I've NEVER properly understood op-amps, and feel I need to... any help? - Page 1
Forum section: electronics / general

author: dryjoints
author_rank: Regular Contributor
timestamp: January 12, 2018, 04:36:29 am
content: I've NEVER properly understood op-amps, and feel I need to... any help would be appreciated. I owned the incredible "Getting Started in Electronics" by the amazing Mr Mims, and even that didn't clarify things. I used to work on a production line, assembling and calibrating 4-20mA rail mounted relays JAMMED FULL of them, but I never quite understood what they do... I suppose it takes the right person to explain simply and concisely, and it will "click" - I know they're something to do with inverting or not...thanks

author: dryjoints
author_rank: Regular Contributor
timestamp: January 12, 2018, 04:44:38 am
content: Quote from: blueskull on January 12, 2018, 04:39:31 amWhoosh!

author: IanB
author_rank: Super Contributor
timestamp: January 12, 2018, 04:59:15 am
content: Quote from: blueskull on January 12, 2018, 04:39:31 amThose words there are the most confusing words you can say to anyone. The op amp will not try to do that at all since the inputs are independent voltages and the amplifier cannot change its inputs, only its output. It's like saying an audio amplifier will try very hard to maintain a signal of 1 V pp at its input. When worded like that it is clearly not reasonable.What you need to say instead is that the op amp approximates an ideal amplifier with an infinite DC gain, so that OUTPUT = BIG NUMBER * (IN1 - IN2). This is not directly useful, but if you feed the output back round to the input you can make the op amp have an adjustable gain of your choice, or do other more elaborate things. The op amp becomes a general purpose building block in utility circuits.

author: rstofer
author_rank: Super Contributor
timestamp: January 12, 2018, 06:19:56 am
content: Dave has some videos about op amps as does w2aew (Google for 'w2aew op amp'). Here is the first in w2aew's presentations:Here is one of Dave's:On the surface, most of this stuff is pretty simple. The devil is in the details so keep searching for Dave's videos because he gets into the small, but important, details in some later videos.

author: EEVblog
author_rank: Unknown
timestamp: January 12, 2018, 08:01:53 am
content: Quote from: rstofer on January 12, 2018, 06:19:56 am850k views! Crazy.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 08:09:22 am
content: Quote from: EEVblog on January 12, 2018, 08:01:53 amMore fundamental Friday's Dave! There are plenty of basic building block circuits/techniques to go, hundredsHint, hint, nudge, nudge, poke.

author: amspire
author_rank: Super Contributor
timestamp: January 12, 2018, 08:42:32 am
content: I will have a go at describing it simply. When the op-amp is acting as an amplifier, the two inputs always be at exactly the same voltage. If one input is at 2.53V, the other input has to be at 2.53V as well. If the two inputs are not at exactly the same voltage, the opamp is no longer acting as an amplifier.Secondly, the inputs draw and source no current. So the only thing that can force the inputs to be the same voltage is the output pin. The output pin will be at whatever voltage is needed to make the two inputs the same voltage.A very simplified explanation but hopefully it is a starting point.If one input has 1V applied and the other input is driven from the output by a 2:1 resistive divider, the output has to be at exactly 2V so that exactly 1V gets to the other input. In this case, the opamp is acting as a 2x amplifier because the output voltage always has to be twice the first input.

author: tszaboo
author_rank: Super Contributor
timestamp: January 12, 2018, 09:10:26 am
content: Quote from: IanB on January 12, 2018, 04:59:15 amThat is how they teach opamps in Signals and Systems I in EE course at uni.You dont need to worry about the inverting and not inverting input first.Just replace the inputs with an open circuit, and have a voltage generator on the output, tweak the voltage until + and - is equal.

author: BrianHG
author_rank: Super Contributor
timestamp: January 12, 2018, 09:40:24 am
content: Animated and visualized:

author: rstofer
author_rank: Super Contributor
timestamp: January 12, 2018, 07:37:46 pm
content: Quote from: paulca on January 12, 2018, 08:09:22 amThe reason that video has 850k views is that it is very thorough. The follow-on videos add in the fringe effects.I really like the Fundamentals Friday videos.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 10:21:29 pm
content: Quote from: rstofer on January 12, 2018, 07:37:46 pmThey have a tendency to tangent off into less-than-fundamental and more intense details, but that's grand too.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 12, 2018, 10:36:09 pm
content: Good analogy is that the input stage or 'long tailed pair' is a bit like an old fashioned set of 'scales' the statue of justice carries. If the voltages (weights on the pans) are not equal, the output swings one way or the other, but by a much larger amount than the difference. All it is doing is comparing voltages; it takes no current in from the input pins.The output stage could be compared to a car's power steering rack. It doesn't increase the signal voltage supplied to it, but it does add 'muscle' to drive more current into a load.The thing most people find it hard to get their head around is that an opamp doesn't have a ground or 0v pin. However, from what I've said above, it only ever compares two voltages, and acts on the difference. It doesn't care what those voltages are in respect to ground.(Although it obviously can't cope with situations where the input voltages are outside of its supply voltage range)

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 10:37:33 pm
content: The harder thing, as a beginner is understanding WTF is going on when they are not working as an amplifier and do not have a common mode voltage.

author: John Coloccia
author_rank: Super Contributor
timestamp: January 14, 2018, 10:51:09 pm
content: http://www.falstad.com/circuit/e-gyrator.htmlThis particular graphic is the single most useful graphic that shows how opamps behave, IMHO, even if it's in a configuration that a lot of people will never actually use.

author: chris_leyson
author_rank: Super Contributor
timestamp: January 15, 2018, 01:24:20 am
content: An op-amp is just an amplifier with a lot of open loop voltage gain something like 120dB or a million at DC give or take. Anyway a hell of a lot of gain at DC but as the frequency increases the gain drops off but don't worry about that just yet. So you've got two inputs labelled + and - but in reality it's just a single differential input, an op-amp just amplifies the difference between the + and - inputs and not the actual voltages applied to the inputs, it's the difference in voltage that gets amplified. So open loop with no feedback applied you've just got an amplifier with a voltage gain of one million but it's not very practical because of manufacturing tolerences the gain will vary, there will be voltage offsets and it will all be temperature dependant, it will drift all over the place so not very practical.To fix all the issues with gain, offset and temperature etc you apply anegativefeedback network around the op-amp and that corrects the errors. Look up closed loop control systems where G is the open loop gain ie the op-amp gain and H is the the feedback gain (or loss), the transfer function will be Output/Input = G/(1 + G*H). If the gain of the op-amp, G, is much larger than the gain, H, of the feedback network then you can approximate the transfer function output/input = G/(1 + G*H) = 1/H if G>>H. If your feedback network has a gain H of lets say 0.01 or 1/100 and G = 1E6 then the transfer funtion output/input = 99.99 or an overall gain of 100.Op-amps just amplify errors and it's the feedback that controls the system gain. Some posters have saidQuoteNo it won't it will just amplify the difference between IN+ and IN- or the error voltage and multiply it by its gain G, it's the feedback network H that does the control.Sorry about the long winded post but go look up closed loop control systems, the op-amp does the gain bit G and the feedback network does the H bit. Once you've got an intuative feeling for DC and op-amps you can can really cool things with AC . This might help with the control aspecthttp://www.electronics-tutorials.ws/systems/closed-loop-system.html

author: mtdoc
author_rank: Super Contributor
timestamp: January 15, 2018, 01:27:58 am
content: Check out the new EEVBlog guest video by Bob Duhamel. It's very good.

author: Brumby
author_rank: Unknown
timestamp: January 15, 2018, 02:45:37 am
content: Quote from: mtdoc on January 15, 2018, 01:27:58 amYes ... I'd start with that - andthenhave a look at the other material.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 15, 2018, 04:26:52 am
content: Quote from: mtdoc on January 15, 2018, 01:27:58 amA really great presentation. Hats off to Bob on that one.

author: fubar.gr
author_rank: Unknown
timestamp: January 15, 2018, 06:31:35 am
content: What Blueskull said is correct, but implies an opamp with a feedback loop.An opamp on its own is nothing more than a differential amplifier. It amplifies the voltage difference between the two inputs by a very high number