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Learning about Operational Amplifiers

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Using the ADALM1000 together with its companion component kit and tutorials to learn about operational amplifiers.

I've recently started a HND in electrical and electronic engineering and as a trainee I still a have a lot of fundamental principles to learn. Over the past couple of years I have gained some experience in digital electronics using microcontrollers such as Arduino. However, in comparison to this I only have minimal experience of analogue circuits.

Analog Devices ADALM1000 and ADALP2000

The ADALM1000, also known as the active learning module, is a USB device which can generate and measure AC/DC voltages and currents. Previously I wrote about my initial experiences using this to learn more about the relationships between voltage, current and impedance. This involved constructing very simple circuits and using the module and pixelpulse2 software to measure the gain of an audio amplifier.

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Since publishing this post I discovered that Analog Devices also produce a companion kit to the module, the ADALP2000, which has 100s of components that you can use in experiments with the ADALM1000. At analog.com/university there are a wide range of examples and tutorials for use with both the module and companion kit.

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I looked at some of the activities on the Analog Devices website and decided to attempt the first, Activity 1:Simple Op Amps. Since I had never used operational amplifiers before I did some background reading from The Art of Electronics and the associated Student Manual.

What is an Op Amp?

An operational amplifier (Op Amp) is fundamentally a voltage amplifying device that almost always uses feedback. They are one of the basic building blocks for many analogue electronic circuits and can be found in everything from mixing desks to instrumentation.

1.1 Op Amp Basics

Unity-Gain Amplifier

A unity-gain amplifier provides the same voltage at the output as it receives on the input, i.e. Vin = Vout. This happens while drawing minimal current from the input signal, therefore it does not put any load on the circuit.

Using a unity-gain amplifier can be beneficial in many situations where you do not want to load a circuit (draw current from it). E.g. a rectifier or filter where the output is across a capacitor. In a situation like this drawing current from the capacitor would discharge it, therefore reducing the voltage.

In scenarios like this it may be very important to ensure that by taking a voltage measurement it does not influence the circuit producing the voltage which is to be measured.

Activity

I started the activity in the Unity-Gain Amplifier (voltage follower) section and as directed I built the circuit following the schematic. I set the channel A supply voltage to a minimum of 1v and a maximum of 4v at 500Hz.

I measured the output voltage using channel B. As expected this was 3v peak-to-peak and was centred at 2.5v. This showed that the circuit was at equilibrium and therefore, working as it should.

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Buffering Amplifier

Very similar to a unity-gain amplifier in the sense that this makes a copy of the input voltage and puts it at the output. This may appear useless at first however, the buffer is an extremely useful circuit since it helps to solve impedance issues. The input impedance of the op-amp buffer is very high - close to infinity. The output impedance is very low and just a few ohms.

This means buffering amplifiers can be used to help chain together sub-circuits while addressing impedance mismatches.

Activity

I proceeded by moving on to the buffering section of the activity and as previously I set up the circuit how it was shown in the schematic. As this type of amplifier is very similar to the previous this showed almost identical results.

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Next the load on the op-amp was changed to see what the affect would be on the current drawn from channel A.

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As expected there was no change in the load on channel A.

1.2 Simple Amplifier Configurations

Inverting amplifier

As suggested by the name of the amplifier this is a circuit which amplifies the input signal and also introduces a phase shift of 180° (inverts), therefore both inverting and amplifying. Here the amount at which the voltage is amplified is known as the amplification factor (gain).

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The measurements show this amplifier working as it should as it both amplifies the signal and also introduces the expected phase shift.

Summing amplifier

A summing amplifier can be used to combine multiple input voltages into a single output voltage.

This is done by connecting more resistors to the input of an inverting operational amplifier – to get an equal mix each should be equal in value.

Activity

I wired up the circuit and tried to configure it with Pixelpulse2. It was at this point that I realised I should have been using another piece of software called ALICE, which is also able to drive the PIO pins!

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A Great Tool for Students

Using Op-Amps was just one of many activities found on the Analog Devices website. In my opinion the two ADALM1000 and ADALP2000 are fantastic learning resources for students; the hardware, software and activities are great for learning the fundamentals of electronics. This enables a hands on approach to self-directed learning with minimal help. For someone like myself I find this far more beneficial than sitting down with a textbook.

There are many more activities which I would love to try as my knowledge and understanding of the fundamentals of electronics develops.

Fortunately I was able to complete these first exercises using Pixelpulse2. In another post I plan to take a look at using ADALM1000 with the ALICE software in order to be able to complete the summing amplifier activity.

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Trainee Electronics Engineer, currently studying towards my degree in Electronic Engineering at the University of Hudderfsield. Completed my HND in Electrical & Electronic Engineering from Bradford College 2017. Love to try new things and build interesting projects!

23 Sep 2015, 9:01

Comments

November 8, 2015 14:50

Nice

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September 24, 2015 00:01

Hi ChelseaBack

A very long time ago I spent a lot of time messing around with op amps. The first ones I touched had about six valves in. They were in an analogue computer donated to my school (John Porte, Etwall, Derbyshire) by Rolls Royce. We only had half of the computer. I think the other half went to a college in Derby which later became the university.

The op amps could be used to sum input voltages. Yes these op amps had a D.C. throughput even with valves. They were all connected to a patch panel. Some of them were connected to Input capacitors so they could perform differentiation and others had feedback capacitors so they could perform integration. Another intriguing device was the Quarter Square Mutiplier (there was only one requiring at least 4 op amps). Anyway that could multiply the input voltages.

All this happened before the invention of integrated circuits but I lived through their invention and was very pleased to get my hands on early chips which had to have small capacitors connected to special pins to keep them stable at high frequencies. Eventually the 748 and 745 chips were invented and by then I was incorporating them in D.C coupled HiFi amps. I've still got a selection of op amps from those days in my shed. They were very useful when my son and two class mates studied electronics at school and required extra tuition from me when their teacher was not available prior to exams.

So perhaps you will connect your op amp as a differentiator and an integrator. The latter is hard to deal with because it's difficult to maintain stability at 0Hz (D.C.) due to a good op amp's high gain. You may have to have some high value feedback resistor regardless and a very finely tuned variable resistor to bias the D.C. input voltage on the differential input. You may need to arrange a course control and a fine control using variable resistors that maintain their setting well (not noisy).

With more op amps you could move onto the quarter square multiplier. See https://en.wikipedia.org/wiki/Multiplication_algorithm

Good luck and happy learning.

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