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How to size the pMOS and nMOS in NOR NAND XNOR gates?

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isaacnewton

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rule of thumb nmos

It's about digital gates design.

For Inverter, we can size the (W/L)p and (W/L)n according to Vth, Delay requirement. But for NOR, NAND, XOR gates, because there are more transistors in those gates, it looks like not easy to choose the values for (W/L)p and (W/L)n.
 

transistor pmos and or nand nor

Normally you design an inverter to have equal rise and fall times. This is the most general approach. Minimum channel length can be used if you want to reach maximum speed. Channel widths depend on the drive strength you want to have for a given maximum speed.

To design a any other gate function, once you have the inverter designed, transistor connected in series must have the same equivalent size ratio than the inverter. For example, for a 2-input NAND, as two NMOS transistors are connecter in series, each individual transistor must have twise the size of the inverter NMOS. The dual case is for the NOR. The EXOR can be build from one AND and a couple of NOR.
 

xnor nmos

you can refer to "CMOS integrated circuits analysis and design" chap.7
 

nor nand speed

This is the most popular question asked in a IC design job interview
 

nmos pmos ratio size

if it use in small load. just use the basic size.
 

nand pmos nmos

Hi

rule of thumb the ratio w/l pmos : w/l nmos = 3:1
 

nand using nmos and pmos

rule of thumb the ratio w/l pmos : w/l nmos = un/up
 

nor pmos nmos

I think it depends on
1. the load. you need large mos to drive large cap load. if load is minimum, you can use minimum mos.
2. the drive capability of your previous stage. it should be able to drive this gate as a load at a given speed. be careful that your previous stage may need to drive many gates at the same time.
3. then you can choose the ratio between n and p. Just simulated it and make the slew rate of 0 to 1 equals to that of 1 to 0.
4. multi-stages may be necessary if one stage can not drive a large load.
 

nand pmos nmos sizing

Start designing with transmission gates.. then see what you need... speed or current? speed --> small, high current --> big. there will be glitch in the ouput of the transmission gates, you can put an inverter to steady the output.. (small size may be okay....). i find that number of transistor somewhat does not matter because of ULSI...
srivatsan
 

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