Most biopotential readout front-ends rely on the g-C lowpass filter (LPF) for forefront signal conditioning. A small g realizes a large time constant (τ = C/g) suitable for ultra-low-cutoff filtering, saving both power and area. Yet, the noise and linearity can be compromised, given that each g cell can involve one or several noisy and nonlinear V-I conversions originated from the active devices. This paper proposes the subthreshold-source-follower (SSF) Biquad as a prospective alternative. It features: 1) a very small number of active devices reducing the noise and nonlinearity footsteps; 2) No explicit feedback in differential implementation, and 3) extension of filter order by cascading. This paper presents an in-depth treatment of SSF Biquad in the nW-power regime, analyzing its power and area tradeoffs with gain, linearity and noise. A gain-compensation (GC) scheme addressing the gain-loss problem of NMOS-based SSF Biquad due to the body effect is also proposed. Two 100-Hz 4th-order Butterworth LPFs using the SSF Biquads with and without GC were fabricated in 0.35-μm CMOS. Measurement results show that the non-GC (GC) LPF can achieve a DC gain of -3.7 ~dB (0 dB), an input-referred noise of 36 μV (29 μV ), a HD3@60 Hz of -55.2 ~dB (- 60.7 ~dB) and a die size of 0.11 mm (0.08 mm). Both LPFs draw 15 nW at 3 V. The achieved figure-of-merits (FoMs) are favorably comparable with the state-of-the-art.