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# HN Question (is this possible?)

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#### dave12

##### New Member
Ok guys can anyone help me answer this question? I'm sure its been typed wrong but Ill see what you think!

Sketch a typical FET circuit operating from a 20V supply, it is required that VDS(Q)=12V. Given that IDSS=6mA and Vp= -4V, determine values for Rs, Rg and Rd = 5K6.

Im not asking for the complete answer, but just a point in the right direction, and for someone to say it can be done. To me the resistive value seems high, but I don't know how to interpret this.

fet circuit

There sure is something wrong. If and external bias of -4V is supplied to
the gate, the transistor would be in saturation. The drain current would be about 3.5 Ma. 20V/5.6K. If a typical value of 1M is put from gate to ground and a resistor is put from source to ground and calculated to provide 4V drop @ 6Ma(4V/6Ma=666 ohms) This provides the gate with an effective bias of -4V. If the source is positive with respect to ground and a resistor is between the gate and ground the gate is - with respect to the source. Vds=12V, means then that Vrd=4V. The drain resistor calculates to same value as the resistor from source to ground.

Thank k7elp60, your help is much appreciated. But I'm still just as confused :?

I just phoned my tutor and asked if there was something wrong with the question and he said No. He says it just takes a little playing around with data sheets. WHAT DATASHEETS?!!

Im not sure what to do now

Thanks for your help anyway.

fet circuit

what your tutor meant was for you to read & understand the datasheet of the fet transistor you are using so you may optimize the function of that particular fet transistor or whatever transistor you may want to use whether unipolar or bipolar. :wink:

Is this a JFET? Do you have a part number? What is Vp?

Russlk said:
Is this a JFET? Do you have a part number? What is Vp?

Vp = Pinchoff Voltage, also known as Vgs(off) -- the gate-to-source voltage at which the drain current is reduced to some small specified level.

Is that VDS(Q) = 12V, or VD(Q) = 12V?

Thanks for all your replys, Russlk I can only assume its a FET and not a JFET because of what the question says. I don't have any data sheets for the FET, so I don't know what type it could be.
mozikluv, do you think I can choose any FET data sheet? What would you recommend?

:shock:
The problem is simple
You must have Vds=12V, result:
20V=Vrd+Vds+Vs=IdRd+Vds+IdRs
Because the gate is grounded by Rg resistance => Vg=0V, so
Vgs=Vg-Vs=0V-Vs=-IdRs
Because the Vgd=Vg-Vd is under Vp the FET is in saturation and the drain current is near independent by Vds, and depend only by Vgs, so using equation Id=f(Vgs) for saturation zone you must obtain the value of Id which allow Vds=12V.
Is this case is not important if FET is J or MOS, and you not need the FET type because the most relevant parameters for your problem is Idss and Vp and you have them.
The equation is Id=K(Vgs-Vp)(Vgs-Vp) in other words (Vgs-Vp) at square.
The K parameter is K=Idss/(Vp*Vp) in other words Vp at square.
This equation is for saturation zone (your case).[/quote][/list]

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