Tunable kΩ to GΩ pseudo-resistor with bootstrapping technique

This paper presents a novel architecture for a kilo-ohm to giga-ohm pseudo-resistor (PR), based on transistors operating in subthreshold with a fixed-Vgs configuration. This PR when used in an RC filter has a very low and constant settling time regardless of the programmed pole frequency. The proposed PR takes advantage of bootstrapping to improve its tuning range. By defining a constant VGs for the transistors using a pre-charged capacitor, voltages from GND to VDD can be used and the transistor can be tuned to work from the cut-off region (for high resistances in the order of giga-ohms) to the deep triode region (for low resistances in the order of kilo-ohms). The disclosed PR can be used in the DC feedback loop of a capacitively coupled amplifier, enabling a bandwidth tuning range of 9 decades (mHz to MHz). Moreover, by using the pre-charge period to connect the gate of the PR transistor to ground, the amplifier settles in less than 1 μs, conforming with applications where electrodes/sensors with different nominal impedances are multiplexed to the amplifier input. The amplifier and the PR were validated in a 0.35 μm CMOS process, with post-layout simulations.

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