Define input-referred noise and how you would minimize it in an amplifier design.

• A. Noise referenced to input; minimize by proper layout, shielding, filtering, short paths, proper op-amp choice, impedance matching, two-stage design.
• B. Noise measured at output only; minimize by increasing supply voltage.
• C. Noise is inherent to any amplifier; cannot be minimized.
• D. Input-referred noise is identical to thermal noise; minimize by using larger transistors only.

Answer: (B)

Input-referred noise is the equivalent input voltage (or current) noise that would have to be present at the amplifier’s input to produce the observed noise at the output. It’s a convenient way to compare amplifiers because it normalizes noise to the input, independent of gain and bandwidth.

To minimize it in an amplifier design, focus on the front end. Use a low-noise input device (or op-amp) and be mindful of source impedance because the input current noise interacts with the source resistance. Keep the signal path physically short, well shielded, and properly grounded to reduce pickup and EMI. Limit the bandwidth to what you actually need, since more bandwidth allows more integrated noise. Consider proper impedance matching so the input sees an optimal load, and a two-stage design where the first stage provides the necessary gain with the lowest possible noise, while the following stages handle the remaining amplification. In essence, reduce noise at the input and at the front end, and contain bandwidth; increasing supply voltage is not a reliable way to lower input-referred noise.