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# n-channel FET to operate as an amplifier

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#### vjee

##### Newbie level 2
Can someone tell me if my choice is correct? I did my reading and from what I understand this is the answer- 1.Gate voltage positive with respect to source.
The question says- One condition required for an n-channel FET to operate as an amplifier is
1. gate voltage positive with respect to source
2. drain voltage negative with respect to gate
3. gate voltage negative with respect to source
4. drain voltage positive with respect to gate.

correct!

Vds must be greater than Vgs-Vth to insure the transistor working at saturation region, but not necessarily greater than Vgs. Thus, only 1 is correct.

Assuming your using a depletion mode transistor which is typically the case. Essentially, the simple nFET model consists of three modes: i) cutoff ii) linear and iii) saturation. In the cutoff your off and in saturation your on => its a switch (typically what they are used for). If you need it as an amplifier you want the linear region.

1.) Vgs>0 but nothing on Vt so it is insufficient and nothing on Vgd
2.) Vgd> 0 a possibility in combination with option 1
3.) Vgs<0 no channel i.e. kaput mode.
4.) Vgd<0 combined with condition 1 could mean it is in saturation

essentially the condition you need for the linear mode operation is Vgs>Vt (no Vt mentioned in your list) , so assume Vgs>0 => Vgs>Vt
When Vds=0 then Vgd=Vgs, and If Vgs>Vgd>Vt>0 for nFET is also the linear case.

Just keeping the Vgs>0 doesn't mean much by itself.

3. gate voltage negative with respect to source

N-channel JFET, the gate must always be at a lower potential
than the source (Vcs < 0). The channel is as fully open as it
can get when the gate and source voltages are equal (VGS = 0).

kak111: Though what you posted was ok for a JFET. typically, MOSFET are the most common i.e. nMOS,pMOS.
So if I was asked for a n-FET I would still go for the nMOS transistor and not the Junction FET which has different device characteristics.

I think the essence of the question is that very assumption. We had JFETs back even when I was in school (circa 1968 ). The mosfet is used more often as a switch, not as an amplifier in the context of this question. At least, that is how I interpreted it. Not a trick question per se, but if the instructor had wanted an answer for mosfets, s/he would have said so.

John

Yes i know the difference between field effect transistor types.

how to bias different fet-types-

It is there...................

: Look Biasing Fet`s

Regards KAK

Obviously, the multiple-choice question is related to MOSFETs, and it's implicitely assuming, that all NMOSFETs have a positive threshold voltage (are utilizing enhancement mode).

As already mentioned, this assumption isn't justified, so strictly spoken, none of the four choices would be fully correct.

If we assume in addition, that the respective lecture didn't tell about depletion mode or JFETs at all, then we can imagine, which answer is thought to be the right one.

P.S.: It may be the case, that the question wasn't completely quoted in post #1...

Isn't it funny?

There is one guy asking an uncomplete question that allows different answers - and five people
puzzle over what it might have been. But the originator sees no reason to clarify the point.
LvW

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