PROBLEM 023 · Medium · Source degeneration for small currents
Ten to One
Widlar current source: 10 µA from a 100 µA reference
The problem
Build a Widlar current source from two NMOS. One is diode-connected and carries a 100 µA reference from VDD. The other has its gate on the same net, its drain on “out”, and a resistor from its source to ground. The resistor makes it carry only about a tenth of the reference current.
The output transistor’s Vgs is smaller than the reference’s by exactly Iout·R, which is why a modest resistor (a few kΩ) can make the current ten times smaller. Degeneration also raises the output resistance by roughly (1 + gm·R).
The test bench
- Supply: VDD = 1.8 V.
- Your reference is the Current source you place: set it to 100 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 9 µA and 11 µA
- Output resistance (0.5 V to 1.5 V) at least 2 MΩ
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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