Or op amp, is basically a two-input analog comparator. It usually uses feedback to force amplification of a signal, but is also used in analog summing, differentiation, latching, and more.

There are also CMOS versions of the op amp available which allow analog switching in CMOS circuits, as well as phase-locked loops, all with direct CMOS logic compatibility!
Op amp is short for operational amplifier, a useful little component in many analog circuits. An ideal op amp has a positive and a negative input that draw no current, and an output port that can source CV current, where V is equal to the voltage difference between the positive and negative inputs, and C is a very large constant. In regular op amps, C can vary by up to 50%, but as you'll see, it doesn't matter much.

To use the op amp as an amplifier, tie the positive input to ground, and stick a resistor R1 between your input and the negative input. Then, tie the output back to the negative port with a resistor R2. The resulting amplification should be equal to -R2/R1. The best part is that R2 and R1 can usually be substituted with impedance elements like capacitors and inductors, and the equation will still hold after the appropriate Laplace Transformations have taken place.

As long as op amps are used within the spec for frequency and aren't expected to output past their source voltages, they will perform very nicely.

The symbol below represents an op amp. The top input is the inverting input VI, the bottom is the non-inverting input VNI. The two vertical lines marked VCC and -VCC are the op amp's power supply, and are usually connected to the positive and negative power rails (voltages range from around 4-15V), although a couple of circuits (eg the integrator) have the power supply connected to positive and zero voltage rails. VCC is an abbreviation for 'supply voltage'.

        |\   |
        |  \ |
VI -----|-   \
        |      >--- VO
VNI-----|+   /
        |  / |
        |/   |

There are many different configurations of op amp. Here are a few configurations that may come in useful.

The bastion of analog computing. "op-amp" is short for "operational amplifier."

An op-amp is a circuit whose output is the sign of the difference of the two voltage levels applied to the inputs.

Here's a schematic diagram:

    | -  |
    |   -|
    |     -
----|+      -
    |xxx      >---Vout
----|-      -
    |     -
    |   -|
    | -  |

As I said, a lone op-amp will only output the sign of the difference between the inputs. The sign is determined by subtracting the voltage level at the "-" input from the "+" input. If the sign is positive:

Vout = V+

If the sign is negative:

Vout = V-

If you want to use an op-amp to amplify by a certain factor, you need to use negative feedback.

There are many differences between a real op-amp and an ideal one. These differences are quite important for the correct development and operation of any circuit involving the devices.

A real op-amp (specifically the 741 model of op-amp chip) has the following properties:

The properties of an ideal op-amp are:

On a related note, my Electronics class at school was recently set the following question for homework:

Outline three important differences between an ideal op-amp and a real op-amp.

In answer to this question, a friend gave the following:
One of the differences between an ideal op-amp and a real op-amp is that an ideal op-amp is difficult to manufacture.

When marking this homework, my Electronics teacher simply burst out laughing at this, and promptly read it out to the class. Needless to say, very little work was done in the next few minutes while we laughed. And just in case folk were wondering, this was not stupidity on my friend's behalf. He put down the three correct answers, but in his usual style, this was simply added to the end.
Any of the ideal op-amp properties listed above would have been suitable answers to the question with the exception of the final entry on the list - the saturation levels of the op-amp are not normally considered an important aspect of the system.

The exam paper the question was taken from is the General Certificate of Education Advanced Level 12 June 1992 Electronics Paper 1 from the University Of Cambridge Local Examinations Syndicate - Oxford And Cambridge Schools Examination Board. The question is 9(ii)
The information on the properties of real and ideal op-amps is taken from my A Level Electronics course notes.

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