Nfet calculator

Home

With this calculator you can obtain the values for the Source and Drain resistors to bias a nfet to work as a common-source amplifier .
You need 3 parameters:

• the supply-voltage (Vss)
• the fet's pinch-off voltage (Vp)
• the fet's maximum current (Idss)

On runoffgroove/fetzervalve you can find an easy way to measure those parameters off the fet.

Common-Source amplifier

Fill in Vss (Volt):                Fill in Vp (Volt):                Fill in Idss (mAmp):       

Show list                 Show graph

As an example the values off a J201 are filled in.
If you take Vgs = -0.32 volt and want 1/2 Vss on the Drain you get Rs=1k6 en Rd=21k.

table

As you can see, you get a list of 10 values for the Gate-Source voltage Vgs.
For each Vgs the Id (current through the fet) is calculated, and the appropriate Source-resistor Rs.
For each Vgs you also find two Drain-resistors Rd.
The first Rd biases the fet's Drain halfway the supply-voltage. (eg Vss=9v => Vdrain = 4,5v)
The second Rd biases the fet's Drain 2/3 the supply-voltage. (eg Vss=9v => Vdrain = 6v)
For the Rg resistance you can use 1 megaOhm (a typical value). If you use a capacitator Cs, you boost up the gain.
The graph shows the 10 Vgs values in relation to the matching Id.