Op-amps

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Contents

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.

  • 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.

Theory

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

Virtual-Ground concept

When the positive input of an op-amp is grounded, the negative input undergoes a state called the virtual-ground. In this state the DC level at the negative input is extremely close to ground.

Example.jpg

This is the basis for many for many functions preformed by an operational amplifier, and many of the equations are derived from this mode. R1 is the feedback resistor. Because the negative input appears to be grounded R2 is the basis for input impeadance. The closed loop gain can be derived by the following equation.

Op-amp-virtual-ground-equ.gif

where a equals open loop gain

Voltage Follower

Voltagefollower.jpg

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. Noninverting.png

To calculate output use the following formula.

Closedloop1.jpg

R3 can be calculated by the following.

Closedloop2.jpg

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

Closedex1.gif

and for R3

Closedex2.gif

lets graph it.

1vin5vout.jpg

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.

Closedex3.gif

and for R3

Closedex4.gif

lets graph it.

12vin3v3out.jpg

Summing amplifier

The summing amplifier will take voltages on the inputs and output the negitive of sum of the voltages.

Summing amplifier.jpg

In the above circuit you will find a basic summing amplifier, with 4 inputs and a single output. What this circuit does is adds the voltages V1 to V4. Lets assume for now that there is unity gain(no gain) and R1 through R4 equal 1000 ohms and the feed back resistor Rf = 1000 ohms and Rom = 200 ohms. This will add the voltages in the following way.

V1v2v3v4.gif

the following is the equation for the above circuit.

Summing math.gif

This would be useful but if the op-amp is powered by a single +5V, the output would not swing above 5v, But what if you want to add and scale the sum to be within 0 to 5 volts? This is where the math gets more complicated, lets assume the same values as before except Rf=250 and Rom=125 and the previous formula for Vout will change to

plug in the values for V1 through V4 = 5V Summing ex1.gif

Why does Rom and Rf change, and what are their relationships to the output? Rf is the feed back resistor and is one half of how the gain is determined. R1 through R4 could have been changed just as easily.

The values for Rom can be derived by

Summing rom.gif

Latex equations

This is a history of the LATEX formatted equations on this page:

  • Virtual Ground
\mathit{gain}=\left({R}_{1}/{R}_{2}\right)\left\lbrack : 1/\left(1-\left(1/a\right)\right)\left(1+{R}_{1}/{R}_{2}\right)\right\rbrack
  • Non inverting
example 1
math>\frac{5000}{1000}\ast \mathrm{1V}=\mathrm{5V}</math>
math>\frac{1000\ast 5000}{1000+5000}=833</math>
example 2
math>\frac{220}{860}\ast \mathrm{12V}=\mathrm{3.06V}</math>
math>\frac{860\ast 220}{860+220}=175</math>
  • summing

by no means is this a complete list.