So suppose we have two E-MOSFETS connected in series across a 10V supply, and the upper MOSFET has its drain connected to the gate, and the lower one is unbiased. (The input is taken to be the gate of the lower MOSFET and the output is taken to be the drain of the lower MOSFET) The Upper MOSFET has a resistance of 2k ohms, and its gate-source voltage is 10V. The lower MOSFET has a resistance of 150 ohms when input voltage = 3V. What is the output of the circuit when the input is 0V and 3V?

We learned in class how to do calculations with one mosfet but on the sample exam this question was asked. I'm pretty hopeless at this (as I'm a CS student and we are forced to take this module for some reason) and was wondering how to do the question. (The lecturer literally repeats the same question every year)

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    \$\begingroup\$ A schematic of this circuit setup would be very useful. \$\endgroup\$ – user2943160 Dec 15 '19 at 19:49
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    \$\begingroup\$ What is an "E-MOSFET"? \$\endgroup\$ – The Photon Dec 15 '19 at 21:35
  • \$\begingroup\$ @ThePhoton Enhancement-mosfet \$\endgroup\$ – Huisman Dec 15 '19 at 21:49
  • \$\begingroup\$ What channel type? N, becaue you write down positive Vgs voltages? \$\endgroup\$ – Huisman Dec 15 '19 at 21:50
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    \$\begingroup\$ The lower MOSFET IS NOT unbiased !!!! It is biased such that Vgs is EITHER 0V or 3V. || Draw the diagram. \$\endgroup\$ – Russell McMahon Dec 16 '19 at 2:54

The question APPEARS straightforward, if unrealistic.
The upper FET has gate connected to drain. Nothing is going to change how it is biased :-).
It's resistance is given.

It's gate source voltage is SAID to be 10V (full supply).
If this is true in both input cases then the output MUST always be zero.
ie you or they have not got the question quite correct.
I take it to mean that you have a fixed resistance FET channel come what may.
If that is NOT what they mean then you may have to ask them.

But, assuming a 10V supply and a 2K resistor where the top FET goes.

The bottom FET resistance is 150 Ohms or infinite.
Go from there ... .

  • \$\begingroup\$ thank you. Helped \$\endgroup\$ – Daniel Nugent Dec 16 '19 at 18:25
  • \$\begingroup\$ What answer did you get? I'd EXPECT 10V or 150/(150 + 2000) x 10 = 10V or 0.698 V (say 0.7V) BUT YOU need to arrive at that answer (or the correct one if this one is wrong) yourself. \$\endgroup\$ – Russell McMahon Dec 20 '19 at 4:06

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