Author Topic: Poles and Zeros  (Read 6083 times)

0 Members and 1 Guest are viewing this topic.

Offline exe

  • Supporter
  • ****
  • Posts: 2562
  • Country: nl
  • self-educated hobbyist
Re: Poles and Zeros
« Reply #25 on: June 17, 2018, 02:48:14 pm »
The only article about negative feedback that I was able to comprehend: https://www.allaboutcircuits.com/technical-articles/negative-feedback-part-1-general-structure-and-essential-concepts/ .

Although, even after many hours reading articles on the subject and doing SPICE simulations and experimenting with a signal generator and oscilloscope I still nowhere near to be able to design, e.g., a power supply compensation network. Sadly, most EE textbooks I saw do not really cover opamp stability issues.
 

Offline rstofer

  • Super Contributor
  • ***
  • Posts: 9889
  • Country: us
Re: Poles and Zeros
« Reply #26 on: June 17, 2018, 03:30:39 pm »
Basically, for any given opamp there is a fixed time delay between a signal being input and arriving at the output. This is mainly due to R/C time constants in the signal path, created by parasitic capacitance in transistor junctions etc.

Mostly we want less than the max gain, so we apply negative feedback.

As signal frequency increases, the inherent delay becomes a larger proportion of a signal cycle. When it exceeds half a cycle, the negative feedback becomes positive feedback.. and all hell breaks loose.

<snip>

As to working out what values to use, sorry but that does require all those funny squiggles. Or else just try and see. Which I suspect is what's used in most cases. Square wave testing will typically show ringing or overshoots if the stage is undercompensated. 


A good discussion of the problems related to poles moving as a function of frequency.  Circuits that are stable at DC can go unstable as the frequency is increased.

I think those squiggles are the difference between engineering and technicianing (I know it's not a word!).  Most feedback systems are not simple op amp circuits, we can't plunk down feedback capacitors.  Lots of times there are hydraulics, motors, valves, temperature controls and other physical phenomenon that resist tinkering.  The frequencies are lower but the lag is considerable.  Just getting a PID loop correct can be a lot of knob twisting (not withstanding 'Autotune').

In school, we had two courses related to controls.  One instructor worked on the controls for the rocket nozzles on the Atlas missile and the other worked designing controls for nuclear power plants.  It isn't as easy as you might think to get the Atlas away from the launch pad without falling over.

 

Online IanB

  • Super Contributor
  • ***
  • Posts: 11858
  • Country: us
Re: Poles and Zeros
« Reply #27 on: June 17, 2018, 04:12:28 pm »
It isn't as easy as you might think to get the Atlas away from the launch pad without falling over.

I don't necessarily think it's easy. Imagine balancing a pencil vertically on the end of your finger, and then pushing the pencil upwards fast without it falling over. This is kind of what a rocket is.
 

Offline rstofer

  • Super Contributor
  • ***
  • Posts: 9889
  • Country: us
Re: Poles and Zeros
« Reply #28 on: June 17, 2018, 05:01:48 pm »
It isn't as easy as you might think to get the Atlas away from the launch pad without falling over.

I don't necessarily think it's easy. Imagine balancing a pencil vertically on the end of your finger, and then pushing the pencil upwards fast without it falling over. This is kind of what a rocket is.

Put a blob of clay on the sharp end of the pencil (top) and while the system seems more unstable (mass at a distance), the inertia at the nose makes it easy to balance on the palm of your hand.
 

Offline IDEngineer

  • Super Contributor
  • ***
  • Posts: 1925
  • Country: us
Re: Poles and Zeros
« Reply #29 on: June 17, 2018, 05:03:38 pm »
This lecture series by Roberge is one of great ones, imo.
https://ocw.mit.edu/resources/res-6-010-electronic-feedback-systems-spring-2013/course-videos/
Unfortunately, every video in that series comes up with "YouTube video could not be played".  |O First time I've ever seen that message.
 

Offline rstofer

  • Super Contributor
  • ***
  • Posts: 9889
  • Country: us
Re: Poles and Zeros
« Reply #30 on: June 17, 2018, 05:14:26 pm »
You have to follow along where they discuss downloading the videos as MP4s or viewing with iTunes.
I just downloaded the first video and it seems to work fine.
 

Online IanB

  • Super Contributor
  • ***
  • Posts: 11858
  • Country: us
Re: Poles and Zeros
« Reply #31 on: June 17, 2018, 05:17:49 pm »
This lecture series by Roberge is one of great ones, imo.
https://ocw.mit.edu/resources/res-6-010-electronic-feedback-systems-spring-2013/course-videos/
Unfortunately, every video in that series comes up with "YouTube video could not be played".  |O First time I've ever seen that message.

The playlist on YouTube is linked below. Unfortunately the content seems to be blocked in the USA. In which countries do people find it to be available?

https://www.youtube.com/playlist?list=PLUl4u3cNGP62in17jH_DiJMkCGNM6Xni-
 

Offline ferdieCX

  • Regular Contributor
  • *
  • Posts: 212
  • Country: uy
Re: Poles and Zeros
« Reply #32 on: June 17, 2018, 05:21:23 pm »
This lecture series by Roberge is one of great ones, imo.
https://ocw.mit.edu/resources/res-6-010-electronic-feedback-systems-spring-2013/course-videos/
Unfortunately, every video in that series comes up with "YouTube video could not be played".  |O First time I've ever seen that message.

The playlist on YouTube is linked below. Unfortunately the content seems to be blocked in the USA. In which countries do people find it to be available?

https://www.youtube.com/playlist?list=PLUl4u3cNGP62in17jH_DiJMkCGNM6Xni-


I am now in Germany, here you get "This video contains content from MIT. It is not available in your country."
 

Offline Cerebus

  • Super Contributor
  • ***
  • Posts: 10576
  • Country: gb
Re: Poles and Zeros
« Reply #33 on: June 17, 2018, 05:46:39 pm »
All those videos can be downloaded from archive.org, there's a link to them from the MIT open course site linked above - http://archive.org/download/MITRES.6-010S13/. Also it's worth noting that the whole textbook for that course, written by the lecturer, can be legitimately downloaded from the course site https://ocw.mit.edu/resources/res-6-010-electronic-feedback-systems-spring-2013/textbook/. Looks quite decent but it might still need more mathematical background than some here would be comfortable with.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
The following users thanked this post: ferdieCX

Offline Kremmen

  • Super Contributor
  • ***
  • Posts: 1289
  • Country: fi
Re: Poles and Zeros
« Reply #34 on: June 17, 2018, 06:45:00 pm »
The only article about negative feedback that I was able to comprehend: https://www.allaboutcircuits.com/technical-articles/negative-feedback-part-1-general-structure-and-essential-concepts/ .

Although, even after many hours reading articles on the subject and doing SPICE simulations and experimenting with a signal generator and oscilloscope I still nowhere near to be able to design, e.g., a power supply compensation network. Sadly, most EE textbooks I saw do not really cover opamp stability issues.

You need wade through this: https://www.youtube.com/channel/UCq0imsn84ShAe9PBOFnoIrg
I don't know how it could be made easier, althoug 3blue1brown also has good material.
Nothing sings like a kilovolt.
Dr W. Bishop
 
The following users thanked this post: exe

Offline Wimberleytech

  • Super Contributor
  • ***
  • Posts: 1133
  • Country: us
Re: Poles and Zeros
« Reply #35 on: June 17, 2018, 09:18:15 pm »



I am now in Germany, here you get "This video contains content from MIT. It is not available in your country."

I ran into this as well in USA.  Here is where I found them available.

http://teachingexcellence.mit.edu/category/inspiring-teachers/james-k-roberg-6-302-electronic-feedback-systems

 

Offline Karlo_Moharic

  • Regular Contributor
  • *
  • Posts: 100
  • Country: hr
Re: Poles and Zeros
« Reply #36 on: June 17, 2018, 10:01:16 pm »
Hey ,

Start with this (don't worry it's easy to understand :))
https://www.dropbox.com/s/5lcyzu70hdqcoeb/Engineering%20mathematics.pdf?dl=0

If you study every day , it will take you about a month to go through the book.

When you are finished with that , take on this book:
http://s1.nonlinear.ir/epublish/book/Circuit_Analysis_For_Dummies_1118493125.pdf
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf