PROBLEM 080 · Hard · Noise against power
Whisper Budget
Low noise on a power budget
The problem
Build the same common-source amplifier (an NMOS with its gate on “in”, a resistor from VDD to “out”), but with only 100 µW to spend. The noise must stay under 14 µV rms (10 Hz to 1 MHz) and the gain over 10 dB.
With a fixed current, gm is largest when the transistor is wide and works near weak inversion (gm/Id close to its maximum of about 25 per volt). That is the way to buy noise performance without power. The cost is area and the larger gate capacitance.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor from “out” to ground, applies the AC stimulus to “in”, and runs a noise analysis from 10 Hz to 100 MHz with “out” as the output and “in” as the reference input.
- Input-referred noise is the output noise divided by the gain: what an ideal noiseless amplifier would need at its input to produce the same output noise.
Numbers to hit
- Gain at least 10 dB
- Input-referred noise, 10 Hz to 1 MHz at most 14 µV
- Power at most 100 µW
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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