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Trying to understand the SPICE output for this basic MOSFET circuit:

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The input (left) is a crude square wave:

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A simple spice simulation shows the output voltage (right) as follows:

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Why does the capacitor level off at 7V? My thinking was that as it charges up, the current through the transistor would decrease until VDS = 0, and at that point the voltage across the capacitor would be 9V, but SPICE thinks differently.

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    \$\begingroup\$ nmos cuts off when \$V_{GS} \lt V_{th}\$, so the maximum voltage the capacitor charges to is \$V_{DD}-V_{th}\$. replace it with pmos to get full output \$\endgroup\$
    – across
    Commented Feb 28, 2022 at 5:46
  • \$\begingroup\$ ohhhh, right. can you please make this an answer? i will accept it now. \$\endgroup\$
    – broken.eggshell
    Commented Feb 28, 2022 at 5:51
  • \$\begingroup\$ @broken.eggshell Put another way, you accidentally built an NMOS source follower instead of a PMOS high-side switch, which is what it looks like you wanted. \$\endgroup\$
    – Ste Kulov
    Commented Feb 28, 2022 at 6:08
  • \$\begingroup\$ Yeah I saw that immediately following the first comment, but it took someone to point it out to me. Still trying to internalize this stuff! \$\endgroup\$
    – broken.eggshell
    Commented Feb 28, 2022 at 6:15

1 Answer 1

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nmos cuts off when \$V_{GS} \lt V_{th}\$, so the maximum voltage an nmos can "pass" is \$V_{DD}-V_{th}\$.

pmos cuts off when \$V_{GS} \gt -V_{th}\$, so the minimum voltage an pmos can "pass" is \$V_{SS}+V_{th}\$.

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