A low power, charge-sensitive preamplifier integrated with a silicon nanowire biosensor

Charge amplifiers, characterized by their charge-sensitivity or charge gain, are essential components of a transducer-interfacing system that amplify charge signals emerging from various sensors and convert them into voltage signals. Today, with increased scaling of MOSFETs, it becomes challenging to obtain the charge amplifiers with high charge sensitivity and low power. Furthermore, if the charge amplifier is to be operated at lower bandwidth, then obtaining low noise at higher charge sensitivity is also difficult. In this paper, the authors propose a novel design of a charge-sensitive preamplifier in 90 nm CMOS technology that can be operated at the frequency range of 10 Hz-10 kHz, suitable for biosignals at lower frequencies. The opamp in the preamplifier is designed to have a folded-cascode structure with composite cascoding at its input transistors for low power operation. A feedback resistance of 185.20 GΩ for the preamplifier is actively realized using long-channel cascode MOSFET stage in the opamp, thereby eliminating the need for a large value of passive resistance on-chip. The preamplifier has a high charge sensitivity of 8.875 mV/fC at a low power consumption of 214.32 nW with input-referred noise of 256.89 μV for the bandwidth 10 Hz-10 kHz. The preamplifier operation is verified by interfacing the model of the preamplifier with the small-signal equivalent of a SPICE model of a Silicon Nanowire Field-effect Transistor (SiNW-FET) based biosensor which was proposed for impedimetric sensing of biomolecules.