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 21st November 2008, 03:52 AM #121 ostripper   diyAudio Member     Join Date: May 2008 Location: Smoky Mountains , Tennessee Yeah, I saw a test run for loopgain and loopgain2 in (edu) folder.. I'm still on the learning curve ..I have ver. 4.000c At least we are running the same thing..
 21st November 2008, 03:55 AM #122 ostripper   diyAudio Member     Join Date: May 2008 Location: Smoky Mountains , Tennessee So "loopgain' is a module of LT that you can insert into any circuit?? I know how to get the circuit built on LT, run an efficiency test on it to simm hoe component choices would behave, I can scope a point, measure a point, etc. I am weak on all the options allowing me to do more advanced analysis on the circuit... Os
 21st November 2008, 03:57 AM #123 andy_c   Banned   Join Date: Apr 2003 To see the simplest possible way to simulate loop gain in LTspice, have a look at the audioamp.asc example in the educational folder. When you put that floating voltage source in the feedback loop and test the loop gain using the formula provided in that example, be sure that the AC values of all other voltage sources in the circuit are set to zero.
andy_c
Banned

Join Date: Apr 2003
Quote:
 Originally posted by ostripper So "loopgain' is a module of LT that you can insert into any circuit??
Try the simplest approach first by opening up the audioamp.asc and using its technique and formula in your circuit. That is, put the floating voltage source in the feedback loop of your amp, use F4 to name the "A" and "B" nodes on either side of the voltage source, and plot V(A)/V(B) to see the loop gain.

 21st November 2008, 04:12 AM #125 infinia   diyAudio Member     Join Date: May 2005 Location: SoCal Hi OLG with Bode plot? Is C13 miller cap for dominate pole compensation? __________________ . .
 21st November 2008, 04:17 AM #126 ostripper   diyAudio Member     Join Date: May 2008 Location: Smoky Mountains , Tennessee Thanks ,Andy , but you are wayyyy more advanced than me... I have the amp.asc open stopped the input (I know that). tried to rename a/b to t1/t2,i've been successful at the rename... ???? only thing I need to know is how to plot V(A)/V(B) to see the loop gain....
ostripper
diyAudio Member

Join Date: May 2008
Location: Smoky Mountains , Tennessee
Quote:
 By infinia...Is C13 miller cap for dominate pole compensation?
yes, thats why "dum dum" me is learning more on spice, to
get appropriate value. C12/13 both.

I guess they both gave up on me..

 21st November 2008, 04:34 AM #128 andy_c   Banned   Join Date: Apr 2003 I'm not sure why you're renaming A and B. Try this. Open up audioamp.asc file in the educational folder. Choose "Simulate, Edit Simulation Command". Click on the "AC Analysis" tab. For "type of sweep" choose "decade". For "points per decade" enter 100 For start frequency, enter 100 For stop frequency, enter 10e6. Click OK. After doing so, the cursor will have a box attached to it. Move the cursor to somewhere on the schematic and left-click it. This will place an AC analysis command in the sim. Press the "run" button. A blank graph will appear. Right click on the graph and choose "Add Trace" In the "expression to add" box, enter V(A)/V(B). Click OK or press Enter. You should see a graph of the loop gain. Once you get this working with audioamp.asc, try it with your own circuit.
 21st November 2008, 04:38 AM #129 infinia   diyAudio Member     Join Date: May 2005 Location: SoCal Hi OS Spice is nice, like any software it's good to know what to expect beforehand. >That's why I asked if anyone still relies on Bode plots by hand using 5 cycle semilog paper. Calculate max gain of each stage (OLG=3 stages added in dB) locate and plot poles and zero's from that. __________________ . .
 21st November 2008, 05:03 AM #130 Greg Erskine   diyAudio Member     Join Date: Jan 2002 Location: Sydney/Australia thanks andy_c for the clear instructions, much appreciated. __________________ Greg Erskine

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