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
COPY INTO BOOKS
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
Pages;5354555657