week 1 - chapter 1 (part i)

7/27/2019 Week 1 - Chapter 1 (Part I)

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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

2


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1.1 Introduction

3

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.

4

Definition


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1.1 Introduction

5

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

6

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

7

INPUT

STAGE

OUTPUT

STAGE

GAIN

STAGE


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1.1 Introduction Typical op-amp packages

8


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1.1 Introduction The 741 op-amp

9

Literally a black boxReal op-amp : 741


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1.2 Ideal Op-Amp

10

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

11

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

12

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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13

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

14

1.2 Ideal Op-Amp


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15

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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16

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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17

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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18

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.

19

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

20


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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.

21


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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.

24

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

25


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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

26


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1.4 Op-Amp Parameters Input Offset Current

Is the difference of input bias currents

21 IIIos

27

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.

28


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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.

30


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1.4 Op-Amp Parameters Example

Determine the slew rate:

t

VSlewRate out

31

sVs

VVSlewRate

/181

)9(9


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1.5 Operation

Types of Op-amp Operation

32

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



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1.5 OperationDC bias of differential amplifier circuit

0)( EEEEBE VRIV

E

BEEE

E R

VV

I

34


DC ANALYSIS

0since2

21 BE

CC VI

II

C

E

CCCCCCCCRIVRIVVV

221


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1.5 OperationExample : Differential Amplifier Circuits

35

Calculate the dc voltages and currents


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1.5 OperationExample

36

E

BEEEE

R

VVI


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

37


Connection to calculate : Av1 = Vo1 / Vi1

AC ANALYSIS Single-Ended


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1.5 Operation

38


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


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

21

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


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

41


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

42


AC Analysis - Common-mode

Common-mode connection


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1.5 Operation

iVI

g,Rearrangin

co

v

RVA


AC Analysis - Common-mode

i

Ebib

r

RIVI

)1(2

Ei

ci

cbccoRr

RVRIRIV

)1(2

1

week 1 - chapter 1 (part i)

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