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Author Topic: EEVblog #600 - Opamps Explained  (Read 27290 times)

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Offline EEVblog

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EEVblog #600 - Opamps Explained
« on: April 06, 2014, 09:51:24 AM »
The most often requested video! Dave explains what Operational Amplifiers (OpAmps) are and how they work. The concepts of negative feedback, open loop gain, virtual grounds and opamp action. The comparator, the buffer, the inverting and non-inverting amplifiers, the differential amplifier, and the integrator circuit configurations are also explained.
Then a practical breadboard circuit to demonstrate a virtual ground and the effect of voltage rail limitations.
All EEVblog Opamp related videos are here:
http://www.youtube.com/playlist?list=PLvOlSehNtuHu2FviAaZaiyXwN41G4b1Lf

 

Offline w2aew

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Re: EEVblog #600 - Opamps Explained
« Reply #1 on: April 06, 2014, 10:36:58 AM »
This is going to be one of your most popular ones Dave.  My video that discusses "how to understand most op amp circuits" is far and away the most viewed one on my YouTube channel. Let's see how many hours it takes for #600 to surpass the view count on mine!
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Offline c4757p

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Re: EEVblog #600 - Opamps Explained
« Reply #2 on: April 06, 2014, 11:02:24 AM »
You must have stopped about a quarter of the way through, subolg123, as he pointed out that it was just an approximation.

I do wish he elaborated on the comparator bit though. Some op amps have diodes between the inputs and can be destroyed by a large input differential...
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Offline dentaku

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Re: EEVblog #600 - Opamps Explained
« Reply #3 on: April 06, 2014, 11:29:58 AM »
Great stuff. When looking at the inverting amplifier circuit I wondered how it worked.
Am I correct in saying it's because current can actually flow INTO the output? I didn't realise that.
I can't see where else it would go.
 

Offline c4757p

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Re: EEVblog #600 - Opamps Explained
« Reply #4 on: April 06, 2014, 11:36:59 AM »
Am I correct in saying it's because current can actually flow INTO the output?

Yes, the output can both source and sink.
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Offline Macci

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Re: EEVblog #600 - Opamps Explained
« Reply #5 on: April 06, 2014, 12:34:06 PM »
I keep scratching my head at that last-minute problem Dave left us with.
All I noticed is that those weird "virtual ground" oscillations happen when the input signal is more-less 0V, and they are about 500mV.
Moreover, lower resistor values mean more current runnig through them. Perhaps the output saturates at some point and can't take in any more current?
Or there is some current that all of a sudden flows in reverse, from output to signal input, thus forcing the "artificial ground" to jump for a moment?
And bigger resistors simply make that current too small for this to happen? I don't know, but i want to know!
 

Offline EEVblog

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Re: EEVblog #600 - Opamps Explained
« Reply #6 on: April 06, 2014, 12:53:09 PM »
This is going to be one of your most popular ones Dave.  My video that discusses "how to understand most op amp circuits" is far and away the most viewed one on my YouTube channel. Let's see how many hours it takes for #600 to surpass the view count on mine!

It won't have a patch on my soldering videos though.
Within 10min of it uploading it was already on the first page search results for "opamps", so that helps greatly. It's all about the magical "google foo".
 

Offline EEVblog

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Re: EEVblog #600 - Opamps Explained
« Reply #7 on: April 06, 2014, 12:59:31 PM »
PLEASE don't even suggest using opamps as comparators to newbies, or they might actually think it is okay and you know full well that it is not okay.

I think it's an important aspect to realise, so that's why I mentioned it. I did say they made bad comparators. They are actually ok for some circumstances. I used one in my uCurrent test jigs for example. So I really don't see the problem mentioning it, as you may very well see then used as crude comparators in circuits. So I think failing to mention it is actually worse than mentioning it.
If you disagree of course, that's fine, but that's not the way I see it.

Quote
Second, you might want to emphasize that opamp input currents can be very very low but there is no such thing as ZERO. Conversely, there is no such thing as infinite.  Just wait until your subscribers find out that their photodetector transimpedance amplifier has unacceptable offset or noise or some other problem because they did not consider the perfect opamp with those zero Ibias currents.  I still spend countless hours sifting through hundreds of opamp data sheets looking for some unobtainable spec my client thinks he wants  (referred to as the component merry go round-- as you call it)

I mentioned we were dealing with an ideal opamp, and that in practice it's not zero, and I have done other videos in input bias currents, and said I might do another video on the practical limitations.
I can't have everything in one video.
Thinking of opamps as ideal opamps for the purposes of teaching is a very common and well accepted way to do it.
Do you start out by teaching that resistors are noise devices?
 

Offline EEVblog

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Re: EEVblog #600 - Opamps Explained
« Reply #8 on: April 06, 2014, 01:02:27 PM »
Am I correct in saying it's because current can actually flow INTO the output? I didn't realise that.

Yes, they are totem pole outputs, they can both source and sink current (I thought I mentioned that somewhere?). As you said, there is no where else the current can go.
 

Offline codeboy2k

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Re: EEVblog #600 - Opamps Explained
« Reply #9 on: April 06, 2014, 01:07:16 PM »
I keep scratching my head at that last-minute problem Dave left us with.

:) I know the answer.. but that's because I've been using opamps for over 25 years already.. so I'll leave it for the young players to figure it out
 

Offline Macci

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Re: EEVblog #600 - Opamps Explained
« Reply #10 on: April 06, 2014, 01:12:42 PM »
I just started electronics as a hobby/career 2 years ago, and I knew most of the stuff Dave put in this video, but nothing beats real-life experience I guess.
Still looking forward to solving this. Am I even close? :P :-DMM
 

Offline EEVblog

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Re: EEVblog #600 - Opamps Explained
« Reply #11 on: April 06, 2014, 01:49:40 PM »
Your approach is certainly the right one for those wanting to understand opamp basics.

And of course that's precisely who the intended audience is, no one else. Of course those more experienced with find countless practical holes in any such basic explanation.
But beginners have to start somewhere, and bamboozling them all sorts of conflicting practical limitations in a basics video is just going to scare them away from electronics.
 

Offline Jianan Li

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Re: EEVblog #600 - Opamps Explained
« Reply #12 on: April 06, 2014, 03:18:15 PM »
So what was going on in the last circuit with the 1k and 10k resistor? I noticed something similar when I was playing with a different op-amp from TI the other day, and the resistors I used were 1k and 5.1k.
 

Offline Kirigozo

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Re: EEVblog #600 - Opamps Explained
« Reply #13 on: April 06, 2014, 03:25:12 PM »
Dave,

Is there any chance you'll make the videos available for download from a server a bit better than a 286 on a dial-up modem? Download times that exceed 45 min regularly fail and have to be restarted. Some of us pay for internet access and wasting it is not an option.

How about it?
 

Offline EEVblog

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Re: EEVblog #600 - Opamps Explained
« Reply #14 on: April 06, 2014, 03:59:14 PM »
Is there any chance you'll make the videos available for download from a server a bit better than a 286 on a dial-up modem? Download times that exceed 45 min regularly fail and have to be restarted. Some of us pay for internet access and wasting it is not an option.
How about it?

Unfortunately bandwidth to do this is ridiculously expensive.
Before anyone jumps in and shouts Amazon S3 or something similar, no, it won't work, go and do the math using real numbers for video files.
Sorry, this is one of those problems that has no easy solution. Just enough people (a few thousand) want to down the podcast videos to make almost any solution slow or really expensive (take your pick), but too few for me to justify spending the crazy amount of money required to make as fast as people expect.
I have in fact considered simply dropping the podcast files altogether.
 

Offline jadew

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Re: EEVblog #600 - Opamps Explained
« Reply #15 on: April 06, 2014, 04:59:50 PM »
My first guess for the little spikes there is that they're caused by the input capacitance since they happen when the polarity changes.
 

Offline vlad777

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Re: EEVblog #600 - Opamps Explained
« Reply #16 on: April 06, 2014, 05:36:40 PM »

I suspect that totem pole (push pull) output goes to high(er) impedance near zero volts, and that is why
you need less current through your feedback.
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Offline vlad777

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Re: EEVblog #600 - Opamps Explained
« Reply #17 on: April 06, 2014, 05:40:27 PM »

At virtual ground, voltage signal has vanished but current did not.

You can have a node at zero volts but still current flowing through it.
Just imagine the ground rail of a power supply, it is at zero volts but
there is a lot of current flowing through it.
« Last Edit: April 06, 2014, 05:42:15 PM by vlad777 »
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Offline miguelvp

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Re: EEVblog #600 - Opamps Explained
« Reply #18 on: April 06, 2014, 05:53:56 PM »
Is there any chance you'll make the videos available for download from a server a bit better than a 286 on a dial-up modem? Download times that exceed 45 min regularly fail and have to be restarted. Some of us pay for internet access and wasting it is not an option.
How about it?

Unfortunately bandwidth to do this is ridiculously expensive.
Before anyone jumps in and shouts Amazon S3 or something similar, no, it won't work, go and do the math using real numbers for video files.
Sorry, this is one of those problems that has no easy solution. Just enough people (a few thousand) want to down the podcast videos to make almost any solution slow or really expensive (take your pick), but too few for me to justify spending the crazy amount of money required to make as fast as people expect.
I have in fact considered simply dropping the podcast files altogether.

Just saying hostmonster.com used them for many years but my sites where not as popular as yours. They do claim no bandwidth limit as long as it's content to your site.

Encode the videos with Quicktime Pro Hinted (even the PC version does a good job and that's what I used to use) and they actually stream at a good rate and compress very well for downloads as well.
 

Offline BravoV

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Re: EEVblog #600 - Opamps Explained
« Reply #19 on: April 06, 2014, 05:57:07 PM »
But beginners have to start somewhere, and bamboozling them all sorts of conflicting practical limitations in a basics video is just going to scare them away from electronics.

This, Dave is hitting the nail on the head on this part.  :-+

Its like when we're at school in early days when we're just starting to learn how to calculate a circle's area or circumference but was hammered heavily on how the Pi was derived from with all those calculus or advanced math.

Remember kids, this is where the Pi is came from .... 



Now .. calculate this 1st before we proceed with the calculation of the circle's circumference.  :palm:


As an electronics enthusiast, I'm one of those that was trapped & scared with those so called "technically correct" limitations that confused the hell out of me when I was learning op-amp at the beginning, and it consumed & wasted a lot of my productive hours. :'(
« Last Edit: April 06, 2014, 05:58:57 PM by BravoV »
 

Offline EEVblog

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Re: EEVblog #600 - Opamps Explained
« Reply #20 on: April 06, 2014, 06:05:24 PM »
Just saying hostmonster.com used them for many years but my sites where not as popular as yours.

I am using Hostmonster for those files.
The problem is simply one of bandwidth. You have a few thousand people wanting to download the file at once, no single server is going handle that kind of streaming traffic. You need a massive CDN.
We had the same issue with the Amp Hour show. But we were able to use LibSyn at an affordable price. The video podcast is 10 times the file size on average, and at least double the frequency of episodes.
And before any says Torrents, I've tried it, hardly anyone used it.

Quote
Encode the videos with Quicktime Pro Hinted (even the PC version does a good job and that's what I used to use) and they actually stream at a good rate and compress very well for downloads as well.

I already use Handbrake with a high RF value, there is only so much magic you can do. And that's at 640x360, imagine if I did a higher res.
 

Offline andtfoot

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Re: EEVblog #600 - Opamps Explained
« Reply #21 on: April 06, 2014, 06:26:10 PM »
Dave,

Is there any chance you'll make the videos available for download from a server a bit better than a 286 on a dial-up modem? Download times that exceed 45 min regularly fail and have to be restarted. Some of us pay for internet access and wasting it is not an option.

How about it?

Is there something wrong with just using JDownloader to download the videos directly from youtube?
 

Offline Kirigozo

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Re: EEVblog #600 - Opamps Explained
« Reply #22 on: April 06, 2014, 06:27:11 PM »
Is there any chance you'll make the videos available for download from a server a bit better than a 286 on a dial-up modem? Download times that exceed 45 min regularly fail and have to be restarted. Some of us pay for internet access and wasting it is not an option.
How about it?

Unfortunately bandwidth to do this is ridiculously expensive.
Before anyone jumps in and shouts Amazon S3 or something similar, no, it won't work, go and do the math using real numbers for video files.
Sorry, this is one of those problems that has no easy solution. Just enough people (a few thousand) want to down the podcast videos to make almost any solution slow or really expensive (take your pick), but too few for me to justify spending the crazy amount of money required to make as fast as people expect.
I have in fact considered simply dropping the podcast files altogether.

Thanks Dave,

I'll try again tomorrow. Might be a bit quieter.
 

Offline zimzom

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Re: EEVblog #600 - Opamps Explained
« Reply #23 on: April 06, 2014, 07:01:50 PM »
I figure this is going to be a very popular video, just a slow burner. Lots of guys are looking for quality beginner info presented in English. :-+ I thought it was great, hope there are more in the series - but understand there are more views on the other types of videos so can understand why you don't do them so often.
 

Offline ali80

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Re: EEVblog #600 - Opamps Explained
« Reply #24 on: April 06, 2014, 07:30:46 PM »
Hi, is the last minute event caused by transition between output transistor pairs? If so, I guess adding a constant current load on the output would fix the problem
 


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