An image of the operational amplifier
amplifier is the fundamental part of a great number of electronic
systems. It is used in small assemblies as well as in high technology
This text proposes a way to apprehend its functioning.
The most known opamp is the LM741. You can enter an electronics
store and buy one for 50 ¢. The picture below shows what it
resembles, magnified three times. Each of the eight legs is a small
electric wire :
This is how it is represented in electronic diagrams:
And here's how it is represented in the handbooks of the
The wire marked NC is used for nothing : NC means "Not Connected".
The wires marked "OFFSET NULL" are sometimes useful for precise
Here follows an "image" of how an opamp works. We will consider that
an opamp contains four things: two voltmeters, an adjustable power
supply, and a robot.
Four remarks about these four objects:
- So that the adjustable power supply can output current, it is
necessary for the opamp to be connected to a power source. You
can for example connect V+ to the positive pole of a 9 V
battery, and V- to its negative pole (0 V).
- The power supply is not very powerful. It will not provide a
strong electric output. At most a few tens of milliamperes.
(Unless you are using a power opamp, of course.)
- When the power supply is tuned to the maximum, it will output
almost V+ (9 V). At the minimum, almost V- (0 V). Between the
two, it can output any tension (1.3 V, 2 V, 4.73 V, 6.89 V...).
The adjustment of the power supply can change from the minimum
to the maximum in about one millionth of a second.
- The two voltmeters have the names "+" and "-". They are not
rigorously completely identical. And, one should not try to make
them measure tensions higher than V+, or lower than V-; they
would function badly or could even burn.
What does the robot? It constantly looks at the two voltmeters and
- If the voltmeter "-" measures a tension higher than the
voltmeter "+", then the robot decreases the tension that the
power supply outputs.
- If the voltmeter "-" measures a tension smaller than the
voltmeter "+", then the robot increases the tension that the
power supply outputs.
- If the voltmeters measure the same, the robot changes nothing
to the tension that is being ouput by the power supply.
A practical example: the tension follower. One simply connects the
output of an opamp to its "-" entry:
To understand the operation of the tension follower, ask yourself
these three questions:
- If at the time one switches this circuit on, the output
produces 4.5 V, and U1 is worth 2 V, how will the opamp react?
Answer: the opamp will have 4.5 V on its input "-", and 2 V on
its input "+". As 4.5 is more than 2, it will lower the output
voltage quickly. When the output voltage passes by 2 V, there
will be 2 V on the entry "-". Having the same tension on the two
entries, the opamp robot does not touch any more to anything and
keeps 2 V on the output. U2 = U1.
- If suddenly the U1 signal passes from 2 V to 2.001 V, how will
the opamp react?
Answer: the opamp will have 2 V on its "-" input, and 2.001 V on
its "+" input. As 2 is smaller than 2.001, it will increase the
output voltage quickly. When the output voltage passes by 2.001
V, there will be 2.001 V on the "-" input. Having the same
tension value on the two inputs, the opamp leaves things like
they are and keeps 2.001 V on its output. U2 = U1.
- Imagine one connects the output of the opamp to an apparatus
which is a strong consumer of current. Suddenly there is a
tension drop on the output of the opamp, from 2.001 V down to
say 1.997 V. How will the opamp react?
Answer: the opamp will have 1.997 V on its input "-", and 2.000
V on its input "+". As 1.997 is less than 2.001, it will
increase the output voltage quickly. When the output voltage
passes by 2.001 V, there will be 2.001 V on the input "-".
Having the same thing on the two inputs, the opamp leaves things
like they are and keeps 2.001 V on its exit. U2 = U1.
The tension follower is a very useful circuit: the U1 signal can
come from an apparatus very sensitive and delicate, the input "+" of
the opamp will not make him undergo any load. The output, on the
other hand, can be connected to just anything. The opamp will fight
as a cow-boy in a rodeo to guarantee that U2 will be a certified
copy of U1. Within a millionth of a second.
If you understand this picturesque view of an opamp, you will be
able to apprehend or conceive yourself 95% of the electronic
circuits using an opamp. For the 5% remaining, and for courses in
high school, you will rather have to use the idea that an opamp is a
machine used to make the following formula true:
Now follow some general diagrams of applications of an opamp. You
can choose yourself the values of the resistors (or potentiometers).
Commonly, one uses values between 1 K and 100 K.
Consult the catalogues of the manufacturers to find operational
amplifiers with the characteristics or qualities you need: power,
speed, precision, low energy consumption... You will also have to
learn sometimes, how to place condensators or "shock absorbers" so
that an opamp does its work without running off the line. You will
also have to take into account the limits of each opamp: maximum
speed, maximum amplification, background noise, maximum or minimal
tensions, technical characteristics... Note a cheap opamp is
sometimes easier to operate and yields better results than an
expensive one, because it is "calmer".
I recommend the LM386. Caution: the tension levels allowed on its
inputs and on its output are special. Closely read its
characteristics inside a data sheet before using it.
The diagrams below are relatively "academic". Do not hesitate to
adapt them. You can feed an opamp with the output of another. You
can use a separate battery for each opamp. You can short-circuit the
outputs. To put it short: make things simple and enjoy yourself.
That way you will get a better understanding and become more alert
to the many precautions to be taken for the realization of reliable
The amplifier - inverter
The threshold detector
The threshold detector with hysteresis
An current injector
A powerful tension follower
Eric Brasseur - 1994