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i.e. Why don't we make both the input and the load transistors as NMOS?

M.A.
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1 Answers1

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If the load transistor were an NMOS, it would act as a common-drain amplifier and try to hold the output at a constant voltage. (\$V_{GS}\$ would be the difference between the NMOS gate voltage and the output voltage.) With a PMOS in the common-source configuration, a constant gate voltage gives you a constant \$V_{GS}\$. A MOSFET driven with a constant \$V_{GS}\$ acts roughly like a constant current source, which is a good load for an amplifier.

Adam Haun
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  • But won't an NMOS load have a constant Gate Voltage? Which can then be configured like the PMOS (Source would be connected to VDD providing constant Vgs), and both drains would be connecting to each other. – M.A. Mar 03 '23 at 16:27
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    @M.A. If you use an NMOS, the terminal at the VDD rail isn't going to be the source. It's going to be the drain. If you use a discrete nFET, you'll have to place it in that orientation to avoid shorting it through the body diode. If you use an integrated nFET, your source/drain is controlled by the [relative voltages, not the fabrication](https://electronics.stackexchange.com/a/567651/9612) because it's a symmetric device. You may benefit from reading [this answer](https://electronics.stackexchange.com/a/535499/9612) that I wrote on a very similar question in the past. – nanofarad Mar 03 '23 at 16:34