I have been trying to design an opamp to obtain alpha and beta waves from brain.
So far, I've designed multistage differential amplifier that provides x100 gain approximately. Inputs are 50uV and 70uV at 30 Hz, the output is about 2mV.
The problem is related to gain stage. Whenever I try to amplify the signal coming from the output, all gain gones.
What is the root of this problem ?
Schematic : https://i.hizliresim.com/rJANdP.jpg