EEVblog #572 - Cascading Opamps For Increased Bandwidth - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: January 25, 2014, 06:32:18 am
content: Fundamentals Friday.Dave explains Gain Bandwith Product and how it's possible to increase your system bandwidth by cascading opamps in series. Also, a discussion on the associated noise issues.A breadboard example shows how variable GBWP can be, and how it can relate to distortion.

author: tree
author_rank: Regular Contributor
timestamp: January 25, 2014, 06:58:41 am
content: I'm rather interested in the derivation of that formula in the upper right hand corner.

author: Rerouter
author_rank: Super Contributor
timestamp: January 25, 2014, 08:57:57 am
content: A more interesting point to me was that the phase shift seemed to radically increase when you lowered supply voltage,(phase margin and feedback loops is one i have really wanted to see for a while for an upcoming video)

author: PeterL
author_rank: Regular Contributor
timestamp: January 25, 2014, 10:31:46 am
content: Nice video, I really like this fundamental friday series.You mention that it can be hard to find the right combination of resistors to get a precise gain. I made an excel sheet for this long time ago. Just put in the ratio you need, and the tolence you permit, and all possible combination turn green.Also the android app 'ElectroDriod' is very handy for this.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 25, 2014, 11:29:03 am
content: For 9K, surely 2x18K in parallel is better than 2K2+6K8 as it reduces your BOM.

author: electronics man
author_rank: Frequent Contributor
timestamp: January 25, 2014, 11:33:51 am
content: Yay more fundermentals Fridays thanks dave

author: EEVblog
author_rank: Unknown
timestamp: January 25, 2014, 12:13:52 pm
content: Quote from: mikeselectricstuff on January 25, 2014, 11:29:03 amAt these volumes and item count it makes no difference.Series can have some advantage in terms of single sided SMD layout, and I found they were in stock at the time so just went with that.

author: andtfoot
author_rank: Unknown
timestamp: January 25, 2014, 12:35:00 pm
content: Quote from: EEVblog on January 25, 2014, 12:13:52 pmDo you mind expanding on that? Having recently started looking at designing SMD boards, I'm curious...

author: Rerouter
author_rank: Super Contributor
timestamp: January 25, 2014, 12:41:05 pm
content: you can jump 2 traces instead of just one, equally if you rotate one 180 degrees your trace follows the same line it entered on, (or so i assume based on how i have been using them)

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 25, 2014, 12:55:49 pm
content: Quote from: EEVblog on January 25, 2014, 12:13:52 pm...but you don't know what size the next batch might be.

author: Melkor
author_rank: Contributor
timestamp: January 25, 2014, 01:13:48 pm
content: You said that most OpAmps have a constant GBWP. However for that particular OpAmp you tested you got values between about 2.4 and 1.4Is there any way to determine from the datasheet if an OpAmp has a constant or variable GBWP ?

author: Wytnucls
author_rank: Super Contributor
timestamp: January 25, 2014, 01:14:03 pm
content: Quote from: tree on January 25, 2014, 06:58:41 amHave a look at these notes for cascading inverting opamps:http://opencourseware.kfupm.edu.sa/colleges/ces/ee/ee303/files%5C3-Lesson_Notes_Lecture_12.pdfThe logic should be the same for non-inverting opamps, to work-out the right formula.

author: EEVblog
author_rank: Unknown
timestamp: January 25, 2014, 01:38:54 pm
content: Quote from: mikeselectricstuff on January 25, 2014, 12:55:49 pmBut I know what it'snotgoing to be!The point is these precision resistors don't plummet in price when you jump up a volume notch. You only save 3 cents jumping from 1000qty to 100,000qty for example. It becomes a non-issue unless you are after every last reel space on a machine.

author: EEVblog
author_rank: Unknown
timestamp: January 25, 2014, 01:42:59 pm
content: Quote from: Rerouter on January 25, 2014, 12:41:05 pmYes and yes. Two in series gives you the option to jump two other traces, and in different locations, and/or returning to the direction you started.That can mean the difference between making a design realisable on a double sided front panel board (like in the uCurrent) or not.Of course on regular double sided board this might not matter at all, but when you are doing a front panel you don't want any traces on the other layer.

author: robrenz
author_rank: Super Contributor
timestamp: January 25, 2014, 01:57:43 pm
content: Great video Dave, I learned a lot

author: daqq
author_rank: Super Contributor
timestamp: January 25, 2014, 03:15:33 pm
content: Thanks for the video Dave, useful.For a 9:1 divider in one application we used this "IC":http://www.farnell.com/datasheets/572234.pdfIt's pretty precise, but pricey. There are many other ratios available from various makers.

author: Rufus
author_rank: Super Contributor
timestamp: January 25, 2014, 03:17:13 pm
content: Quote from: mikeselectricstuff on January 25, 2014, 11:29:03 amIf you don't mind E24 200R and 1k8 or 300R and 2k7 does it with 2 resistors.

author: free_electron
author_rank: Super Contributor
timestamp: January 25, 2014, 05:18:33 pm
content: GBW is not to be confused with unity gain bandwidth !GBW is a meaningless number. it is mainly determined by how high the internal stabilising capacitor is between the longtail and the output driver. This capacitor is responsible for the gain collapse over frequency. A far more interesting number is the slew rate of an opamp.Unity gain bandwidth tells you up to what frequency an opamp can 'follow' the incoming signal when set in unity gain.unity gain frequency is only specified for 1 input level : 0dB. Send a larger signal in there and UGB will go down. Send a smaller signal in and UGB may go up. You can't just arbitrarily send a level into the opamp.There are other problems to be faced : the feedback divider will have stray capacitances due to the PCB material , soldermaks ,greasy fingers etc .. This may have an impact on the frequency/gain response. If you are working in the khz range you may not notice this , go to higher frequencies and it becomes problematic. That is why there are current feedback opamps for high frequencies. These do not care about stray capacitances on the pcb.

author: tree
author_rank: Regular Contributor
timestamp: January 25, 2014, 07:10:36 pm
content: Quote from: Wytnucls on January 25, 2014, 01:14:03 pmThanks. I worked through it and found out how the formula is derived.

author: tree
author_rank: Regular Contributor
timestamp: January 25, 2014, 07:11:35 pm
content: Why would you want to increase the bandwidth in an application such as this (uCurrent)? I though this would measure ~DC current consumption, especially if you're measuring it with a handheld meter (which don't have 100s of kHz of bandwidth).

author: Marco
author_rank: Super Contributor
timestamp: January 25, 2014, 07:15:19 pm
content: Quote from: free_electron on January 25, 2014, 05:18:33 pmTake the first opamp in Dave's cascade ... why would slew rate be interesting for it at all?I can see how it's important to know it can add an additional limit on the bandwidth, but I don't see why it would necessarily be more interesting.

author: robrenz
author_rank: Super Contributor
timestamp: January 25, 2014, 07:20:02 pm
content: Quote from: tree on January 25, 2014, 07:11:35 pmI signed up for a µcurrent just to use as a current input to a scope. I don't need it for measuring DC current. In this case the bandwidth increase makes it much more useful.

author: tree
author_rank: Regular Contributor
timestamp: January 25, 2014, 07:25:16 pm
content: Quote from: robrenz on January 25, 2014, 07:20:02 pmI thought about that, but then wouldn't its [the measurement setup] accuracy be limited by the scope's accuracy? I suppose when you need to make a rough measurement, then it would be fine, but when you're trying to make a measurement with .05% accuracy then I don't think it would work well.

author: robrenz
author_rank: Super Contributor
timestamp: January 25, 2014, 07:39:01 pm
content: Quote from: tree on January 25, 2014, 07:25:16 pmThe µcurrent only guarantees a .05% conversion accuracy from current to voltage. The voltage measuring accuracy contribution to the max error has to be considered whether scope or DMM. But it is a very nice pre amplified shunt for a scope.

author: rfbroadband
author_rank: Unknown
timestamp: January 26, 2014, 01:19:03 am
content: Nice video. To gain more insight on these kind of topics one could also use a simulator like LTSpice/PSpice, especially when you want to look at noise and distortion. A comment on non-linearity: If one can see non linearity of CW signals in the time domain, it is usually too late. It is more accurate to characterize non linear effects in the frequency domain (-> simulator or a scope with decent FFT capability, or even an SA).I like the series.

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 26, 2014, 02:00:37 am
content: Dave,That's been your best Fundamental Friday:Storytelling is very straightforward, every aspect very well explained.A minute critique:You did not mention, that by cascading those OpAmps, your initial/superior goal of high precision DC gain will deteriorate.One stage only will give 2 x 0,05% DC max. error, 2 stages will double that worst case value by 2 because you will have 4 instead of 2 uncertain resistors.The probable error might follow a SQRT law, therefore 2 stages will increase uncertainty by a factor of 1,41 at least.Anyhow, a fundamental error analysis, plus data from your series production / testing would make up a nice next-next-next fundamental Friday, I suppose.Frank

author: Echo20-127
author_rank: Newbie
timestamp: January 26, 2014, 01:36:26 pm
content: Quote from: Dr. Frank on January 26, 2014, 02:00:37 amThis gain accuracy might actually be a problem, but I expect Dave to know what he is doing with this...On the bandwith problem, I guess this is a rather clever solution. I'm not very experienced in this domain but I would expect the improvement in the bandwidth domain to come at the detriment of an increased phase delay as the signal propagates through one more OpAmp. (tell me if I'm wrong)It would be an improvement when you are trying to read the amplitude of some short transient current, but the delay would be something to keep in mind.

author: casper.bang
author_rank: Frequent Contributor
timestamp: January 26, 2014, 03:22:01 pm
content: Great video... entirely too few fridays in Dave's calendar.

author: qno
author_rank: Frequent Contributor
timestamp: January 26, 2014, 07:37:07 pm
content: I normally use slew rate.The GBW is nice but says nothing about the amplitude of the output.Before the amplitude of the opamps drops to -3 dB the output sinus waveform has beenchanged to a triangle due to the slew rate limit.See wikihttps://en.wikipedia.org/wiki/Slew_rate

author: electrocat
author_rank: Newbie
timestamp: January 27, 2014, 02:28:28 pm
content: HiA method I used to get a 10:1 divider is a 8 resistor network.----R---R---R---R---R---R-! ! !--R---R--!2R in parallel is the 1R in the divider2R in parallel is series with 4 R's is the 9RUse a 1% network with 8 individual resistorsGenerally the match between individual resisters is much better then 1%, for a radiometric divider the exact value does not matterThe resistors are all from the same mix of resistive material therefore they temperature stability tracks very well.I have used the method over the years with great success, if space is not a problemGenerally get better then 0.1% ratios with high temperature stability, only have to by 1 part plus not very expensive.Regards

author: BravoV
author_rank: Super Contributor
timestamp: January 27, 2014, 03:04:12 pm
content: Quote from: electrocat on January 27, 2014, 02:28:28 pmInteresting & noted down, thanks.

author: German_EE
author_rank: Super Contributor
timestamp: January 27, 2014, 05:25:14 pm
content: Sorry but I'm missing something here, I cannot see why the bandwidth of an op amp will change depending on what gain you set it at. All that is in the feedback path is two resistors so what causes a device that works at over 2 MHz at unity gain to only work at a few KHz?Peter

author: Bored@Work
author_rank: Super Contributor
timestamp: January 27, 2014, 06:29:57 pm
content: Quote from: German_EE on January 27, 2014, 05:25:14 pmIn simplified terms:To get a stable opamp it is internally frequency compensated with what is called a dominant pole. The result (again simplified) is that an opamp behaves like a first order low-pass filter with -20 dB/decade frequency response. I.e. the low-pass behavior limits amplification at higher frequencies.Actually, as an EE you should know that from your introduction course to opamps.

author: German_EE
author_rank: Super Contributor
timestamp: January 27, 2014, 07:49:37 pm
content: EE, Electrical Engineer, normally I work on bigger stuff such as 10Kw motor drivers but I do the smaller stuff like this for fun.Peter

author: vigsgb
author_rank: Contributor
timestamp: January 28, 2014, 12:13:52 am
content: I think you should create a video on bandwidth as it applies to electronics as I don't even know why you needed more bandwidth in your micro current.. was it for response time of the input? Make tests quicker. When I search online and YouTube I find so many variants but nothing a newbie like myself can understand.

author: mexakin
author_rank: Contributor
timestamp: January 29, 2014, 11:44:06 am
content: You need more bandwitdh in opamp circuits to see fast changing frequencies on your test signals,for example try to measure a rectangle signal with a too small bandwitdh, you will see a sinus of it, so you must always know what to expect from your test and measurment equipments, otherwise there is no need to measure at all

author: The Chump
author_rank: Contributor
timestamp: January 29, 2014, 10:07:01 pm
content: Quote from: mikeselectricstuff on January 25, 2014, 11:29:03 amFor a two resistor ratio of 9:1, what could be better than 1k8 and 200R?

author: vigsgb
author_rank: Contributor
timestamp: January 30, 2014, 12:11:32 am
content: Quote from: mexakin on January 29, 2014, 11:44:06 amThanks! That answers my question. Tomorrow I will be playing with some op amps. Need to get a scope soon so I can see what the wave form looks like and such.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: January 24, 2019, 07:30:28 pm
content: in the video, Dave mentions that it is better that we apply the whole gain in one opamp for a couple of reasons, what are the reasons behind this?