In this instructable I hope to cover the fundamentals of DC circuits involving operational amplifiers, otherwise known as op-amps. The material I will present has been adapted from my Electronics op amp circuit analysis pdf materials at Pomona College. For a textbook explanation of these concepts, I highly recommend either Scherz and Monk’s Practical Electronics for Inventors, or Curtis Meyer’s Basic Electronics: An Introduction to Electronics for Science Students.
The good news about op-amps is that there are a few relatively simple first principles from which the behavior of any circuit can be readily deduced. To explain them, it helps first to get familiarized with the op-amp itself.
The first thing you’ll notice is the hefty number of inputs and outputs, eight to be exact. For the purposes of this instructable, we can safely ignore the two labeled balance, as well as the one labeled NC, which should hopefully make things a little less daunting.
The most important part of building any op-amp circuit is. This is because the op-amp is an active circuit element, which basically means that it generates energy in your circuit.
This is as opposed to it being a passive circuit element, such as a resistor, which strictly draws energy circuit. As such your op-amp always needs to be connected to a power source.
These inputs are not always drawn in circuit diagrams, so it is important to remember that it is implied that your op-amp is powered via these two inputs in any circuit diagram. The next thing about your op-amp that should jump out at you is that it has two inputs, but only one output. The output is, well, the output. The output signal is, in general, some modified version of the input signal, the details of which will be covered soon.
The upper of the two inputs, denoted by the minus sign in the triangle, is known as the inverting input, while the lower input, denoted by the plus sign, is known as the non-inverting input. Op-amps, for the most part, only get interesting when the output is directly linked to one of these two inputs, producing what is known as feedback. Negative feedback, which we will soon see is the “good” kind, refers to when the output is directly linked to the inverting input. Positive feedback, on the other hand, refers to when the output is directly linked to the non-inverting input.
Now that we have these terms hammered down, we can progress to the so called “golden rules” of op-amps, and then to circuit analysis. As you may have guessed, and operational amplifier amplifies! But what does it amplify, and by how much? V-, multiplied by an amplification constant A.
This constant is generally on the order of one million or more! We surely aren’t getting a million or more volts out of our op-amp are we?