PROBLEM 019 · Easy · The basic current mirror
Copy Paste
Basic current mirror
The problem
Build a 1 : 2 current mirror from NMOS transistors. A 50 µA current source from VDD feeds a diode-connected transistor (gate tied to drain). A second transistor shares its gate voltage and should deliver twice the current from its drain, which is the net named “out”.
A perfect mirror would deliver exactly 100 µA whatever voltage its output sits at. Real transistors have finite output resistance, so the copy drifts as the drain voltage moves away from the diode’s own. Your choice of channel length decides how well it holds.
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 97 µA and 103 µA
- Change between 0.5 V and 1.5 V at most 6 %
- 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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