I was looking at figure 33 of the OP77 datasheet, and the JFET labeled D2 has an odd connection. Based on the way the circuit works that JFET must be a diode right? So why is the JFET connected as it is in the figure?
The JFET is acting as a diode. So why not just use a diode? The main reason for using a JFET as a diode is that it has low leakage compared to ordinary diodes.
It is acting like a diode. Because the circuit in question is an absolute value amplifier it has to handle positive signal excursions differently to how it handles negative signals because basically it's a precision full-wave rectifier.