This paper presents a small-area, ultra low-power, low-mismatch differencing amplifier for use in transient pixels of micro-bolometer based temporal contrast IR sensors. The two-stage capacitive-feedback amplifier works in the sub-threshold domain, has a voltage gain of 46 dB, a 3 dB bandwidth of about 10 kHz and consumes 85 nW of static power. The amplifier circuit has been fabricated in a 0.35 mum standard CMOS process and consumes less than 2000mum of silicon area, enabling a square pixel size of 50 mum x 50 mum.