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Op-Amps: A Beginners Guide

Sam Sattel

eagle-academy

We’re Going to Amp You Up! The Beginner’s Guide to Operational Amplifiers (Op-Amps)

We all live in a world surrounded by the wonders of amplifiers. If you’ve ever been to a concert and heard the loud electric squeal of a guitar blast through the stadium, that’s an amplifier at play. Or the speakers pouring music through your radio on a lazy Sunday afternoon, amplifiers again. In this world of amplification, the goal is simple – to boost the electric current and voltages up a notch. But are all amplifiers created the same, or used for the same purposes? Definitely not. We’re here to learn about the mysterious black box of the amplifier family, and how it plays a way bigger role than just with sounds. It’s the operational amplifier, and it’s here to amp you up!

Amplifiers at a Glance

Before diving into the intricacies of the op-amp, let’s first understand what amplifiers as a general category of components do for the world of electronics. Ever get your hands on a hearing aid? They’re a perfect example. Hearing aids use a microphone to pick up sounds from the external environment, which then gets turned into an electrical signal. Inside this hearing aid, there’s an amplifier that takes that signal, boosts it up to make it louder, and sends it to a speaker resting inside your ear canal. It’s not magic, just engineering!

how-hearing-aids-work

Amplifiers are what make hearing aids possible. (Image source)

This entire process of taking an input signal, amplifying it, and sending it onwards as an output signal is the essence of amplifier circuits. The boost that an amplifier produces for a given signal is measured in gains, or gain factor. This is simply the difference in voltage between an input signal and an output signal. For example, if you start with 1 volt at your input, and get 5 volts at your output, then you have yourself a gain of 5. For sound related amplification, this gain is measured in decibels (dB).

So What Makes an Op-Amp Special?

While all amplifiers might have the same general purpose, when you need an ideal amplifier then you want to use an op-amp. Within analog electronics, nothing comes close to matching the ideal performance of an amplifier as this device. The beautiful thing about an op-amp is that you can mix and match active parts like transistors with passive components like resistors, capacitors, etc. to get some useful amplification characteristics, like:

  • High gain. One of the most well-known features of op-amps is their very high gain, which can range from 10,000 to 100,000! Granted, this level of gain used in an open loop amplifier is a bit useless and overkill, which is why you’ll be adding sources of feedback to control gain levels and distortion.
  • High input impedance. Another key attribute is a high impedance, and the op-amps in production these days come with input impedances that are almost infinite, measuring in at 0.25M ohms or even hundreds of millions of ohms.
  • Low output impedance. In a perfect amplifier, you’ll get zero output impedance, and an op-amp is the only physical component that comes close to achieving this. You’ll find most IC-based op-amps having an output impedance of less than a hundredth of an ohm.
  • Limited bandwidth. Op-amps also have a limited bandwidth, which can work in your favor. Many op-amp ICs used for audio applications only experience their complete gain over a small bandwidth. But in other circuits, you’ll be wanting to reduce that gain, which is where a limited bandwidth comes in handy.

The Inner Workings of an Op-Amp

The great thing about op-amps is that they aren’t just used to amplify sound like a traditional amplifier. You’ll also see them being used for:

  • Audio and video frequency pre-amplifiers and buffers
  • Voltage and current regulators
  • Analog calculators
  • Precision peak detectors
  • Analog-to-digital and digital-to-analog converters
  • And a whole lot more!

Regardless of its particular purpose, an op-amp always aims to deliver an output voltage raising or lowering input voltages until the are equal. But how does it make that happen? Let’s take a look at a typical op-amp schematic symbol to explain how it works. Here’s what you need to know:

ideal-op-amp-schematic-symbol

An ideal op-amp schematic symbol on its own with no feedback added.

Input Signals

There are two input signals, an inverting input marked with a negative (-) symbol, and a non-inverting input indicated with a positive (+) symbol. When you have a positive signal on your inverting input, then you’ll get an amplified output that is the inverse, or the opposite of this signal. So if a positive signal goes in, then your output will be negative. The non-inverting input works the opposite way. If you send a positive signal into a non-inverting input, then you’ll get an output that matches the input signal, just amplified.

Op-Amp-Inverting-and-Non-Inverting-Inputs

The type of input used has a direct effect on the signals output in an op-amp circuit. (Image source)

Output Signals

On the other side of the schematic symbol for this op-amp is the output. This output takes the difference between both your inverting and non-inverting input signals to produce an amplified output signal until the input voltages are equal. This is why an op-amp is commonly referred to as a differential amplifier because it provides an output result based on the difference between the two input signals.

Power Signals

You need to power your device. At the top and bottom of the op-amp symbol, you have a V+ and V-, which notes the positive and negative sides of a DC power connection. These power supply rails often won’t be shown in circuit diagrams since it’s assumed that they will always be connected. Whether you see them or not, you’ll find op-amps connected to a +5-15V and -5-15V power rail, all based on the specifications of the op-amp chip you plan to use.

The Most Common Op-Amp Circuits

Op-amps have an extremely high gain that you won’t be able to change. This is why you’ll be adding feedback into your op-amp circuit with the addition of resistors, capacitors, or inductors to control the gain and get different results from your circuit. This addition of feedback loops also allow you to easily create variations on an op-amp circuit to get some widely different results. Here are the most common circuits you’ll be building when you first start out:

  • Op-amp Schmitt trigger. This circuit configuration provides immunity to noise and different switching levels that are dependent on your op-amp circuit being in a high or low voltage state.
  • Op-amp summing amplifier. This circuit configuration is perfect for summing audio inputs, and you’ll find it commonly used in audio mixers.
  • Op-amp comparator. This is the configuration of choice when you need to provide a high or low signal depending on the state of your two inputs.
  • Op-amp inverting amplifier. This circuit is the most common op-amp configuration and is well known for providing gain while also being used as a virtual ground amplifier.
  • Op-amp non-inverting amplifier. This circuit configuration produces high gain and high input impedances and is used in many amplifier input stages.
inverting-non-inverting-op-amp-circuit

An inverting and non-inverting op-amp circuit, side by side. (Image source)

Shopping for Op-Amps

You’ll find many different kinds of operational amplifier packages out there from your favorite parts supplier. Most are categorized by many values, including:

  • Nominal slew rate
  • Number of channels
  • Maximum input offset voltage
  • Maximum supply voltage
  • Packaging type
  • Nominal gain bandwidth

The most common op-amp nominal gain bandwidths you’ll find are 1 MHz, 1.3 MHz, and 4 MHz. You’ll also find the number of channels being anywhere between 1 and 8, with the most common op-amps having 1, 2, or 4 channels.

For package types, the most famous op-amp you’ll encounter is the 741, which comes in an 8-pin mini-DIP package. This op-amp packs in 20 transistors and 11 resistors, and has been the op-amp configuration of choice since 1968. It also happens to be the cheapest of the bunch, costing less than a dollar.

ua741-op-amp

UA741 op-amp IC, ready to be snapped into your breadboard or soldered!

You’ll also see op-amp ICs available in SOIC packages, which makes them easy to add to a circuit without taking up too much space. More often though, op-amps will be available as DIP packages with eight, fourteen, or sixteen pins. This package makes them easy to solder by hand on your next prototype or snap quickly into a breadboard.

Getting Amped Up

There you have it, everything you might need to know about operational amplifiers as an electronics design beginner! This IC is the component of choice when you need a high-performance amplifier that provides high gain, high input impedance, and low output impedance. And by being able to swap out the various feedback components like resistors, capacitors, and inductors, you can fit together a ton of different variations to produce your own multivibrator, analog-to-digital converter, or precision timer circuit.

Are you getting amped up to design your very own op-amp circuit? Try Autodesk EAGLE for free today!

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