Isaac Posted on February 4, 2021
This is to open the op amp to make it in the working range.
Lucas Posted on February 4, 2021
The op amp is not rail-to-rail, even if it is rail-to-rail, the input and output voltage ranges will not be completely equal to the power supply voltage of the op amp;
So string 3 diodes to make it work within the normal working range of the op amp;
Jackson Posted on February 4, 2021
It seems that these tubes only serve to make the second operational amplifier avoid the dead zone voltage between the power supply and gnd. It is not good for in4148 to work at dozens of ma for a long time.