PROBLEM 001 · Beginner · Drain current, overdrive and gm/Id
Sweet Spot
Bias a transistor for efficiency
The problem
Connect one NMOS with its source at ground, its drain on VDD and its gate driven by an Input source named “in”. The drain current is the current the supply gives, so it is easy to measure.
Choosing W, L and the gate voltage is a trade. A narrow device needs a big gate voltage (large overdrive) to carry the current and gives little gm per microamp. A wide device carries it at low overdrive and gives much more. The ratio gm/Id is the efficiency of the bias.
The test bench
- Supply: VDD = 1.8 V.
- The test bench reads the supply current, which is the transistor’s drain current, and sweeps the source named “in” from 0 V to 1.2 V to draw the transistor’s transfer curve.
Numbers to hit
- Drain current between 95 µA and 105 µA
- Efficiency gm/Id at least 12 1/V
- 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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