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Considering the output characteristic of the n channel JFET, can we somehow obtain the same characteristic only for the p-channel type of this transistor? N channel JFET transfer characteristic

In R.Jaeger's book about microelectronics circuit design, only a vague description was given on how only the polarities are changed, and everything else is left untouched. I suppose that means, that we will either be graphing the same curve in the first quadrant, or in the third also, only using the Vsg voltage.

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  • \$\begingroup\$ I've never heard of a p channel jfet having a matching n channel equivalent. Not saying there isn't so maybe you could give an example? \$\endgroup\$
    – Andy aka
    Commented Jun 12, 2015 at 14:46

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Considering the output characteristic of the n channel JFET, can we somehow obtain the same characteristic only for the p-channel type of this transistor?

It's not practical because the manufacturing tolerances for any specific N channel JFET are so wide to make it a worthless exercise. See below from the 2N5486 data sheet: -

enter image description here

The above describes the channel resistance against temperature WHEN the voltage across gate and source is zero.

If you picked out one device that had Vgs(off) = -1V then you'd use the top curve but who knows what device you have grabbed from the handy bag of JFETs. It could easily be a device whose Vgs(off) was -8V

Because of this, Fairchild (and other suppliers) have basically created three part names for the same device because the tolerances are so wide. If you read the data sheet there are: -

  • 2N5484 having a Vgs(off) between -0.3 and 3.0 volts
  • 2N5485 having a Vgs(off) between -0.5 and 4.0 volts
  • 2N5486 having a Vgs(off) between -2.0 and 6.0 volts

Irrespective of all that, they are still anticipating that some may have a characteristic that is -8.0 volts! It's not unknown on Fairchild data sheets to have made a typo and actually meant -6.0V.

So, trying to characterize a P ch JFET from an N ch device is fruitless

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  • \$\begingroup\$ If that is so, what DOES the p-ch JFET characteristic look like? \$\endgroup\$
    – Shemafied
    Commented Jun 12, 2015 at 15:26
  • \$\begingroup\$ It's a mirror image about the vertical axis i.e. the drain current is controlled by a positive gate-source voltage. The higher the voltage the lower the current. A P ch device and an N channel device are the same when Vgs is zero. \$\endgroup\$
    – Andy aka
    Commented Jun 12, 2015 at 15:30
  • \$\begingroup\$ These transfer characteristics can be really helpful when doing circuit approximations, so that's why it comes in handy to have them nerby. \$\endgroup\$
    – Shemafied
    Commented Jun 12, 2015 at 17:39
  • \$\begingroup\$ The problem is that reality for jfets is a wet finger in the air. When it comes down to predictability jfets are awful compared to the humble bjt as I was trying to point out. \$\endgroup\$
    – Andy aka
    Commented Jun 12, 2015 at 17:45

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