Why we should always connect N-mos transistor to ground

Discussion in 'General Electronics Chat' started by maheshg, Jul 31, 2011.

  1. maheshg

    Thread Starter New Member

    Aug 11, 2010
    Hi friends,I have a simple doubt that why we always connect n-mos to ground and p-mos to Vdd??explain
  2. tom66

    Senior Member

    May 9, 2009
    There is no need to, but it does make driving it easier.

    For example, say we have a 1 ohm load resistor and a 12V power supply. If we put the NMOS on the high side - connected between 12V and the resistor - we need to provide 12V (supply voltage) + Vgs (about 10V) to turn it on, because the gate is relative to the supply voltage of the resistor. This means we need to find a 22V power supply. Whereas with a low side switch, we only need to provide the gate with 10V; we could even use our 12V, because the gate is relative to ground, always.

    WIth PMOS it's different. A PMOS is turned on by a negative gate voltage less than its negative threshold. Using the same example; on the high side, we could connect the gate to ground, giving it a -12V drive and turning it on, because the gate is relative to the highest voltage i.e. the 12V supply; on the low side, we need to find or make a -10V/-12V power supply, because it's now relative to ground.

    Now, as I showed in my examples, you could put NMOS on the high side and PMOS on the low side. There's nothing in particular stopping you, but you need to make more complicated circuitry to drive them. Gate drivers are made for high-side NMOS driving. The most common use of a high-side NMOS is to replace the high-side PMOS which is less efficient and more expensive; a high-side MOSFET adds an element of safety, as nothing can be shorted to ground to blow anything up.