Dear robert21, [quote userid="360385" url="~/cadence_technology_forums/f/rf-design/42966/test-signal-iprobe-rout-calculation/1364011#1364011"]every small signal analysis uses this method to find rout.[/quote] Andrew is correct, robert21. Your test bench does not appear to be consistent the circuit topology used to determine output impedance of a three terminal device. Why? Consider what the impedance looks like as you look into the unlabeled node of the drain of the MOS device. Referring to Figure 1 below, the input impedance consists of the parallel impedances of your vdd source and the rout of the MOS device whose output impedance you are interested. Since the DC source vdd has zero impedance, the impedance shown as Zin will be zero. Your topology shunts the rout of the MOS device by the zero impedance of the voltage source vdd. I hope this makes sense robert21. Let me know if it does not. What is needed to measure rout with a test bench similar to yours is a series impedance in the vdd source that will not significantly impact the device rout. Of course, the added vdd series impedance must also be small enough in order that the DC drop from the DC MOS device drain current does not result in a significant voltage drop. Two possibilities to consider: 1. Include a resistor from the analogLib in series with your vdd ideal supply and assign it a small DC resistance (1 milliohm?) and a large AC resistance (Rac >> rout, see Figure 2). 2. Include a very large inductor from the analogLib in series with your vdd ideal supply. With a large enough value of inductance, at low frequencies its impedance may be set as >> the expected value of rout. The series inductor will not reduce the DC voltage of your vdd source, but at finite freuqencies, its impedance will prevent most of your input current from flowing through the DC source vdd. In both cases, change your DC source V2 to an AC current source with an AC value of 1 and a DC current of 0. Run the AC simulation. However, when you analyze the result, measure the real component of the voltage looking into the MOS drain node. Since the AC current source has a value of 1, the real part of the measured voltage will be the real impedance. You can extrapolate the real impedance, which is effectively rout(f), to its value at DC from your measured result. There are other ways to measure rout, but this came to mind given your existing test bench. Let me know how you make out... Shawn FIgure 1 Figure 2
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