1. Operational Amplifiers
Mr Stockley HoD

2. 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 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).

3. What is an Op Amp? A high gain voltage amplifier
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

4. Why use them? Check out the transistor Small Current Larger Current
Transistor current gain
(hfe) = Ic
Small Current Ib
Larger Current Ic Ib
MAX gain
= 150 Ie

5. - Anatomy of an op amp + It is an 8 pin DIL IC. Positive supply
Copy this
3140 pin numbers -
Inverting Input 7 2 6
Output
741 +
Non Inverting Input 3 4
It is an 8 pin DIL IC.
Negative supply
Compares the voltage difference between the two inputs
Page

6. Two common types
LM324

7. Comparing objects 1 1 + - 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’ 1 1

8. A Practical Comparator Circuit
LM318
Fixed Potential Divider
Variable Potential Divider
Copy this
Amplifies x
Needs 9 / v at input to make LED light (= 90µv)
90µv change
Output current is only 10mA
4.5v
Create a Yenka circuit and print a copy

9. Dark Detecting Circuit+ -
Pin 1 NC
Pin 2 inv Input

10. 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.
Vin R1
Vout
COPY INTO BOOKS R2
Vout = x Vin R2
R1 + R2
Equation
Page 103, 107, 108, 109

11. The Maths ? 1v R2 Vout = x Vin R1 + R2 1K Vout = x 9v 8K + 1K 1
Try this one

12. Potential Dividers Alternative arrangements;
A variable resistor enables the voltage to be adjusted.
Other resistive devices can be used (LDR / thermistor)
Using a potentiometer (3 legs)

13. How sensitive is it? The gain of the op amp is 100 000
Does that mean that a voltage difference will give v?
No, it means that to get 9v at the output, a voltage change of 9/ v is needed = v or 90µv !!!!
A very small voltage change indeed – (much less than with a transistor)

14. Resistance changes with temperature
Thermistor
Resistance changes with temperature
NTC = negative temperature coefficient
As temperature increases, it’s resistance decreases
At 100ºC, what is the resistance of the thermistor?
72Ω

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

16. Breadboard

17. Making the output more powerful
Variable potential divider
Fixed potential divider
Transistor
Op Amp
Problem – the transistor always remains on – WHY?
Current limiting resistor
The output only drops to 2v – transistor switches on at 0.6v

18. The solution
The FET turns off at a higher voltage.
FET

19. - Inverting Amplifier Rf Rin Vout Vin Rf Rin Gain = Vout Vin
Feedback resistor
Rin
Gain =
Vout
Vin Rf
Rin - 60 10
Gain = -
= -6
Input = 1v
Vout
Vin
Output = -6v

20. - More Maths !!!! Rf Rin - Rf = - Rf Rf Rin Vout Vin Gain = -10 x 5k6
If the gain of the amplifier is to be -10, calculate the value of the feedback resistor.
Gain =
- Rf
- Rf =
-10 x 5k6
Gain x Rin =
=56k
- Cancel out

21. Questions
Pages; 53 54 55 56 57