo1 / Vi1

AC ANALYSIS Single-Ended

1.5 Operation

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Differential Amplifier Circuit

AC ANALYSIS Single-Ended

AC equivalent of differential amplifier circuit

B

E

C

1.5 Operation

Scan figure 10.11 & 10.15

ibibi rIrIV 1

21

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Differential Amplifier Circuit

AC Analysis - Single ended

i

ib r

VI

21

bbb III 21iii rrr

21

KVL

i

ibc r

VII

21

1

1

22 ie

c

i

cicco V

r

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

1.5 Operation

7521

AI

I EC 5.96

2

VkAV 5.4)47)(5.96(9 2690965.0

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CQ

Te I

Vr

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Differential Amplifier Circuit

Example

krr ii 2021

Ak

VV

R

VVI

E

EEE 193

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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 mV 26TV

1.5 Operation

i

c

d

od r

R

V

VA

2

21 where 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

1.5 Operation

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Differential Amplifier Circuit

AC Analysis - Common-mode

Common-mode connection

1.5 Operation

Ei

ib Rr

VI

)1(2

g,Rearrangin

Ei

c

i

ov Rr

R

V

VA

)1(2

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Differential Amplifier Circuit

AC Analysis - Common-mode

i

Ebib r

RIVI

)1(2

Ei

cicbcco Rr

RVRIRIV

)1(21