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Old 1st January 2013, 11:03 PM   #1
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Default How to model stability analysis of a linear regulator such as this?

In simulation, what is the proper method to model a stability analysis of the following circuit?

Meaning where do I inject the AC signal and where do I look at the loop gain/phase? Do I need to inject the AC signal into vdc and look at vb? If so, what other modifications do I need to make to my testbench to properly model the analysis?
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Old 2nd January 2013, 07:34 AM   #2
Elvee is online now Elvee  Belgium
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The most straightforward way (not 100% theoretically accurate, but more than sufficient) is to insert your AC source between FB and the NI input, and look at the ratio of VFB/VNI
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Old 2nd January 2013, 05:00 PM   #3
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Originally Posted by Elvee View Post
The most straightforward way (not 100% theoretically accurate, but more than sufficient) is to insert your AC source between FB and the NI input, and look at the ratio of VFB/VNI
What inaccuracies arise from this method?
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Old 2nd January 2013, 05:14 PM   #4
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use forum search for Tain Middlebrook LoopGainProbe for some discussions

you can also force the output with a Isource - the impedance curve is related to the feedback factor - severe peaking is a warning sign

load step response gives similar info
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Old 2nd January 2013, 05:30 PM   #5
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Fusion916 View Post
What inaccuracies arise from this method?
The loading of the divider by the amplifier's input is not taken into account
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Old 3rd January 2013, 02:52 AM   #6
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Elvee, do you mind posting a simulation schematic of your test circuit for stability analysis?
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Old 3rd January 2013, 06:54 AM   #7
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I bet MicroCAP does it. If you install it and go to EXAMPLES folder than you find something similar ( stability over time, outside temperature, parts parameters deviations and transition events). Must be here.
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Old 3rd January 2013, 08:23 AM   #8
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Fusion916 View Post
Elvee, do you mind posting a simulation schematic of your test circuit for stability analysis?
Here it is; the circuit had to be adjusted to display something usable as the original values were too crazy, but the principle remains the same whatever the exact circuit.
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Old 3rd January 2013, 05:30 PM   #9
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V3 is the AC source correct? And what purpose does the 1n4148 serve?
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Old 3rd January 2013, 06:48 PM   #10
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Fusion916 View Post
V3 is the AC source correct?
Yes
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And what purpose does the 1n4148 serve?
Displaying a nice downwards sloping curve as expected, instead of an ugly flat one (not connected with simulation issues).
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