# Biasing with Negative value of Vgs in NE76038 GaAs MESFET

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

• 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! Mar 10, 2017 at 10:13

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 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.

• what about the value of RG? Mar 13, 2017 at 8:18
• 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). Mar 13, 2017 at 8:43