So I'm building something and I figured I needed a mid-voltage Comparator or symmetrical Schmitt Trigger (by losing the MOSFET half of the V2 side). For reasons of "just-because", I decided it should be MOSFET based. So I thought I wouldn't run to any problem, but just when I started, there are values that popped up I don't think is correct.

Below is the stripped-down version:


simulate this circuit – Schematic created using CircuitLab

Now, I'm just wondering as to why CMP1 would be "15V", as M1 should not even switch on as the gate is less electropositive than the source.


This is just the MOSFET implementation of this BJT comparator: enter image description here


Also, I gave the title "CMOS Comparator", what I meant was "MOSFET Comparator". I corrected it now.

  • \$\begingroup\$ To clarify your schematic a bit I suggest removing input generators and labeling the inputs. I don't think you can make a comparator with only two transistors anyway. \$\endgroup\$ – Vladimir Cravero Jun 12 '15 at 9:55
  • \$\begingroup\$ I'm starting to think people in this site lack imagination. Sure I'm not showing you the whole project, but this one is just a weeny, tiny, small step from the previous... \$\endgroup\$ – Dehbop Jun 12 '15 at 9:58
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    \$\begingroup\$ They don't lack for imagination. They've got more experience, and much MORE imagination, and so can (usually) see multiple things that might be in or around your circuit if it isn't complete enough. Rather than provide an answer that might be based on a different view of your problem, they ask for clarification. \$\endgroup\$ – JRE Jun 12 '15 at 10:05
  • \$\begingroup\$ ^ this. And please note that your first circuit is not a mos implementation of the second, it's just vaguely similar. \$\endgroup\$ – Vladimir Cravero Jun 12 '15 at 10:06
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    \$\begingroup\$ My stripped down version answer is "Your circuit does not work because of reasons.". Good luck! \$\endgroup\$ – Vladimir Cravero Jun 12 '15 at 10:25

First, your stripped-down circuit is pretty worthless without specifying the loads on the M1 and M2 drains.

Second, even though the Vgs of M1 is -5, the only current available at the drain will be leakage currents from the source. Since the drain is floating, regardless of the amplitude of those leakage currents, since there is no alternative current path, a voltage reading at the drain will equal the voltage at the source - in this case, 15 volts.

  • \$\begingroup\$ Never considered Drain-Source leakage, good thing that CircuitLab also simulates that. \$\endgroup\$ – Dehbop Jun 12 '15 at 23:47

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