Op-Amp Experimentation 1: Op-Amp Basics

Op Amp Experimentation 7: PID Controller – coming soon

Introduction

In my post series about building a torque sensor I briefly covered the usage of op-amps. Now, one thing I don’t think I mentioned is that a large amount of the information I wrote was stuff I learned specifically for writing that post – or at least stuff I had forgotten, and had to review. The reason I mention this is because I firmly believe the absolute best way to really learn something is to teach it. In that same spirit, I really want to truly nail down op-amps, so I’m writing this post to further my own understanding as well as provide a good resource of information for others. So without further ado, I present “Op-Amp Experimentation!”

Op Amp Basics

Let’s begin by starting with the basics. Op-amp is short for Operational Amplifier. Op-amps are used in practically everything, especially in analog circuits. Without wasting too much time discussing terminology, I’ll just just post a picture of the circuit schematic of an op-amp.

Some quick definitions:

$\displaystyle {{V}_{{S+}}}$ – Positive power supply

$\displaystyle {{V}_{{S-}}}$ – Negative power supply

$\displaystyle {{V}_{{+}}}$ – Non-inverting input

$\displaystyle {{V}_{{-}}}$ – Inverting input

$\displaystyle {{V}_{{out}}}$ – Output voltage

Ideal Op-Amps

It is at this point that every class I have taken talks about ideal op-amps, and lists a bunch of rules for ideal op-amps. I’m going to try and simplify this, but for the most part this op-amp page is just a rehash of every other op-amp introduction. However, I’m just going to post the rules and move on – it is much easier to learn op-amps from a practical application side than from a theoretical side. Anyways, here’s another picture from Wikipedia:

The main rules are (from Wikipedia):

Infinite open-loop gain: $\displaystyle G=\frac{{{{v}_{{out}}}}}{{{{v}_{{in}}}}}=\infty$
Infinite input impedance: $\displaystyle {{R}_{{in}}}=\infty$ so zero input current: $\displaystyle {{i}_{{v+}}}={{i}_{{v-}}}=0$
Zero input offset voltage: $\displaystyle {{v}_{+}}={{v}_{-}}$

Quote from Wikipedia:

In a closed loop the output attempts to do whatever is necessary to make the voltage difference between the inputs zero.

The inputs draw no current

Now the basic equation for an Op-Amp is ${V_{out}} = G\left( {{V_ + } - {V_ - }} \right)$ .

Open Loop vs. Closed Loop

So open-loop and closed-loop have been mentioned and not explained. In the future I will be making a series of pages on Control Systems, but for now I’ll just give a simple explanation. Generally speaking, closed-loop implies feedback while open-loop does not. A common example of an open-loop system is a washing machine – you tell the machine to wash and it just washes. There aren’t any sensors to determine if the clothes are “clean”, there is nothing sending data back to the washing machine from the clothes, it just runs out the clock and stops. A closed-loop system example is an oven – you set the oven to 400°F and the oven starts heating up – it heats up until the thermometer reads 400°F at which point the heater shuts off. A closed-loop system has a feedback from the output to the input.

Fancy description, how exactly does that apply to op-amps? Simple. The output is literally connected electronically to the input in a closed loop application. Here’s some more Wikipedia pictures:

Closed Loop Op-Amp = G is calculated from external circuitry

On the next page I will cover the math behind op-amps.

Next page: Op-Amp Experimentation 2: Basic Circuit Math

Op-Amp Experimentation 1: Op-Amp Basics

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Op-Amp Experimentation 1: Op-Amp Basics

Introduction

Op Amp Basics

Ideal Op-Amps

Open Loop vs. Closed Loop

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