Low-Cutoff Frequency Reduction in Neural Amplifiers: Analysis and Implementation in CMOS 65 nm

Area of Science:

• Electrical Engineering
• Biomedical Engineering
• Microelectronics

Background:

• Scaling down CMOS technology degrades neural amplifier performance, increasing low-cutoff frequency due to short-channel effects and reduced input resistance.
• Increasing feedback capacitors to lower cutoff frequency is undesirable as it reduces input impedance and increases area.
• Reduced operational transconductance amplifier (OTA) input resistance, caused by gate oxide leakage, is identified as the primary cause of increased low-cutoff frequency.

Purpose of the Study:

• To analytically investigate the small-signal behavior of AC-coupled neural amplifiers and identify the root cause of increased low-cutoff frequency in advanced CMOS nodes.
• To propose and evaluate two novel solutions for reducing the low-cutoff frequency without increasing feedback capacitor values.
• To demonstrate improved neural amplifier performance for in vitro brain recordings.

Main Methods:

• Analytical analysis of neural amplifier small-signal behavior to understand the impact of CMOS scaling on input resistance.
• Simulation of a cross-coupled positive feedback technique using pseudoresistors to increase OTA input resistance.
• Design and fabrication of a neural amplifier utilizing thick-oxide MOS transistors in the input differential pair of the OTA within a 65 nm TSMC CMOS process.

Main Results:

• The cross-coupled positive feedback method achieved a simulated low-cutoff frequency of 1.5 Hz with a midband gain of 30.4 dB and power consumption of 0.6 μW.
• The fabricated neural amplifier using thick-oxide transistors demonstrated a bandwidth of 2 Hz to 5.6 kHz, a midband gain of 34.3 dB, and consumed 3.63 μW at 1 V.
• The fabricated amplifier achieved an input-referred noise of 6.1 μV and occupied a silicon area of 0.04 mm².

Conclusions:

• The study successfully identified reduced OTA input resistance due to gate oxide leakage as the cause of increased low-cutoff frequency in scaled CMOS.
• Two effective methods, cross-coupled positive feedback and the use of thick-oxide transistors, were proposed and validated to reduce low-cutoff frequency without compromising amplifier performance.
• The developed neural amplifiers show significant potential for high-fidelity in vitro neural recordings with low power consumption and small area.