MOS Trigger Circuits

Standard Schmitt Trigger

Most CMOS designs use the follwing general purpose schmitt trigger to shape signals with slow edges.

The circuit is very small and therefore it is cheap to implement. Accuracy however is poor for the following reasons:

    - Thresholds depend on matching of NMOS and PMOS transistors
    - PMOS and NMOS transistors have different bulks. So they don't match
    - hole and electron mobility differ.
    - Thresholds depend on supply voltage VDD
        => spread of trip points usually is in the range of +-25% of VDD

Advantages of the circuit:

    - fast
    - no current consumption if Vin<Vthnmos or Vin>VDD-Vthpmos
    - cheap

Suggested applications: wafe shaping, simple noise rejection circuits (requires stable VDD).