The paper reports a tunable interface electronics for High Electron Mobility Transistors (HEMTs) based Field Effect Transistor (FET) sensor. The approach presented in the work can estimate both the ‘on resistance’ and drain current of the FET sensor while maintaining a constant drain-source voltage (VDS). The interface electronics consist of a full Wilson current mirror with a floating voltage-controlled resistance which is controlled by the output of an integrator forming a closed-loop feedback control. The bias point for the sensor is set by the external voltage, Vbias and the difference between the sensor drain-source voltage, VDS and Vbias is fed to the integrator as an error signal. The output of the integrator, Vc varies the tunable resistance which in turn varies the current in the current mirror branch. This current is mirrored to the sensor branch and continues to vary until the drain-source voltage, VDS matches the applied external bias voltage, Vbias. At this point, the circuit is at the balanced state and the corresponding control voltage, Vc represents the sensor signal. The circuit is evaluated with SPICE simulation and experimentally verified using a prototype PCB. Experimental results revealed that the circuit can work for a range of 200Ω to 2000Ω with an absolute relative error of less than ±1%. The circuit can also be tuned for the desired working range and allows the user to apply external bias potential based on the sensor’s requirement. The proposed circuit has great potential as a bio-chemical interface circuit for FET based sensors.
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