PROBLEM 029 · Easy · A resistor-programmed bias current
Poor Man's Bias
A bias current from a resistor
The problem
Build a bias current generator from a resistor from VDD to a diode-connected NMOS (its gate on its own drain, source on ground), and a second NMOS that copies the gate voltage and delivers the current to the net “out”. Set the resistor so the copy carries about 50 µA.
The reference current is (VDD − Vgs)/R. Vgs barely changes, so the current follows the supply almost one-for-one: a 100 mV droop in VDD changes the current by about 9%. The test bench sweeps the supply and shows how much. The later problems fix this.
The test bench
- Supply: VDD = 1.8 V.
- The test bench holds “out” at 0.9 V with a voltage source and measures the current your copy delivers, then repeats the measurement as the supply sweeps from 1.4 V to 2.2 V.
Numbers to hit
- Output current at 1.8 V between 45 µA and 55 µA
- Supply sensitivity at most 110 %/V
- Power at most 220 µW
- 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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