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A general question regarding switches in CMOS. Have a look at the schematic below (symplified current mirror)

Symplified schematic of a current mirror

There is a input reference current, on/off switch, a PMOS mirror and the resulting output current. In literature I noticed that the switch is implemented as a PMOS, but sometimes also as a NMOS. Beside the difference in logic functionality, what are the differences? Which is prefered? Thanks in advance.

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    \$\begingroup\$ Do you have a circuit diagram or a link to where you have read this? \$\endgroup\$ – Andy aka Jan 8 '15 at 16:12
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I know that NMOS transistors are generally faster than PMOS transistors since electrons have a higher mobility than holes. Therefore NMOS would be preferred over PMOS for applications requiring higher switching frequencies.

However, PMOS transistors have less flicker noise than NMOS because of their smaller transconductance, which means that PMOS conduct less current than NMOS for the same voltage bias and thus generate less noise. Therefore, for applications requiring low noise (such as a precision current mirror), PMOS might be better.

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