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?
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\$\begingroup\$ I thought this was a duplicate (electronics.stackexchange.com/questions/16966/…) But now I see that question asked about the BJT, while your question asks about the JFET. \$\endgroup\$– The PhotonCommented Mar 27, 2013 at 20:53
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2 Answers
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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.
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\$\begingroup\$ any recommendations on a SMD or through hole JFET that would work? \$\endgroup\$– DavidCommented Mar 27, 2013 at 23:02
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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.
answered Mar 27, 2013 at 21:55