First amplify, then filter, because the signal is small and difficult to filter, and the amplifier itself has noise, so post-filtering can filter out the noise of the amplifier.

Active amplifier circuit is much better, so differential amplifier is used to suppress * * * mode interference. In fact, while magnifying, it is also filtering. We can refer to some circuits and find that the active filtering of the circuit also has amplification function.

You can choose to use the chip op07, which is cheap and has low noise.