EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Aeternam on November 21, 2016, 08:48:03 pm
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I was clicking around LTSpice while reading up on Thevenin equivalent circuits and was wondering if it was feasable to determine Vthevenin and Rthevenin (read, output impedance) using the simulator. Turns out it's pretty straightforward:
1. Determination of Vthevenin is easy. Capture the circuit with no load attached, then run an operating point analysis. Take note of Vout, this is Vthevenin (here: 15V)
2. Determination of Rthevenin is not very complicated either. We know that, if we add a load equal to our circuit's output impedance, its output will drop exactly by half. So let's add a load to the circuit, make its resistance a variable, and probe Vout. The screenshots below show the result (plus the syntax for the parameter sweep). All you have to do now is find Vthev/2 (7.5V) on the chart and read off the corresponding Rthev (5k). Easy :-+
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Simpler to short the output through a 0V voltage source to ground, do another .op and divide Vthevenin by the current through the short. (Its quite common to add a 0V source when you need to measure the current in a wire and there's no convenient single component to measure the current through.)
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Two calculation methods:
http://seventransistorlabs.com/Calc/Thevenin.html (http://seventransistorlabs.com/Calc/Thevenin.html)
Tim
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Or you could use the ".tf V(Vout) Vin" spice directive which will directly tell you the output impedance.
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Maybe I am missing something (usually am), just stim the
output with a current source, plot V and V/Istim vs F, you
will have your Zout.
Regards, Dana.