MOSFET

In a pass transistor (switch) made of a single nMOSFET with its gate at the supply voltage VDD, how high does the output rise when passing an input of VDD?

  1. AUp to VDD
  2. BUp to VDD + Vth
  3. CUp to about VDD − Vth
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Answer: C. Up to about VDD − Vth

An nMOS turns off when its gate-source voltage falls below the threshold voltage. As the output (source side) approaches VDD − Vth, the gate-source voltage approaches Vth and almost no current flows, so the output rises no further. A 0 is passed fully but a 1 is weakened, which is why transmission gates combining nMOS and pMOS are used.

  • A. Reaching VDD requires measures such as adding a pMOS in parallel.
  • B. The output cannot exceed the gate voltage.
  • C. Correct. It stops one threshold voltage below VDD.
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