Raising the value of Rs serves to increase negative biasing the gate, which raises the value of Vds. Lowering it brings Vg closer to 0V, but increases Id, and increases the voltage drop across Rd. Opening Rs pulls Vg to ~-5V, and the transistor is in cutoff. In this case I do get 15V at Vd since there is virtually no current through the transistor (spice simulation shows 177pA) but then Vds becomes ~10V. -- Sincerely, James Burkart *Filmmaker & Documentarian* *Burkart Studios* 925.667.7175 | Personal 415.738.2071 | Office *Web:* burkartstudios.com *Facebook:* facebook.com/burkartstudios On Mon, Sep 11, 2017 at 1:10 PM, Dwayne Reid wrote= : > Hi there, Sean. > > You are forgetting about current that flows through the gate resistor > to Ground. This is a J-FET, so the gate diode junction is forward > biased if Rs is open. > > Another possibility that Rs is very high value. I haven't bothered > to work out the math for the possible high value. > > dwayne > > > At 11:58 AM 9/11/2017, Sean Breheny wrote: > >I might be missing something but I don't think this problem is well > >designed. I think we can narrow it down to Rs being open but technically > it > >isn't possible, as far as I can see, to get exactly the voltages > mentioned. > >Vd=3D15V implies that absolutely no current is flowing from the supply, = so > >there shouldn't be ANY voltage drop across the drain-source connection. > > > >Here's my train of thought: > > > >If Rd were open, Vd would be zero. > >If Rd were shorted, the current would be limited by the negative Vgs whi= ch > >would develop (I am defining negative Vgs as Vs>Vg, which reverse biases > >the gate junction for an N JFET), so there would be some Vds drop of > >several volts. > > > >If Rg were open, I think the FET behavior would tend toward the Vgs=3D0 > >behavior where almost the maximum current flows, but then Vd would be le= ss > >than 15V. > > > >If Rg were shorted, the circuit would behave almost as if nothing were > >wrong, unless 15V was enough to cause reverse breakdown of the gate > >junction, but in that case Vd would be less than 15V since an excessive > >leakage current would flow. > > > >If Rs were shorted, Vs would be zero. > >If Rs is open, then we would expect Vds=3D0 (Vd=3DVs=3D15). This is the = closest > >to what we observe. In the real world, the 10Meg input impedance of a > >typical multimeter would probably produce something similar to what is > >claimed here (it would act as a very high value of Vs) but Vd would not = be > >identically 15V. > > > > > >On Mon, Sep 11, 2017 at 12:57 PM, Dwayne Reid > >wrote: > > > > > Hi there. > > > > > > Consider what happens if Rs goes open. Also consider that the gate > > > junction becomes forward-biased when Vgs goes negative. > > > > > > dwayne > > > > > > > > > At 06:40 AM 9/11/2017, James Burkart wrote: > > > >Hello all! > > > > > > > >I am working on a lab for a class and we are given a simple > self-biased > > > >N-channel JFET circuit: Rd =3D 4.7k, Rs =3D 1k, Rg =3D 2M, Q1 =3D 2N= 4860, Vdd > =3D > > > >+15V. > > > > > > > >Then we are given the scenario: Vds =3D 66.5mV, Vd =3D 15V, Vs =3D 1= 4.9V. > > > > > > > >Assuming only one component can be bad, what is the likely problem? > > > > > > > >I built the circuit in MPLab and tried everything I could think of t= o > > > >recreate the voltages, but everything I tried does not work. Shortin= g > Rd > > > >was my first thought, but that also results in Vds being 12.6V. What > am I > > > >missing? Can anyone get me on the right track? I have shorted and > opened > > > >all resistors, the only thing I can't do (or don't know how to do) i= s > > > >recreate a faulty FET. > > > > > > > >-- > > > >Sincerely, > > > > > > > >James Burkart > > > >*Filmmaker & Documentarian* > > > > > > > >*Burkart Studios* > > > >925.667.7175 | Personal > > > >415.738.2071 | Office > > > > > > > >*Web:* burkartstudios.com > > > >*Facebook:* facebook.com/burkartstudios > > > -- > Dwayne Reid > Trinity Electronics Systems Ltd Edmonton, AB, CANADA > 780-489-3199 voice 780-487-6397 fax 888-489-3199 Toll Free > www.trinity-electronics.com > Custom Electronics Design and Manufacturing > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .