Op-amps

From DP

Contents 1 The Operational Amplifier 1.1 Introduction 2 Theory 2.1 Virtual-Ground concept 3 Voltage Follower 4 Simple non inverting 4.1 Example 1 4.2 Example 2

The Operational Amplifier

Introduction

The operational amplifier is one of the most useful analog IC available, and one of the most daunting for beginners and experienced engineers. This page is to introduce some of the uses, and how to calculate values for the various configurations.

Theory

The operational amplifier can operate in many difrent ways because of its gerral characteristics.

Virtual-Ground concept

Voltage Follower

A voltage follower uses the the properties of the op-amp to make the input equal to output. It does this because the op amp wants the negative and positive inputs to be equal. It will adjust the output according to the values on the inputs, since the output is directly tied to the negitive(or inverting) input, the op-amp acts like an analog buffer.

Simple non inverting

Below is a diagram of an op-amp in a closed loop non inverting configuration. Probably the most used configuration. Lets assume this is an ideal amplifier. R₁ is the input and R₂ is the feedback. When R₁ = R₂ the circuit is in unity(meaning input = output).

To calculate output use the following formula.

<math>{V}_{\mathit{out}}=\frac{{R}_{2}}{{R}_{1}}\ast {V}_{}</math>

R₃ can be calculated by the following.

<math>{R}_{3}=\frac{{R}_{1}{R}_{2}}{{R}_{1}+{R}_{2}}</math>

Example 1

In order to bring a 1V peak input to a 5v range we would need R₂=5k and R₁=1k and R₃=833

<math>\frac{5000}{1000}\ast \mathrm{1V}=\mathrm{5V}</math>

and for R₃

<math>\frac{1000\ast 5000}{1000+5000}=833</math>

lets graph it.

Example 2

Lets say that you want to measure a 12 volt max signal with a 3.3v a/d converter. Using the same formulas as before and standard resistor values.

<math>\frac{220}{860}\ast \mathrm{12V}=\mathrm{3.06V}</math>

and for R₃

math>\frac{860\ast 220}{860+220}=175</math>

lets graph it.

• While for these examples it is fine to assume an ideal amplifier. Many op-amps do not go rail to rail and have varying amounts of linearity. In the next section we will address these issues, and look at some other properties of common op-amps.