PROBLEM 070 · Medium · Output resistance of a follower
Stiff Output
A follower with an output resistance under 150 Ω
The problem
Build a source follower: an NMOS with its drain on VDD, its gate on an Input source named “in” (DC level of your choice), and its source on “out”, biased by a Current source to ground.
The test bench injects a 1 A AC current into “out”: the AC voltage that appears there, in volts, is the output resistance in ohms. For a follower it is about 1/(gm + gmb): to lower it you need more gm, which means a wider transistor and more current.
The test bench
- Supply: VDD = 1.8 V.
- The test bench injects a small AC test current into “out” (1 A in the simulator’s units, so volts are ohms). “in” has no AC signal.
Numbers to hit
- Output resistance at most 150 Ω
- Output DC level between 0.3 V and 0.9 V
- Power at most 1500 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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