Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
Well your question is very general, and I'm not sure if I understand what you mean, but I'll give it a try:
NMOS transistors have a positive threshold voltage, which means that in order to "turn the transistor on," a gate-source voltage larger than the threshold voltage should be applied.
PMOS transistors on their turn, have a negative threshold voltage. This means that the gate-source voltage should be smaller than the threshold voltage in order to "turn the transistor on."
For this reason, NMOS transistors are very often connected to ground potential with their sources, whereas PMOS transistors are very often connected to Vdd with their sources.
It would be of no use to connect a PMOS source to ground, because the gate voltage needed to turn the PMOS in its active region would be smaller than 0V.
I hope this helped, if not; please rephrase your question.
Perhaps the question is "why do pmos input pairs of differential amplifiers work better when the input is close to ground, and why do nmos input pairs of differential ampliers work better when the input is close to the power supply"
The answer is that if your common mode input is close to ground for some reason, you wont be able to use nmos as your input because you will not have enough vgs to overcome the vt + the current source below it. The same answer is true in reverse for the pmos.
The only problem you have when using the pmos as an input pair with a low input common mode is of putting the input transistor into triode. Normally this is much easier to stop than the problem mentioned above.
Perhaps the question is "why do pmos input pairs of differential amplifiers work better when the input is close to ground, and why do nmos input pairs of differential ampliers work better when the input is close to the power supply"
The answer is that if your common mode input is close to ground for some reason, you wont be able to use nmos as your input because you will not have enough vgs to overcome the vt + the current source below it. The same answer is true in reverse for the pmos.
The only problem you have when using the pmos as an input pair with a low input common mode is of putting the input transistor into triode. Normally this is much easier to stop than the problem mentioned above.
in general the pmos input pair has lower 1/f noise, but lower gm. Thus they are good when the input common mode is low, or the circuit has difficulties with 1/f noise at the input pair. They are generally worse if speed/input loading is an issue or higher input common mode.
one good thing about pmos input pair is that they can, in most digital processes tie their substrate to their source to remove the body effect, which is not possible unless a twin tub process is paid for (for the nmos).
As the supply voltage is higher than the ground voltage ,so the current should pass downwards(from the supply to the ground) and so the input to the diff. pair should be such that the vgs is + for nmos input - diff. pair (- for pmos input diff. pair ) (either lower than Vth for sub-threshold operation OR larger than Vth for saturation operation )
So in order to have the diffpair transistors , their active loads, & the current tail in subthreshold or saturation you will get the required input CM levels for pmos is lower than mid supply while that for nmos higher than mid supply.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
