Three novel differential amplifier topologies using double gate a-IGZO TFTs on flexible substrate are presented in this paper. The designs exploit positive feedback and a load with self-biased top gate to achieve the highest static gain in single stage a-IGZO amplifiers reported to date. After fabrication, the three amplifiers exhibit respectively a static gain of 14 dB, 21.5 dB, and 30 dB, with a bandwidth of 2 kHz, 400 Hz, and 150 Hz. Also, for each circuit the input referred noise has been measured to be 420 mu V-rms, 195 mu V-rms, and 146 mu V-rms, respectively. Based on these results, the a-IGZO amplifier providing the highest gain is suitable as front-end for heart rate measurements and, with some further optimization verified in simulation, can also be used for other bio-potential applications, like electro hysterogram and electro cardiogram.