Unified Robotics I: Actuation

Operational Amplifier

Introduction

You were first introduced to operational amplifiers or op-amps in RBE 1001. They are used in many different kinds of devices such as:

    • Comparators
    • Amplifiers
    • Battery Monitors
    • D/A and A/D Converters
    • Linear motor controls
    • Other Applications

When you buy an op-amp today it generally comes as an integrated circuit or IC. Here you can see some eight-pin DIPS or dual in-line packages.

Figure 1 - Pictures of Operational Amplifiers

But the question still stands, "What is an op-amp?"

Well the answer is that an op-amp is just a really high gain voltage amplifier!

But before using an op-amp we must first understand how it works. And to understand how they work we're going to need to build a model. We'll begin with a visual model and then work up to a mathematical model.

 

Modeling an Op-Amp

We'll start with the symbol for an op-amp.

Figure 2 - Op-amp symbol

Looking at the left hand side of the triangle we can see that there are two inputs: one with a positive sign and one with a negative sign. The input with the positive sign is called the non-inverting input and the input with the negative sign is called the inverting input. Both inputs may have different voltages connected to them. We'll come back to the inputs momentarily.

Now let's look at the right hand side of the op-amp. There we have a single output.

Unlike some of the circuits we've looked at previously, an op-amp is an  active circuit. It therefore needs to be powered. The power is connected to the Power Rails; the voltage applied to the these will vary depending on the particular op-amp you are using and the application.

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