The CMOS inverter is the smallest useful bridge from MOSFET switches to logic.
It turns one input voltage into the opposite output voltage range.
The Two Networks
A CMOS inverter uses:
- a PMOS pull-up path to the supply voltage
- an NMOS pull-down path to ground
flowchart TD vdd["Supply voltage"] --> pmos["PMOS pull-up"] pmos --> out["Output"] out --> nmos["NMOS pull-down"] nmos --> gnd["Ground"] input["Input"] --> pmos input --> nmos
The same input controls both transistors.
Input Low
When the input is low:
- PMOS is on
- NMOS is off
- the output is pulled up toward the supply voltage
So a low input produces a high output.
Input High
When the input is high:
- PMOS is off
- NMOS is on
- the output is pulled down toward ground
So a high input produces a low output.
Why This Is Logic
The inverter is physical before it is symbolic.
The logical rule NOT appears because the circuit’s transistor paths create opposite voltage ranges:
| Input Voltage Range | Output Voltage Range | Logical Meaning |
|---|---|---|
| Low | High | NOT 0 = 1 |
| High | Low | NOT 1 = 0 |
Boundary
During switching, both transistors may briefly conduct partly, and the output node has capacitance that must charge or discharge.
The durable model is:
A CMOS inverter works because one input selects either a pull-up path or a pull-down path, making the output settle into the opposite voltage range.