cmoslogic-gatesmosfetnmospmos
Below you can see a CMOS XOR gate. I wonder why we do not change extra inverters like A' or B' with opposite MOSFETs.
For example, could not we just put the green construction in the place of red one?
Here is my design after switching all MOSFETs according to description I have provided.
As Tom hints this is reverse voltage protection on the +12V line, however you may find it confusing for two reasons:
The MOSFET symbol is wrong - the part is a p-channel MOSFET, but an n-channel symbol is used. Try redrawing it with a p-channel symbol that includes the body diode.
I'm just going answer one part of your question that I don't think anybody has answered head-on yet.
Why is it that the output voltage doesn't change rapidly in the range of the threshold voltage of the MOSFETs?
Say you have 1.5 V on Vgate. Now the NMOS is fully turned on, because you've exceeded the threshold voltage.
But what's Vgs of the PMOS?
It's still -10.5 V, also far in excess of the threshold voltage.
You have to drive Vgate up to near 11 V before the PMOS Vgs gets near its threshold voltage.
Best Answer
NMOS can't really pull up that well, PMOS can't pull down.
An NMOS is controlled by \$V_{GS}\$, the voltage between the MOSFETs gate and source. In the original schematic, whenever the output should be low, all of the NMOS sources are pulled to ground (the transistors with floating sources will have had the sources pulled to ground by other NMOSs if the output will be low). Therefore, there will not be any problems getting \$V_{GS} > V_{th,N}\$.
In the second diagram, the top left NMOS has a floating source. If the source is at \$V_{DD}\$, the input \$A\$ would need to be at \$V_{DD} + V_{th,N}\$ to turn that transistor on. This is problematic.