An orthogonal current-reuse amplifier for multi-channel sensing

We demonstrate a micropower low-noise CMOS amplifier array that reuses bias current to improve the fundamental noise-power tradeoffs of fully-differential amplifier designs. The presented circuit implements current-reuse by stacking the differential input pairs of four amplifiers. The output drain currents of each channel's differential pair are used as the tail currents for the differential pairs of the succeeding channel. Orthogonal current-reuse improves the noise and power tradeoff by sharing bias devices to conserve headroom. With four channels (n = 4), there are 16 unique output currents (2ⁿ) from the stack, each of which is a linear combination of the four inputs. Amplified versions of the original input signals are reconstructed by appropriately combining the small-signal output currents in output stages that operate at much lower bias currents. With an input-referred noise of 3.7 μV_rms and a bandwidth of 19.9 kHz, a single channel achieves a noise efficiency factor (NEF) of 3.0. Amortizing the bias current across the amplifier's four channels yields an effective NEF of 1.64. The total power consumption is 15.6 μW, or 3.9 μW per path from a 1.5 V supply. Orthogonal biasing suppresses crosstalk between the channels, providing an isolation of 40 dB under 3-σ mismatch. The implemented circuit was fabricated in a standard 130 nm CMOS process and occupies an area of 0.125 mm². The proposed technique is useful for a variety of applications ranging from low-power neural recording arrays to multiphase radio baseband amplifiers.