PROBLEM 028 · Medium · Mirror accuracy and area
One Percent Club
A mirror accurate to 1%
The problem
Build a 1 : 1 NMOS current mirror from two identical transistors: one diode-connected carrying a 50 µA Current source from VDD, the other delivering the copy to “out”. The copy must be within 1% of the reference at 0.9 V and change by less than 1% between 0.9 V and 1.5 V, while the total gate area W·L·M of your two transistors stays under 200 µm².
A basic mirror’s error comes from the copy’s Vds being different from the diode’s (0.9 V against Vgs ≈ 0.7 V): the copy carries more current by about λ·ΔVds. A long channel makes λ smaller, but area grows with L, so the transistors can be made narrower again to hold the current density. That is the trade-off between accuracy and area.
The test bench
- Supply: VDD = 1.8 V.
- Your reference is the Current source you place: set it to 50 µA.
- The test bench holds the net named “out” at 0.9 V with a voltage source, sweeps it from 0.1 V to 1.7 V, and measures the current your mirror exchanges with it.
Numbers to hit
- Output current at 0.9 V between 49.5 µA and 50.5 µA
- Change between 0.9 V and 1.5 V at most 1 %
- Gate area W·L·M of both transistors at most 200 µm²
- Every transistor in saturation
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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