I understand why when there is a ground you need a resistor or an NMOS but what if there were no ground at all and simply A goes high, B is 0, A goes low, current flows from VDD to B through the p channel. What's the reason this won't work? Forgive me but I'm relatively new to this.

  • \$\begingroup\$ I used our schematic tool that we have on our website to replace the image that you had originally. Consider using this when you want to draw a circuit. \$\endgroup\$ – user103380 Sep 30 '18 at 4:00
  • \$\begingroup\$ What does it mean for Vb to be zero, if there's no ground in your circuit? \$\endgroup\$ – The Photon Sep 30 '18 at 4:42
  • \$\begingroup\$ Ground or no ground, you need a + and a - in a circuit to have some sort of current flow. The mos will conduct based on the voltage difference between it's gate and the source, doesn't matter ground or not. \$\endgroup\$ – Damien Sep 30 '18 at 5:42
  • \$\begingroup\$ Lewis, it is hard to answer your question without asking a lot of other questions first. But in general, high-side PMOS operates with respect to VDD, not ground. If your node 'A' is at a lower voltage than VDD, that will tend to turn on the PMOS, meaning that node 'B' will get pulled up toward VDD. But there is probably a ground somewhere, right? VDD is defined with respect to GND (normally). \$\endgroup\$ – mkeith Sep 30 '18 at 6:23
  • \$\begingroup\$ If you measure the output voltage between B and VDD the non-ideality becomes obvoius. \$\endgroup\$ – Jasen Sep 30 '18 at 7:00

If "B is 0" then you have implied a "ground". Ground is just a name for an electrical node by which other signals are referenced and need not be connected to physical ground.

If "A goes high", then you have implied both a ground and a positive supply rail.

If "A goes low" ditto.

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  • \$\begingroup\$ So i think i understand; it ensures that B is truly at 0 so there is no stray charge in the next gate (assuming it's in an SRAM cell) or something making it 0<, allowing for a quicker transition possibly \$\endgroup\$ – Lewis Kelsey Sep 30 '18 at 15:12
  • \$\begingroup\$ If B = 0 then that is your voltage reference point. If you do not define a zero point (or ground reference) then how would you make referenced voltage measurements? The international space station doesn't connect to true earth ground but its metal frame is called ground for the sake of having some form of neutral measurement point. \$\endgroup\$ – Andy aka Sep 30 '18 at 15:14

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