Pull-Up and Pull-Down Ratio in nMOS Inverter
nMOS Inverter Overview
In an nMOS inverter, the nMOS transistor acts as the pull-down device, while a load device (resistor or depletion nMOS) acts as the pull-up network.
Pull-Down Network
- The nMOS transistor is the pull-down device
- It pulls the output to 0 (GND) when ON
Pull-Up Network
- The load device pulls the output to VDD when input is LOW
- It provides a weak logic HIGH
Pull-Up Ratio (PU Ratio)
Defined as:
Pull-up ratio = ÎēL / ÎēN
Where:
- ÎēL = transconductance parameter of load device
- ÎēN = transconductance parameter of nMOS pull-down transistor
Meaning: Measures strength of pull-up compared to pull-down.
Pull-Down Ratio (PDR)
Defined as:
Pull-down ratio = ÎēN / ÎēL
Meaning: Measures strength of pull-down compared to pull-up.
Design Condition
For proper operation of nMOS inverter:
- Pull-down must be stronger than pull-up
- ÎēN > ÎēL is typically required
Simple Intuition
| Device | Role | Strength |
|---|---|---|
| nMOS transistor | Pull-down | Strong |
| Load device | Pull-up | Weak |
Summary
- Pull-up ratio = pull-up strength / pull-down strength
- Pull-down ratio = pull-down strength / pull-up strength
- nMOS inverter works best when pull-down dominates