Low-Voltage Mixed-Mode First-Order Universal Filter Using Multiple-Input Operational Transconductance Amplifier

This paper presents an electronically tunable first-order universal filter capable of operating in multiple analog modes, realized through a compact architecture built around multiple-input operational transconductance amplifiers (MI-OTAs). By leveraging the MI-OTA’s ability to accommodate several input signals within a single transconductance stage—allowing direct arithmetic operations such as summation and subtraction—the proposed design minimizes the number of active elements traditionally required for mixed-mode filtering. Consequently, both inverting and non-inverting forms of low-pass, high-pass, and all-pass responses can be generated in voltage mode, current mode, transadmittance mode, and transimpedance mode, enabling a total of 24 distinct first-order transfer functions using one unified circuit topology. The pole for all responses can be conveniently adjusted by electronically tuning the OTA transconductance. The multiple-input capability is realized using a multi-input MOS technique, while subthreshold-biased bulk-driven transistors allow the circuit to function from a 0.5 V supply with an extended input voltage range and ensure ultra-low power dissipation. The filter was designed and evaluated in Cadence Virtuoso using a 65 nm TSMC CMOS process. Under a 7 nA bias current, the low-pass configuration achieves a consumption of 87.5 nW and a dynamic range of 44.7 dB. Additionally, experimental verification was performed using the commercial LM13700 OTA, confirming the correct operation and practicality of the proposed approach.