Input stage zener diode ?

[thylantyr]

Differential input stage;

What is the purpose of D1, a 15V zener diode ?
The schematics I've seen for this type of
input stage don't have this.

Speculation?

 

[megajocke]

It is used together with the cascode transistors to let the input stage work at a lower voltage. Without it there would be about 90V over the input transistors. Now most of the drop is over the cascodes. Maybe you already knew that bit 🙂

I guess the purpose of this is so that you can use low-noise transistors, and it also helps the PSRR as the voltage over the input transistors won't vary much with the supply. There are probably more advantages too.

 

[thylantyr]

Thanks for the info.