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diode-connected NMOS's equivalent resistance issue (Read 5263 times)

trashbox

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diode-connected NMOS's equivalent resistance issue
Aug 01^st, 2007, 12:39am

Hi guys,
When I was deriving a formula from a simple circuit, I run into the diode-connected NMOS's equivalent resistance issue. The details are:

In the attached figure, the formula Vref1=(1+R1/R2)Vgs1-Vgs2 is obvious for the left circuit when two NMOSs M1/M2 are both in saturation and sink the most of current because of large R1 and R2. However, if using diode-connected NMOS M5/M6 instead of the passive resistance, I can not derive the Vref2 from the right circuit. The simulation shows the formula for Vref2 is right. Would you please give me any advice?

P.S., If I use 1/gm5=R1 and 1/gm6=R2 to derive Vref2 from Vref1, the result is not correct. I think 1/gm is a small-signal resistance and here the equivalent resistance should be a DC resistor.

formula.JPG

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boe Community Fellow Offline Posts: 615	Re: diode-connected NMOS's equivalent resistance i Reply #1 - Aug 1^st, 2007, 2:21am Hi trashbox, The gm is indeed a small signal parameter; for your circuit you need the large signal resistance R = VDS / IDS(VGS). BOE
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trashbox Community Member Offline Posts: 66	Re: diode-connected NMOS's equivalent resistance i Reply #2 - Aug 1^st, 2007, 2:53am Hi Boe, Yes, you are right. Thanks for your reply. I have solved this issue now. Best wishes, Trashbox
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hande.vinayak Junior Member Offline Posts: 22 BOM	Re: diode-connected NMOS's equivalent resistance issue Reply #3 - Mar 25^th, 2011, 3:02am Hi trashbox and boe, Even after substituting R = VDS / IDS(VGS), I could not get the final expressions for Vref2 as shown in above figure. Please, can you share the method to solve the problem? Thanks.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: diode-connected NMOS's equivalent resistance issue Reply #4 - Mar 25^th, 2011, 1:13pm Why not just run a DC bias curve and take the resistance from the slope of the line?
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