Hi, I'm building a STM, and this is the circuit I use : http://www.geocities.com/spm_stm/Simple_STM_Rev_C.PDF I'm right now testing the circuit I build. I build a small part (U3A), and then test it, and build another small part (U3B) and test, then connect the two and test it again. This is the way I build it and test it. However, I found that I can't get the gain to be 1. For U3A and U3B, I get around 0.08/2.25 gain for each part. While I conncet U3A and U3B, the gain I get is even smaller. I tried to change R2 to be bigger (it original is R2=10k, R1=10k), like 20k, 100k, 10M, the gain becomes a bit bigger but is still much smaller than 1. I checked all the frequencies I could get from 50 to 5M, and can never get gain = 1. I connect the ground to the ground of oscilloscope and signal generator. Apply voltage to 4 and 11 by power supply. I didn't have any experience about this, and don't know what's wrong with this. I would like to know what would be the possible way to solve this. Thanks for your help. I appreciate it.