Operational Amplifiers

Mr Stockley HoD

What the specification says;

Candidates should be taught to:• understand the function of an operational amplifier and

be able to describe the use of the non-inverting and inverting inputs;

• use operational amplifiers which require a single power supply, i.e. 3140 IC;

• use an operational amplifier as a comparator and an inverting amplifier;

• know how to limit the gain of an operational amplifier by using an input resistor and a feedback resistor (negative feedback only).

What is an Op Amp?

• It is an amplifier in an I.C.• It has two inputs and one output.• It compares the difference between the inputs and

amplifies this.

Can be as much as 1000 x

A high gain voltage amplifier

Why use them?• Check out the transistor

Larger Current

Small Current

Transistor current gain

(hfe) = Ic

Ib

Ib

Ic

Ie

MAX gain

= 150

Anatomy of an op amp

741

Inverting Input -+Non Inverting Input

Output

Negative supply

It is an 8 pin DIL IC.

2 7

6

3 4

Compares the voltage difference between the two inputs

Positive supply3140 pin numbers

Page 117 - 119

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Two common typesLM324

Comparing objects+ - Output If something in the

“+” column is bigger than the “-” column then we produce a logic ‘1’

If something in the “+” column is smaller than the “-” column then we produce a logic ‘0’

0

1

0

1

A Practical Comparator Circuit Fixed

Potential Divider

Variable Potential Divider

4.5v

90µv change

Amplifies 100000 xNeeds 9 / 100000v at input to make LED light (= 90µv)

Output current is

only 10mA

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Create a Yenka circuit and print a copy

LM318

Dark Detecting Circuit

+

-Pin 1 NC

Pin 2 inv Input

Potential Dividers• It is two resistors connected in

series across a potential (or voltage)

• The voltage at the mid point of the two resistors is determined by the ratio of the resistors and the input voltage.

Vout = x Vin R2

R1 + R2

R1

R2

Vin

Vout

Equation

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Page 103, 107, 108, 109

The Maths

Vout = x Vin R2

R1 + R2R1

R2

Vin

Vout

Vout = x 9v 1K8K + 1K

Vout = x 9 = 19 1v

Try this one

?

?

?

Potential DividersAlternative arrangements;

A variable resistor enables the voltage

to be adjusted.

Other resistive devices can be used (LDR / thermistor)

Using a potentiometer (3 legs)

How sensitive is it?

The gain of the op amp is 100 000

Does that mean that a voltage difference will give 100 000v?

No, it means that to get 9v at the output, a voltage change of 9/100 000v is needed = 0.00009v or 90µv !!!!

A very small voltage change indeed – (much less than with a transistor)

ThermistorNTC = negative

temperature coefficient

As temperature increases, it’s resistance decreases

At 100ºC, what is the resistance of the thermistor?

72Ω

Resistance changes with temperature

Temperature alarm

This operates as a ‘cold’ alarm. How could you modify it to work as a ‘hot’ alarm?

Breadboard

Making the output more powerful

Fixed potential divider

Variable potential divider

Op Amp Transistor

Current limiting resistorProblem – the transistor always remains on – WHY?

The output only drops to 2v – transistor switches on at 0.6v

The solution

FETThe FET turns off at a higher voltage.

Inverting AmplifierRf

Rin

Vin

VoutGain =

Rf

Rin

-

Vout

Vin

Input = 1vGain = -

6010 = -6

Output = -6v

Feedback resistor

More Maths !!!!

Rf

Rin

Vin

Vout

Gain = Rf

Rin

-If the gain of the amplifier is to be -10, calculate the value of the feedback resistor.

5K6

Gain x Rin = - Rf-10 x 5k6- Rf =

=56k- Cancel out

Questions

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