PROBLEM 071 · Medium · The PMOS follower
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A PMOS follower: a level shifter that goes up
The problem
Build a PMOS source follower: a PMOS with its drain on ground, its gate on an Input source named “in” and its source on the output “out”, fed by a Current source from VDD. The test bench adds a 5 kΩ load from “out” to ground.
The output is the input plus the source-gate voltage |Vgs|, a shift of about 1 V in this process: it lifts a low-level signal near ground up toward mid-supply. The follower’s gain is slightly below 1.
The test bench
- Supply: VDD = 1.8 V.
- The test bench connects a 5 kΩ resistor from “out” to ground and applies the AC stimulus to “in”.
Numbers to hit
- Gain into 5 kΩ at least -3.5 dB
- Output DC level between 1.2 V and 1.7 V
- Power at most 800 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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