This is the value of Idss (drain current with Vgs = 0) and value of Vp (pinch off voltage) :

value of Vp and Idss

I want to bias the transistor using common source with Ids (drain current) = 10 mA and Vds = 3 V.By using this formula we can calculate the value of Vgs.

formula to calculate Vgs

By using Idss = 30 mA and Vp = -0.8 V from the datasheet, we obtain the value of Vgs = -0.338 V. How can I design the bias circuit with the negative value of Vgs?

  • \$\begingroup\$ If your circuit must ensure that your bias is the same 10mA across dispersion of Idss characteristics, you'll need something smarter than setting Vgs to a constant voltage! \$\endgroup\$
    – bobflux
    Mar 10, 2017 at 10:13

1 Answer 1


Indeed Vgs needs to be negative.

If you insist on making Vs (potential at the source) zero (connect it to ground) then you'd have to use a negative voltage to bias the gate. This can be cumbersome if you have no negative supply rail.

But that's not needed !

What if you'd bias the gate to 0 V (GND) and "lift" the source instead. So bias the source at 0.338 V.

That also gives Vgs = -0.338 V. That is easy as you want 10mA flowing so 0.338 V / 10 mA gives .....ohms (you do the calculation).

For the signal it is still a common source due to the capacitor in parallel with Rs

enter image description here

You have to apply your signal around ground level so if that is not the case (it has a DC level larger than zero) use an AC coupling capacitor to apply the signal to vG. Unfortunately the circuit does not show this.

  • \$\begingroup\$ what about the value of RG? \$\endgroup\$
    – uulwake
    Mar 13, 2017 at 8:18
  • 1
    \$\begingroup\$ What about the value of Rg ? Think about how much current is flowing through Rg, assuming the signal to vG is coupled in via a capacitor (the AC coupling capacitor). \$\endgroup\$ Mar 13, 2017 at 8:43

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