cmosnmospmosswitches
A general question regarding switches in CMOS. Have a look at the schematic below (symplified 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.
The two MOSFETs are drawn incorrectly in the question's picture. That is why you are confused. This is how they should be i.e. sources tied together: -
I'm not aware that there is a decent non-isolated gate-drive version.
Simplest way to remember current direction is by the little arrow indicator on the transistor, for NMOS it is pointing out of the drain thus current flows from source to drain. And for PMOS the arrow is into the source, so flows from source to drain. It is important to remember that the source is indicated by the arrow location. So in the case drawn, NMOS source is on the bottom and PMOS source is on the top.
Best Answer
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.