Nowadays, detection of trace concentration gases is still challenging for portable sensors, especially for the low-cost and easily operated metal-oxide-semiconductor (MOX) gas sensors. In this paper, a widely applicable amplification circuit is designed and fabricated to evidently enhance the signal of the MOX sensors by adding a field effect transistor (FET) into the conventional circuits. By optimizing the FET parameters and the loading resistance, this amplification circuit enables the commercial Figaro TGS2602 toluene sensors response effectively to the highest permissive limit (0.26 ppm) of toluene in indoor air of cars, with the detection limit of similar to 0.1 ppm. Furthermore, this circuit can also make the commercial Hanwei MP502 acetone sensors and MQ3 ethanol sensors response to the 1-2-ppm acetone in breath of diabetes and 2-ppm ethanol for fast and effectively drinker driver screening. The mechanism is investigated to be the gate voltage induced resistance change of the FET, with the highest theoretically estimated and experimentally measured magnification factor of 5-6. This FET amplifier can effectively enable the ppm level commercial MOX sensors response to sub-ppm level gases, promising for MOX gas sensor integration and also for other kind of resistive sensors.