PROBLEM 005 · Easy · The triode region
Fake Resistor
A transistor as a resistor
The problem
Connect one NMOS with its source at ground, its gate held high by an Input source named “in” and its drain on a net named “out”. Do not use a drain resistor: the transistor itself is the resistor.
At a small drain-to-source voltage the channel behaves like a resistance R = 1 / (k′·(W/L)·(Vgs − Vth)). The resistance is only constant while Vds is small: as Vds grows the channel pinches at the drain and the curve bends toward saturation.
The test bench
- Supply: not needed.
- The test bench holds “out” at 50 mV and measures the current; the resistance is 50 mV divided by it. It sweeps “out” from 0 V to 0.3 V so you can see where the curve bends.
Numbers to hit
- On-resistance at 50 mV between 800 Ω and 1200 Ω
- Rise of resistance at 0.3 V at most 45 %
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.
Sign in or create a free account →More on device fundamentals
- Sweet SpotSet a drain current of 100 µA, then make every microamp count: the lower the overdrive, the more gm you get from it.
- Self-TiedTie the gate to the drain and feed it a current: the transistor sets its own voltage. Choose its size to set that voltage.
- Upside DownA PMOS conducts when its gate is below its source. Pick the gate voltage and size for a 50 µA current.
- Long Story ShortA transistor’s best possible voltage gain is gm·ro. Find out what channel length buys you.
- Barely OnBelow threshold a transistor’s current is exponential in Vgs and its efficiency is at its peak. Reach 5 µA at a gate voltage under 0.5 V without wasting area.