Spice oddity. Is it me or a glitch ? - Page 2 - diyAudio
Go Back   Home > Forums > Design & Build > Software Tools

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

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 16th April 2012, 11:27 AM   #11
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
That 30V is the output voltage after the amplifier has applied it's DC gain.

The input error could be very much smaller, just a few tens of millivolts of input error, could cause the output offset to rocket.
__________________
regards Andrew T.
  Reply With Quote
Old 16th April 2012, 11:29 AM   #12
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by Mooly View Post
It's minus 3
Is it?
Trace the current flow from Q1 emitter through the transistor to it's base and then to source/sink/ground.
To complete that route you must add that ground referencing resistor.
What voltage would you expect on the base PIN?
__________________
regards Andrew T.
  Reply With Quote
Old 16th April 2012, 11:41 AM   #13
diyAudio Member
 
Join Date: Nov 2005
Hey Mooly if you don't get it right away maybe post an asc file...
  Reply With Quote
Old 16th April 2012, 02:10 PM   #14
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Thanks Andrew E, here's the file.

Lol, I haven't got it Is it something obvious !
Attached Files
File Type: zip Glitch.zip (6.4 KB, 6 views)
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 16th April 2012, 02:16 PM   #15
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by AndrewT View Post
That 30V is the output voltage after the amplifier has applied it's DC gain.

The input error could be very much smaller, just a few tens of millivolts of input error, could cause the output offset to rocket.
The amp has a gain of 1 at DC and so any change in the bias voltage is reflected in the output. It doesn't sit on a knife edge at all like a differential amp.

Bias,
-3 volt gives 0 volt output
-2 volt gives + 1 volt output
-1 volt gives +2 volt output

and

-4 volt gives -1 volt output
-5 volt gives -2 volt output

The circuit works. It's why the totally unconnected and unrelated components upset the simulation.

I tried the 47K to ground and it works fine, just with a small DC offset. [Going back and using the same circuit I started out with, I set V8 to give zero volts bias (same as ground) and that was OK too as shown here. As expected it gave the same result as 47K to ground].

Continuing with the circuit and the 47K to ground I then put the bias voltage source back and then it all goes to pot again.
Attached Images
File Type: jpg Picture 5.JPG (170.1 KB, 46 views)
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.

Last edited by Mooly; 16th April 2012 at 02:25 PM. Reason: To try and make it clearer because I'm tying myself in knots trying to explain it :)
  Reply With Quote
Old 16th April 2012, 03:20 PM   #16
RJM1 is offline RJM1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Titusville, Fl.
It comes up after about 240ms (I got rid of your 16 for Ncycles of V5) but it is odd that that LTspice is still thinking that your servo is there until you remove the op-amp (you can remove the voltage V1 and replace it with a wire and it will still start immediately).
You also get the delay if you replace the op-amp with another device,
Attached Images
File Type: jpg glitch.jpg (77.8 KB, 32 views)

Last edited by RJM1; 16th April 2012 at 03:25 PM.
  Reply With Quote
Old 16th April 2012, 04:42 PM   #17
diyAudio Member
 
Join Date: Nov 2005
Leaving ICs placed without supplies can yield some really strange effects. Basically you can't do it. My guess is that it's affecting how the bias points are being calculated in a 'seemingly' beneficial way.

Delete all the hangnails and use something like .ic v(n020)=-1.8 (or zero) and go from there. Might want to consider how the circuit will actually start up.

EDIT: Ooops that .ic doesn't clean things up as much as I thought without leaving C6 at 1u like I had it for test. Somewhere around there is the trouble though..

Last edited by Andrew Eckhardt; 16th April 2012 at 04:46 PM.
  Reply With Quote
Old 16th April 2012, 04:57 PM   #18
diyAudio Member
 
Join Date: Nov 2005
-2.45 looks like more of a settling point.
  Reply With Quote
Old 16th April 2012, 05:33 PM   #19
diyAudio Member
 
Join Date: Nov 2005
Try a 1n4148 to clamp E-B reverse bias on Q1
  Reply With Quote
Old 16th April 2012, 06:07 PM   #20
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by Andrew Eckhardt View Post
Try a 1n4148 to clamp E-B reverse bias on Q1
Mmmm

Top solution Mr E

So whats going on I wonder. Can the program having run with the servo must somehow be leaving something behind even when run afresh after altering things.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Intermittent DCX2496 glitch ComputerJLT Digital Line Level 4 26th January 2012 03:53 AM
JFET CS design, how? (and possible SPICE oddity) StefanTH Solid State 4 23rd October 2008 10:24 AM
P-P resistance oddity Carlp Tubes / Valves 6 30th October 2006 02:49 AM
Is this glitch ? Bernhard Digital Source 4 11th April 2005 01:35 PM
Working amp witha glitch - or two. lazyfly Solid State 5 21st June 2004 03:24 PM


New To Site? Need Help?

All times are GMT. The time now is 09:03 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2