PROBLEM 087 · Expert · Hysteresis
Hysteria
A Schmitt trigger with 300 mV of hysteresis
The problem
Build a six-transistor CMOS Schmitt trigger. Two NMOS in series from “out” to ground have their gates on “in”; a third NMOS feeds back from VDD to the midpoint between them, with its gate on “out”. Two PMOS in series from VDD to “out” have their gates on “in”; a third PMOS feeds back from ground to their midpoint, with its gate on “out”. The output is “out”.
When the input rises, the feedback NMOS holds the midpoint high and delays the switching until the input is well above mid-supply; when it falls, the PMOS feedback does the same in the other direction. The difference between the two thresholds is the hysteresis. Set the feedback transistors’ sizes to place the thresholds at about 1.1 V (rising) and 0.7 V (falling).
The test bench
- Supply: VDD = 1.8 V.
- The test bench drives “in” with a source (do not place one) and hangs a 20 fF load capacitor on “out”. It sweeps “in” upward from 0 V to 1.8 V and then downward from 1.8 V to 0 V, and reports the input voltages at which “out” crosses 0.9 V in each direction.
Numbers to hit
- Rising threshold between 0.95 V and 1.3 V
- Falling threshold between 0.5 V and 0.8 V
- Hysteresis at least 300 mV
Design it yourself
Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.
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