FET Practical DC Analysis Problems
If you have a one or a few JFET's in your disposal you can try to measure them.  

You can use this setup:  



AS you can see you can use a voltmeter to measure Vgs(off) and ammeter to measure Idss at once, all you need is to switch your multimeter between voltmeter/ammeter.   

And you know Vgs(off) and Idss you can solve for Vgs: 

$$V_{GS} = Vgs(off)* \left(1 - \sqrt{\frac{I_D}{I_{DSS}}} \right)$$ 

I have two JFET in my workbench
First one is BF245A and I measure it and got

Vgs(off) = - 1.9V and Idss = 5mA

And BF245C:

Vgs(off)= -4.8V and Idss = 15mA 

(the typical values given in datasheet are -5.6V/17mA).

So, If I want Id = 5mA I would use BF245C. 

And Rd = 10V/5mA = 2.2kΩ 

$$g_{m0} = \frac{2*I_{DSS}}{|V_{GS}(off)|} = \frac{30mA}{4.8V} = 6.25mS$$

Therefore the maximum gain passible for a given drain resistor is
\$R_D = 2.2kΩ\$ is:

\$Av = g_{m0}R_D = 13.75 \; V/V\$ (without Rs)

For automatic bias you now need to select \$R_S\$ resistor

$$V_{GS} = Vgs(off)* \left(1 - \sqrt{\frac{I_D}{I_{DSS}}} \right) = -4.8V*\left(1 - \sqrt{\frac{5mA}{15mA}} \right) = -2V $$ 

$$R_S  = \frac{2V}{5mA} = 400Ω = 390Ω$$ 







simulate this circuit &ndash; Schematic created using CircuitLab

And now we can test the circuit on the workbench.