Eric Brasseur Home    |    Links    |    Contact    |   
   



An image of the operational amplifier
En français
В українському on prchecker.info

The operational amplifier is the fundamental part of a great number of electronic systems. It is used in small assemblies as well as in high technology devices. 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 :


opamp lm741


This is how it is represented in electronic diagrams:


ampo2


And here's how it is represented in the handbooks of the manufacturers :


manufacturer
        representation


The wire marked NC is used for nothing : NC means "Not Connected". The wires marked "OFFSET NULL" are sometimes useful for precise adjustments.

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:

ampo4


What does the robot? It constantly looks at the two voltmeters and does this:
A practical example: the tension follower. One simply connects the output of an opamp to its "-" entry:


follower of tension


To understand the operation of the tension follower, ask yourself these three questions:
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:


formula


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 circuits.




       reverser

The inverter



follower

The follower



amplifier reverser

The amplifier - inverter



amplifier

The amplifier



adder

The adder



subtracter

The subtracter



shunting device

The deriver



integrator

The integrator



       detector of threshold

The threshold detector



detector of threshold to
        hysteresis

The threshold detector with hysteresis



OR gate

OR Gate



NOR gate

NOR Gate



        oscillator

An oscillator



injector of current

An current injector



powerful follower

A powerful tension follower



Eric Brasseur  -  1994
www.000webhost.com