PROBLEM 085 · Medium · Open-loop comparators
Decide Fast
Open-loop comparator: 50 mV overdrive
The problem
Build an open-loop comparator: a five-transistor OTA (an NMOS pair on a tail Current source with a PMOS mirror load, the input on the diode side being “inp”) followed by an inverter that brings the output to the rails. Its output is “out”. The test bench holds “inn” at 0.9 V and steps “inp” from 0.850 V to 0.950 V. Make it decide in under 2 ns.
A comparator is an amplifier used without feedback: its gain makes a small input difference into a full-swing output, and its speed is how long that takes. The delay depends on the overdrive (the size of the input step): the smaller it is, the longer the regeneration. The first stage gives the gain; the inverter restores the logic levels.
The test bench
- Supply: VDD = 1.8 V.
- The test bench holds “inn” at 0.9 V and drives “inp” with a pulse between 0.850 V and 0.950 V (50 ps edges). Do not place sources on “inp” and “inn”.
- It reports the delay from the input crossing 0.9 V to the output crossing 0.9 V, and the output levels.
Numbers to hit
- Delay at the rising step at most 2 ns
- Output high level at least 1.6 V
- Output low level at most 0.2 V
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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