Spice simulation - Page 103 - 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 February 2011, 06:30 PM   #1021
diyAudio Member
 
Join Date: Jan 2011
Quote:
Originally Posted by keantoken View Post
Here is that example I was talking about. I wouldn't build it for myself, but it will get you started.

- keantoken
Hey keantoken,

Per your suggestion, I've been playing around with measuring open loop gain. I don't know for sure, but I think you might have your voltage probe in the wrong place for the type of push-pull amp we've been simulating. I couldn't get it to work right until I moved it like this:

OLG.png

I'm reading the LoopGain2.asc simulation in the examples. Is this also how you read it?

BTW, while figuring out the OLG measurement, I looked at the amp circuit you posted and realized it's exactly the same as the one I was working on except the LTP & driver transistors are the opposite polarity. They're basically identical amps.
  Reply With Quote
Old 16th February 2011, 10:35 PM   #1022
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Buckeye, that is the correct method AFAIK. Maybe OS is measuring it wrong, I got that schematic from him. Although maybe he started doing it that way because he saw me doing it. In any case, I would go by the loopgain files. I was confused at first by those files, I remember doing it the wrong way for a long time.

Many amplifiers are basically the same. The devil is in the details, the choosing of operating points, the compensation, transistors chosen, grounding, etc. But actually I gave you that file because it was included default with LTSpice, guaranteed to work at first startup.

- keantoken

Last edited by keantoken; 16th February 2011 at 10:37 PM.
  Reply With Quote
Old 17th February 2011, 12:39 AM   #1023
diyAudio Member
 
Join Date: Jan 2011
Confused here... what is the correct method, the one I posted or the one you posted? The difference is the feedback path. I think the feedback connection is supposed to be after the probe and from the point of the load. The way push-pull solid state amps are often drawn, it's real convenient to draw the feedback from where the emitter resistors are. But those resistors are what drives the load, so they have to be before the probe. I mean, if you look at the LoopGain2 example, and expand the op-amp, you'll likely see a push-pull pair driving the output. The probe is inserted between the output, which is where the emitter resistors are, and the load. Then, the feedback is taken from the load to the inverted op-amp input. That's the same as the long-tail pair inverting input.

So my point is... you can't put the probe between the LTP (-) base and the emitter resistors. It's got to go on the other side of those resistors. Right?
  Reply With Quote
Old 17th February 2011, 12:52 AM   #1024
Account disabled at member's request
 
MJL21193's Avatar
 
Join Date: Mar 2007
Quote:
Originally Posted by Buckeye View Post
Confused here... what is the correct method, the one I posted or the one you posted?
Correct is what is shown in the Mongrel file. It needs to be in the feedback loop as shown. The input needs to be grounded as well.
The plot parameters (I don't use LT, so the terms might be off) are something like this: -v(10)/V(33) where V(10) is the net after the probe (on the output side) and V(33) is the net before the probe.

Looks like this:
Attached Images
File Type: png 111.png (35.7 KB, 234 views)
  Reply With Quote
Old 17th February 2011, 12:55 AM   #1025
Account disabled at member's request
 
MJL21193's Avatar
 
Join Date: Mar 2007
You get a plot that looks like this:

112.png
  Reply With Quote
Old 17th February 2011, 01:10 AM   #1026
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
John, here is a picture of the LTSpice OLG files.

- keantoken
Attached Images
File Type: png Loopgain.png (127.4 KB, 241 views)
  Reply With Quote
Old 17th February 2011, 01:13 AM   #1027
diyAudio Member
 
Join Date: Jan 2011
No, NO! Please say it isn't so!

Looking at the T.et.al() analysis I thought the probe had to go here:

mongrel.png

Plus you need the current probe. Are we all doing the same type of OLG simulation? Maybe I'm way off base.
  Reply With Quote
Old 17th February 2011, 01:20 AM   #1028
diyAudio Member
 
Join Date: Jan 2011
Quote:
Originally Posted by MJL21193 View Post
You get a plot that looks like this:...]
Those are nice plots, but the reason I'm doing OLG simulation is to find the phase angle at unity gain. I thought if the phase goes past 180 before unity gain (0dB), the system is unstable. So that's why I'm looking at that. I'm a little confused about how that mongrel simulation does that.
  Reply With Quote
Old 17th February 2011, 02:01 AM   #1029
Account disabled at member's request
 
MJL21193's Avatar
 
Join Date: Mar 2007
Quote:
Originally Posted by Buckeye View Post
I thought if the phase goes past 180 before unity gain (0dB), the system is unstable.
I can't comment on the LT sim - I don't know how to use it. I 'grew up' on Multisim and I'm used to it's interface. I'm not an expert, by ANY stretch of the imagination, but here's how I understand it:
In the results, the thing to look for is where the gain hits zero db and when this happens, the phase should be close to -100 degrees.

111.png

The one I posted earlier was hastily done, for illustration only from a 'work-in-progress' amp. The plot above is close to the real amp.
  Reply With Quote
Old 17th February 2011, 05:03 AM   #1030
diyAudio Member
 
Join Date: Jan 2011
Quote:
Originally Posted by MJL21193 View Post
I can't comment on the LT sim ...
The open loop gain analysis should be the same regardless of which tool you use. What is the proper injection point for the probe? I tried it your way, and the results are similar but not identical.

The top trace is your way, the bottom trace is with the probe between the emitter resistors and the load with feedback going from the load.
Attached Images
File Type: png olg1+2.png (26.8 KB, 230 views)
  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
Need help with Spice simulation overmind Everything Else 4 23rd December 2002 05:58 PM


New To Site? Need Help?

All times are GMT. The time now is 12:52 PM.


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