PROBLEM 030 · Easy · A voltage reference from stacked diodes
Diode Duo
A 1.4 V reference from two diode-connected transistors
The problem
Build a voltage reference: a resistor from VDD to the net “out”, and two diode-connected NMOS stacked under it (the upper one’s gate and drain on “out”, its source on the lower one’s drain; the lower one’s gate and drain together, its source on ground). The voltage on “out” is about 2·Vgs.
A diode-connected transistor is a low-impedance element (1/gm), so when the supply moves, most of the change falls across the resistor and only a fraction appears on “out”. That fraction is r/(R + r) where r is the stack’s small-signal resistance. The more current, the lower r and the more stable the reference, but the more power it burns.
The test bench
- Supply: VDD = 1.8 V.
- The test bench sweeps the supply from 1.4 V to 2.2 V and measures the voltage of “out” at each point; it reports the voltage at 1.8 V and its change per volt of supply.
Numbers to hit
- Voltage at 1.8 V between 1.3 V and 1.5 V
- Change of “out” per volt of supply at most 0.35 V/V
- Power at most 130 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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