Download Operational_Amplifiers

Operational Amplifiers
Mr Stockley HoD
What the specification says;
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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?
A high gain voltage amplifier
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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
Why use them?
• Check out the transistor
Transistor current
gain
(hfe) =
Small
Current
Ic
Ib
Larger
Current
Ic
MAX gain
Ib
= 150
Ie
Anatomy of an op amp
Positive supply
Inverting Input 2
-
7
6
741
Non Inverting Input 3
+
3140 pin
numbers
4
Negative supply
Output
It is an 8 pin
DIL IC.
Compares the voltage difference between
the two inputs
Page 117 - 119
Two common types
LM324
Comparing objects
+
-
Output
0
1
0
1
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’
A Practical Comparator Circuit
LM318
Variable
Potential
Divider
Fixed
Potential
Divider
Amplifies 100000 x
Needs 9 / 100000v at input to
make LED light (= 90µv)
90µv change
4.5v
Create a Yenka circuit and print a copy
Output
current is
only 10mA
Dark Detecting Circuit
+
Pin 1 NC
Pin 2 inv Input
Potential Dividers
• It is two resistors connected in
series across a potential (or
voltage)
Vin
R1
Vout
• The voltage at the mid point of
the two resistors is determined
by the ratio of the resistors and
the input voltage.
Equation
Page 103, 107, 108, 109
R2
R2
Vout =
x Vin
R1 + R2
The Maths
Vin
R1
R2
Vout
?
R2
Vout =
x Vin
R1 + R2
1K
Vout =
x 9v
8K + 1K
1
Vout =
x 9 = 1v
9
?
?
Try this one
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)
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)
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
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
Variable potential
divider
Fixed potential
divider
Op Amp
Problem – the transistor always remains on – WHY?
Transistor
Current limiting resistor
The output only drops to 2v – transistor switches on at 0.6v
The solution
The FET turns off at a higher voltage.
FET
Inverting Amplifier
Feedback resistor
Rf
Rin
Vout
Vin
Gain = -
Input = 1v
Gain =
60
10
= -6
Rf
Rin
Vout
Vin
Output = -6v
More Maths !!!!
Rf
Rin
5K6
Vout
Vin
Gain =
-
Rf
Rin
If the gain of the amplifier is to be -10, calculate the value
of the feedback resistor.
Gain x Rin = - Rf
- Rf = -10 x 5k6
- Cancel out
=56k
Questions
Pages;
53
54
55
56
57