EEVblog #600 - Opamps Explained

[ultranalog]

Great video which I'd love to remcomend to friends, but just have one niggle about your inverting amplifier. You are writing Av=Rf/R1 which is true from a magnitude point of view, but for beginners very confusing since you leave out the minus sign.

Av = -Rf/R1

makes more sense here. Those drawing a Bode diagram can work out for themselves that in the magnitude plot the minus sign drops out.

[andtfoot]

Great video which I'd love to remcomend to friends, but just have one niggle about your inverting amplifier. You are writing Av=Rf/R1 which is true from a magnitude point of view, but for beginners very confusing since you leave out the minus sign.

Av = -Rf/R1

makes more sense here. Those drawing a Bode diagram can work out for themselves that in the magnitude plot the minus sign drops out.

This is noted at about 31:30 into the video. 

Thanks Dave for the nice refresher video. It's been a while since I have looked at inverting opamp circuits, and the 'ah hah' moment of remembering for me was when Ohm's Law was applied to the first resistor. 

[dentaku]

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?

Yeah, People should be able to download he video through something like http://offliberty.com/
It works very well.

[opa627bm]

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.

Use torrent bro!

[HighVoltage]

It is very easy to download any youtube video with internet download manager (IDM)
And if the connection breaks, it will continue where you stopped.

Thanks for another great video

[filip_cro]

Can you make video on how are op-amps build?

I have basic knowledge but I want much more.

[mikeselectricstuff]

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.

It would at least have been  useful to say  why they make bad comparators...

[EEVblog]

It would at least have been  useful to say  why they make bad comparators...

Sure, and it also would have been useful to say a lot of other stuff I didn't. Like I said, I can't cover everything in one video, it was long enough already.

[EEVblog]

Can you make video on how are op-amps build?

There are many different topologies of opamp design, almost too may to count let alone describe I think. Ultimately though I don't think it is of major importance. Practicalities of using various types opamps is much more important to know than exactly how they are designed internally I think.

[EEVblog]

Use torrent bro!

I did, hardly anyone used it.

[R_G_B_]

 Dave,

How do you make an operation amplifier operating as an intergrator stable at D.C low frequency?


How do you stabilize cascaded opamp intergrators at low frequency e.g stop the scope  trace from wondering and oscillating.

even with feed back compensation I can not seem to get them to stabilize at frequencies below 30hz?


Thanks

[Christe4nM]

Even though most of the content of this FF was known to me that last question got me to think. In my very first course on electronics we used the LM358 and found that there was a discontinuity in the negative swing of the output. We found that the TL072 didn't have that 'feature' so we used that instead.

Since I actually didn't know what causes this glitch, even though I had some ideas after Dave's video, I went to investigate. So I bread-boarded Dave's little circuit, put 2Vpp into the input and sure enough there it was, even with the 10k / 100k  resistors. (lm358_1.png and lm358_2.png). With the 1k / 10k configuration it is present again, but at a different voltage (lm358_3.png and lm358_4.png).

My first guess was that the glitch has something to do with the output current that the opamps needs to supply. With the 10k/100k configuration this should be about 0.2 mApp. With the 1k / 100k configuration this should be about 2 mApp. Hmm, a first quick glance at the datasheet for Output Current doesn't show a problem yet. The opamp should be perfectly capable of sourcing or sinking 2 mApp / 1mApeak.

Now in the 1k / 10k configuration the glitch sits at about -600mV... Heh! That is a familiar voltage, could it have anything to do with the transistors in the output stage? Let's see where the glitch is in the 10k /100k configuration. Hey, that is about -6.6 V. About 10x (11 actually) as large. That looks to me as if there is indeed something going on with voltage/current and the output stage.

The screen capture in lm358_1 also shows that the glitch is at a slightly different voltage for the negative going swing and the positive going swing. The Datasheet shows the internal schematic output stage. There you can see that there is a small resistor between the top BJT and the bottom BJT. I take it that is used for current limiting. (~0.6 V over the resistor turns on the BJT directly to the left.). Now I also know that a NPN and a PNP of the same 'family' can differ in capabilities. This is seen in the datasheet as that maximum output sourcing current is larger than the maximum output sink current.

All in all my best guess is that the glitch has everything to do with the configuration of the output. I guess that when the input is at 0V the top BJT (darlington) in the output stage is still driving a bit of current through the current limiting resistor. Then, when the input goes positive (thus the output negative) the current going into the output actually goes into the top BJT's before the bottom PNP is turned on. So the output goes negative to about -600mV before the PNP really takes over from the NPN. In the 10k / 100k configuration this is only at -6.6 volts since the current through the current limiting resistor is 10x as small.

Well that's my 2 cents. Hopefully I'm thinking in the right direction.

Attached screen captures:
Ch3: Blue trace: input signal, 1kHz sine, 2Vpp
CH1: Yellow trace: output from LM358
Both: AC coupled, Bandwith limit on, 1M input impedance, normal acquisition mode (not in High Res!)
1) 10k/100k config.
2) 10k/100k config.
3) 1k / 10k config.
4) 1k / 10k config.

edit: added info about screen captures

[nuhamind2]

My bet is that some parasitic capacitance play a bit of role in here. My intuition tell me that any imperfection in open loop characteristic is - to some extent- repaired by the feedback path.

[c4757p]

Hint: It's not due to capacitance!

[arcover1024]

Maybe due to the input bias current. If this is the cause, the spikes should vanish if the non-inverting input was connected to ground through a resistor.

[max_torque]

Eagle eyed viewers should also have noted the non linear effects due to the recovery from saturation during the segment where Dave reduced the rail voltage, which of course are present when an op amp is used as a comparitor (ie, fully saturated except during the transient output change/swing)

[nuhamind2]

The challenge question addressed. AN-116 answers it. Application notes are typically where IC vendors CTA ( cover their as*) and the first place to find those  "gotchas" 

http://www.ti.com/lit/an/snoa662b/snoa662b.pdf . See page 6.

Nice curve ball, Dave!

So,that's basically making the bottom transistor never conducting and eliminate crossover (it never cross actually).  Basically making it a single supply.

It seem that the opamp actually care about it's power supply,my whole life is a lie.

[robrenz]

Excellent video Dave.  You have a great knack for getting concepts across.

[Mavro]

Great video Dave!

It would be great to do a video on how opamps in audio circuits work.   Audiophiles are always swapping out or in search of the best OpAmp.   How does an OpAmp help in that situation?

[hikariuk]

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.

There's a Tampermonkey script that lets you download videos (amongst other functionality changes) directly from YouTube, if you're using Chrome.  I shall leave the discussion of the ethics of such a thing to other people

[Legion]

Just wanted to say thanks for the great video! I just started reading about opamps a few days ago so it was fantastic to have this video go up. I like that you didn't overburden the fundamentals with myriad small details. Kept the core concepts clear and easy to understand. I look forward to future videos on opamps.

Thanks again!

[Christe4nM]

The challenge question addressed. AN-116 answers it. Application notes are typically where IC vendors CTA ( cover their as*) and the first place to find those  "gotchas" 

http://www.ti.com/lit/an/snoa662b/snoa662b.pdf . See page 6.

Nice curve ball, Dave!

Good find. So...
Crossover distortion. Two words saying exactly what I guessed .
Nice to read the reasons behind it though.

[David Hess]

How do you make an operation amplifier operating as an intergrator stable at D.C low frequency?

Either limit the DC gain or do some type of periodic calibration for the offset voltage and bias current.

How do you stabilize cascaded opamp intergrators at low frequency e.g stop the scope  trace from wondering and oscillating.

even with feed back compensation I can not seem to get them to stabilize at frequencies below 30hz?

Adjust the feedback so the gain is less than 1 before the phase reaches 180 degrees.  135 degrees is typical.  Learn to use a bode plot.

[tom66]

Audiophiles will use undefined terms such as "colour", "brightness", "danceable", "sonic redemption", "space clarity", "well-tempered personality" etc. to describe components.

The terms can mean nearly anything.

Fun fact: simply moving a couple of inches in your seat is enough to disrupt any audio comparison due to the "comb effect". So it's very hard to accurately compare any audio system.  A well designed amplifier will do nothing audible to the signal; the rest is up to the listener, and often the headphones or speakers, which introduce tons more distortion than any electronic part does.

[Rutger]

Thanks for the video Dave, it was informative and short enough. I am glad you didn't go into all the detail explanations on why this and that, it would have made the video too long.

My question; what is the most versatile jelly bean op-amp a hobbyists should have in his/her toolkit?