Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Software Tools SPICE, PCB CAD, speaker design and measurement software, calculators

LT Spice LoopGain2.asc Questions
LT Spice LoopGain2.asc Questions
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 3rd January 2019, 06:27 PM   #1
brian92fs is offline brian92fs  United States
diyAudio Member
 
Join Date: Jan 2017
Location: Northern California
LT Spice LoopGain2.asc Questions
Default LT Spice LoopGain2.asc Questions

I have a few questions on how to use the LT Spice “LoopGain2.asc” model for open Loop gain analysis under these scenarios.

Output networks: I seem to remember reading that zobel networks and output inductors should be removed from the model? Is this correct? If they should be included, is it after Vi & Ii (net X)?

Bootstrapped VAS current source: Gain margin is different depending on whether the cap is connected before or after Vi & Ii. Which connection point is appropriate? I believe it is net Y in my example.

Transitional Miller Compensation (TMC): If the TMC resistor is connected after Vi & Ii, the AC plot looks odd. Should this be connected before Vi & Ii (net Y in my example)?

I attached an example from LT Spice XVII illustrating my questions. The example is based on Rod Elliot's P3a & P101. It is just an example for learning purposes. Not meant to be a working amplifier.

Thanks!
Attached Images
File Type: gif LoopGain2-Example.GIF (33.3 KB, 58 views)
Attached Files
File Type: asc LoopGain2-Questions.asc (9.5 KB, 3 views)
File Type: txt Cordell-Models.txt (19.4 KB, 4 views)
  Reply With Quote
Old 4th January 2019, 10:11 PM   #2
Dave Zan is offline Dave Zan  Australia
diyAudio Member
 
Join Date: Nov 2010
Quote:
Originally Posted by brian92fs View Post
I have a few questions on how to use the LT Spice “LoopGain2.asc” model...
...that zobel networks and output inductors should be removed from the model? Is this correct?
Hi Brian
No, not correct, you should keep the Zobel and output inductors.
Your placement of the probe is not quite correct either.
Redraw the circuit wired as it will be used, place the probe components as a "T" with labeled nodes and I will try to help.
Also, you could search my old posts with keyword "Tian", I have had discussions on this, especially with Damir (DadoD) and Paul (MCD99).

Best wishes
David

Last edited by Dave Zan; 4th January 2019 at 10:14 PM.
  Reply With Quote
Old 4th January 2019, 11:11 PM   #3
brian92fs is offline brian92fs  United States
diyAudio Member
 
Join Date: Jan 2017
Location: Northern California
LT Spice LoopGain2.asc Questions
Revised file with Zobel Network added back. I think I see what you mean by the probe placement. I had it on the wrong side of my box indicating where the output networks would be.

I'll take a look at the other threads you mentioned over the weekend.

Thanks David.
Attached Images
File Type: gif LoopGain2-Example.GIF (33.0 KB, 46 views)
Attached Files
File Type: asc LoopGain2-Questions.asc (9.8 KB, 3 views)
  Reply With Quote
Old 5th January 2019, 01:04 AM   #4
Dave Zan is offline Dave Zan  Australia
diyAudio Member
 
Join Date: Nov 2010
Quote:
Originally Posted by brian92fs View Post
I'll take a look at the other threads you mentioned over the weekend.
Now I look back over them I see that it's pretty scattered.
So here is a pic. of the usual placement.
Click the image to open in full size.

Best wishes
David
  Reply With Quote
Old 5th January 2019, 07:35 PM   #5
brian92fs is offline brian92fs  United States
diyAudio Member
 
Join Date: Jan 2017
Location: Northern California
LT Spice LoopGain2.asc Questions
I took a look at your example again. The placement of the probe is reversed from the instructions in the LoopGain2 file from the educational library. Is this intentional?

I attached a screenshot for reference.
Attached Images
File Type: gif LoopGain2.GIF (64.5 KB, 29 views)
  Reply With Quote
Old 5th January 2019, 10:35 PM   #6
Dave Zan is offline Dave Zan  Australia
diyAudio Member
 
Join Date: Nov 2010
Quote:
Originally Posted by brian92fs View Post
...The placement of the probe is reversed from the instructions...
The probe measures a loop gain so it can be placed in different places around the loop and should still return the same results, but most people place it as shown because it looks more like it's in the feedback path.
I wrote the results "should" be the same but reality doesn't quite match theory, because there are second order effects that the Tian probe doesn't capture (need a more complicated, Middlebrook GFT probe for that).
Usually the difference is too small to matter but consistent placement will at least ensure any potential inaccuracy is consistent too.

I picked the example arbitrarily, then a close inspection showed there are a lot of potential feedback paths, not sure if this makes it a bad example because it's too complicated or a good one to show unexpected subtlety.
For instance, a bootstrapped current source is actually an example of positive feedback and may require some care if you have never considered it as such.

Best wishes
David

Last edited by Dave Zan; 5th January 2019 at 10:38 PM.
  Reply With Quote

Reply


LT Spice LoopGain2.asc QuestionsHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
EF184 as ccs / design questions / spice model Rundmaus Tubes / Valves 11 16th February 2017 05:25 AM
Spice model for GU50 (LT-Spice) voltagezone Tubes / Valves 44 19th November 2013 09:05 AM
Spice modeling of speakers, other newbie questions parallax Multi-Way 12 15th March 2013 01:17 PM
Driver stage to finals newbie questions - (I hope not dumb questions) rongon Tubes / Valves 15 13th June 2011 09:29 PM
Free Spice Or Cheap Spice Simulator-Where To Start? kelticwizard Everything Else 29 15th February 2007 02:38 AM


New To Site? Need Help?

All times are GMT. The time now is 04:19 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki