PROBLEM 069 · Medium · Driving a resistive load
Heavy Load
A source follower into 1 kΩ
The problem
Build a source follower: an NMOS with its drain on VDD, its gate on an Input source named “in”, and its source on the output “out”, pulled down by a Current source to ground. The test bench connects a 1 kΩ load resistor from “out” to ground.
The gain of a follower is gm/(gm + gds + 1/R_L). With 1 kΩ the transistor needs a gm of several mS to keep the gain near 1: a wide transistor and a lot of current. The load itself takes current from the transistor, so the bias current is more than the current source alone.
The test bench
- Supply: VDD = 1.8 V.
- The test bench connects a 1 kΩ resistor from “out” to ground and applies the AC stimulus to “in”.
Numbers to hit
- Gain into 1 kΩ at least -2.5 dB
- Output DC level between 0.3 V and 0.7 V
- Power at most 2500 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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