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This is an old schematic, and the MPF102 JFET in the picture is confusing me.

Can someone help me identify the Gate, Source, & Drain connections to this symbol, please!

My guess is that the bottom right pin is gate, but it is not intuitive for me.

Here is the old symbol, from the schematic:

enter image description here

Since the MPF102 is no longer available, I chose to replace it with a J113, and here is how I typically see it drawn:

enter image description here

Would someone be willing to map the equivilent pins of both of these two symbols so I can see whats going on.

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  • \$\begingroup\$ What difference do you expect between drain and source? And yes, the gate is the terminal that is 'alone'. \$\endgroup\$
    – Sredni Vashtar
    Mar 25, 2018 at 18:17
  • \$\begingroup\$ In many cases with today's JFET's the drain and source are interchangeable. \$\endgroup\$
    – user105652
    Mar 25, 2018 at 19:59

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I've never seen a JFET be symbolized as such, honestly. But I did look on Google for images of this MPF102 JFET and this is what I found. enter image description here

As you can see, the symbol matches the second JFET symbol for the J113 JFET. I believe the confusion that you're having is that these transistors will look differently on a schematic, which is not true. However, I believe that the symbol that you're seeing with the MPF102 is different because it's either an older schematic or that's how the software defines how JFETs look.

In terms of particular JFET devices, they will all look the same on a schematic diagram with the exception of the component name on the schematic (assuming that the doped channel is the same, N-Channel, P-Channel, etc.). The pinout of the transistor is going to be the same as well. In other words, with respect to schematic diagrams, the MPF102 and J113 will look the same and will have the same pinouts.

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  • \$\begingroup\$ Thank you Duken for your comment. I agree 100% with everything you have presented. Do you have a suggestion for how to connect this JFET given the information provided? This schematic is from 1998, while its not quite an antique, my guess is that it was drawn in the wild west days of computer aided circuit design. \$\endgroup\$
    – user4467707
    Mar 25, 2018 at 18:49
  • \$\begingroup\$ I would presume that the diagram for the MPF102 that you have shown is relatively the same. The bottom left side being the gate, the bottom right (with the arrow pointing out) being the source, and the top right being the drain. It would be interesting to see the rest of the schematic to really confirm my suspicion. \$\endgroup\$
    – user103380
    Mar 25, 2018 at 20:46
  • \$\begingroup\$ Duken, The full schematic may be seen at this address: raptorzone.co.za/images/noise_canceller_schematic_diagram.gif AND thank you again for the help, I now feel that the gate is bottom left as well. \$\endgroup\$
    – user4467707
    Mar 25, 2018 at 21:20
  • \$\begingroup\$ My suspicion is correct :) looking at the surrounding BJT devices, I can now assume that the orientation of this JFET is similar to the BJTs in the circuit. (be sure to click on that check mark and give a +1 if you find people's answers as helpful to what you're looking for) \$\endgroup\$
    – user103380
    Mar 25, 2018 at 21:29

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