PROBLEM 002 · Beginner · The diode-connected transistor
Self-Tied
A diode-connected transistor as a 0.68 V reference
The problem
Connect one NMOS with its gate tied to its drain and its source on ground. The shared net is called “out”. The test bench pushes a 20 µA current into “out”.
Because the gate and drain are the same net the transistor is always saturated, and it settles at the Vgs that carries 20 µA. A wider device needs less overdrive for the same current, so its Vgs is lower; a narrower one needs more. You are choosing W/L to set a voltage.
The test bench
- Supply: not needed.
- The test bench pushes 20 µA into “out” and sweeps that current from 0 to 60 µA to draw the voltage it produces.
Numbers to hit
- Voltage at 20 µA between 0.66 V and 0.7 V
- Transconductance gm at least 0.3 mS
- 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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