This paper presents a 1.2 V low-power rail-to-rail class AB operational amplifier (OpAmp) suitable for integrated lock-in amplifiers. The proposed OpAmp has been designed in a standard 0.18 μm CMOS technology. For a 1.2 V single supply and 81 μW power consumption, simulations shows a 86 dB open loop gain, 56 phase margin, 17 MHz unity gain frequency with 1 pF load and 97 dB CMRR. Adaptive biasing provides 33 V/μs slew-rate for a 1 pF capacitive load. A compact and reliable lock-in amplifier (LIA) has been designed using the proposed circuit. Post-layout results for noisy signals confirm the capability of the LIA to effectively recover information from signal-to-noise ratios down to -26 dB with errors below 4.2% up to 50 kHz and a power consumption of only 675 μW.