Power-noise trade-off in signal amplifiers

Energy-constrained measurement systems must simultaneously offer good measurement performance and reduced power dissipation. However, noise in op-amp-based signal amplifiers is usually reduced at the cost of increased supply currents, hence power dissipation. Conversely, low-power op amps are noisier than low-noise op amps. We propose the quotient between the signal-to-noise ratio and the dissipated power as a new figure of merit to evaluate that trade-off in signal amplifiers, and name that quotient the signal-to-noise-to-power ratio (SNPR). We have optimized SNPR for op-amp-based voltage and transresistance amplifiers. If the power dissipated by the feedback current is larger than the op amp quiescent power, the equivalent resistance R_p seen from the inverting terminal of the op amp to ground must equal its optimal noise resistance R_op (input voltage noise (e_n) divided by input current noise (i_n)). Furthermore, a small e_ni_n product reduces SNPR sensitivity to resistor values that deviate from the optimal value. If the op amp quiescent power dominates over power dissipated by the feedback current, SNPR can be optimized from its graphical representation. The larger is the quiescent power, the smaller is SNPR and the less sensitive is it to R_p.