PROBLEM 082 · Hard · Noise of a differential pair
Noisy Pair
Noise of a differential pair
The problem
Build a differential pair with load resistors to VDD and a tail Current source: inputs “inp” and “inn” (sources), outputs “outp” and “outn”. The test bench runs a noise analysis on the differential output, referred to “inp”.
Both input transistors add independent thermal noise, so the input-referred noise power of a pair is twice that of one transistor: 8kT·γ/gm. Each load resistor adds its own noise, divided by the gain. Doubling the tail current makes the pair about 1.4 times quieter.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor from each output and applies the AC stimulus to “inp”; it runs a noise analysis from 10 Hz to 100 MHz on the differential output “outp”−“outn”.
Numbers to hit
- Differential gain at least 12 dB
- Input-referred noise, 10 Hz to 1 MHz at most 16 µV
- Power at most 250 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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