Op-amp construction? - Page 1
Forum section: electronics / beginners

author: Jeffrey Eagan
author_rank: Contributor
timestamp: February 17, 2014, 08:08:02 am
content: Evening all!Bit of a stuck moment;I'm trying to get an op-amp speaker amplifier up and going, and I have a circuit all drawn up, (see photo below!) and I'm curious on what op-amp I should use for this. It's mainly going to be a 5" subwoofer amp, and possibly more channels. I'm also not ENTIRELY sure wether or not this design would work, but for the circuit, I just put a 30Hz and a 1kHz test load. But a last question before I head to bed, is this circuit ANY good?Any help is GREATLY appreciated!Cheers!Jeff

author: jeroen79
author_rank: Frequent Contributor
timestamp: February 17, 2014, 04:07:14 pm
content: How did you come to this circuit?I see no connection between the opamp's output and it's inputs.Do you know what an opamp does and why the output must be linked to one of the inputs?I see that the middle graph is jumping from one extreme to another with no resemblance of a sine wave.Do you know why? (you should when you find the answers to the questions above)

author: Nerull
author_rank: Frequent Contributor
timestamp: February 17, 2014, 04:41:49 pm
content: Op-amp circuitry generally looks more like this:How much reading have you done on how op-amps work in a circuit?

author: Rudane
author_rank: Regular Contributor
timestamp: February 17, 2014, 04:44:10 pm
content: You're op-amp is connected open loop. The gain for an open loop op-amp is hugh, something like 100,000 so that's why your output behaves the way it does. You need to close the loop and set the gain. You've got some reading ahead of you.

author: cyr
author_rank: Frequent Contributor
timestamp: February 17, 2014, 05:28:02 pm
content: Your opamp is acting as a comparator, and by connecting one input to a triangle voltage you have managed to create a PWM circuit.Question is, was that intentional or do you want a plain old fashioned linear amplifier?

author: Jeffrey Eagan
author_rank: Contributor
timestamp: February 17, 2014, 06:26:20 pm
content: Honestly, I haven't done much reading. (Whoops!) But I was going more towards a linear amplifier.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: February 18, 2014, 04:33:13 pm
content: Quote from: Jeffrey Eagan on February 17, 2014, 06:26:20 pmWell, time to do some reading. An op-amp is a really simple beast, if you understand it, but fully taming it is a subject worth several huge books.Getting an op-amp to work is simple. Just look up an inverting or non-inverting configuration, and build it on a breadboard. If you're amplifying AC signals (with + and - power inputs), go with inverting. For DC signals or a voltage follower, use non-inverting.To make an op-amp perform WELL, you might need a bit more than that. If you have any education in control theory, now's the time to whip it out. If you don't, I guess any time's a good time to learn to Laplace transform, lol.Anyway, this is the diagram that really made the difference for me in understanding what an op-amp really is:As you see, it's just a difference operator, followed by an extremely high gain. If you create a feedback loop, you have a textbook example control system.

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 18, 2014, 09:41:10 pm
content: Quote from: Jeffrey Eagan on February 17, 2014, 08:08:02 amWhy don't you build something complete, tested and functional like this and its way cooler in many many ways than a standard PWM modulator.http://www.nxp.com/documents/user_manual/UM10155.pdfIts cool cause you will learn everything in once place.The incomplete circuit you showing is a simple PWM modulator using a triangle sawtooth ramp a high gain amplifier in most cases its a high speed comparator a least you can use is a LM311 or better LM319,using an op amp is straight textbook (and sometimes used in bad designs) and not suitable as its not truly a 1-bit (zero or one) quantizer, anyway this is class-d I had so many debates around this topic with some trolling fool I don't have the energy to extend my help, just for now.Comparators are not required to operate in closed loop, thus they don't require any internal frequency compensation (at the expense of reduced slew rate). Also, op-amps may have higher open-loop gain at low frequencies, but comparators have higher gain at high frequencies, due to the absence of compensation.As it has been mentioned, op-amps have evolved a lot in the past decades and hundreds of new models have appeared as opposed to what happened with comparators, but these facts are quite easy to understand. Lets take a LM393 dual comparator and a uA741 dual op-amp as a reference, both are very old devices from 1970s, but while the op-amp is very slow and too noisy for most today's applications, the comparator is fast and is still fine for new designs (my LM393 are swinging from rail to rail in 200ns or so provided adequate input overdrive and output bias).Don't underestimate devices just for being old. There is still a lot of old stuff regarded as industry standard because it still fits today's needs quite well. Personally, I use a lot of LM393, CD4000 CMOS gates, TL431, BC546/556, BD139/140, LM358, 1N4007, 1N4148 etc... and I like these devices.PD. Using op-amps as comparators works but produces poor performance.Source from.http://www.diyaudio.com/forums/solid-state/71430-comparator-vs-opamp-pros-cons.htmlGood luck.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 18, 2014, 10:11:15 pm
content: I don't think he's intending to build a comparator, he just found an example op-amp circuit and tried to tweak it, not understanding the function it performed in the circuit.

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 18, 2014, 10:23:34 pm
content: Quote from: Nerull on February 18, 2014, 10:11:15 pmI don't think he can drive a 5" sub-woofer with a comparator or op-amp.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 18, 2014, 10:57:08 pm
content: No, but presumably that's why he's trying to use it to drive mosfets.

author: AlfBaz
author_rank: Super Contributor
timestamp: February 18, 2014, 11:11:41 pm
content: @NerullHow about editing your post with a "typical" circuit showingnegativefeedback

author: Jeffrey Eagan
author_rank: Contributor
timestamp: February 18, 2014, 11:38:58 pm
content: Thanks for the advice/comments everyone! Very much appreciated!