PROBLEM 081 · Hard · Flicker (1/f) noise
1/f Fighter
A low flicker-noise corner
The problem
Build the common-source amplifier again, and keep its 1/f corner (the frequency where the input-referred noise rises to √2 times the white floor) below 200 kHz. A minimum-size transistor’s corner is above 400 kHz. The reported corner and floor are shown while you design.
Flicker noise power is inversely proportional to the gate area W·L·M and to the frequency; thermal noise is not area-dependent. So the corner moves down when the area goes up. A longer channel works too, because it adds area without adding capacitance per unit gm.
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 8 dB
- 1/f corner frequency at most 200 kHz
- Power at most 300 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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