An　integrated　lock-in　amplifier　(LIA)　with　automatic　phase　tuning　and　second-order　harmonic　frequency　extraction　is　presented　in　this　paper.　The　automatic　phase　alignment　loop　implemented　in　the　proposed　single-channel　LIA　can　directly　align　the　phases　of　an　input　signal　with　a　reference　signal.　Furthermore,　the　proposed　LIA　system　also　includes　an　enhanced　signal　detection　method　in　which　the　2(nd)　harmonics　of　the　carrier　frequency　(omega(ref))　is　applied.　As　opposed　to　conventional　LIA,　this　method　reduces　flicker　noise　and　input　offset　effects　that　limit　the　signal-to-noise　ratio　(SNR).　In　terms　of　signal　power　detection,　the　proposed　single-channel　LIA　architecture　does　not　require　phase　synchronization　between　input　and　reference　signals.　The　circuit　is　designed　for　a　reference　frequency　of　100　Hz,　suitable　for　biomedical　applications.　The　LIA　is　Implemented　using　0.18-mu　m　CMOS　technology　with　a　single　power　supply　voltage　of　1.8　V.　The　system　consumes　360　mu　W　at　an　operating　frequency　of　100　Hz　and　presents　a　high　input　sensitivity　dynamic　range　for　a　detection　bandwidth　of　50　Hz　and　a　FOM　of　116　dB.