a question about MOSFET

crutschow said:

Post a schematic.

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crutschow said:

To turn on a MOSFET the gate voltage must be higher than the source node, positive gate-to-source voltage for an N-MOSFET, and negative gate-to-source voltage for a P-MOSFET. Thus the type you use depends upon the polarity of the signal you are switching and the drive signal. In this case the voltage and drive signals are +5V. Thus for the FETs that control the +5V (M33, M37) you use P-MOSFETs which can be turned on by connecting their gates to 0V and turned off by connecting their gates to +5V (since their source nodes are at +5V). Similarly for the FETs that switch the voltages to ground (M35, M38) you use an N-MOSFET which can be turned on by connection their gates to +5V and turned off by connecting their gates to 0V (since their source nodes are at 0V).

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crutschow said:

If you use only N-MOSFETs, then when switching the +5V, the output from the source is reduced by the turn-on gate voltage of the FET since it is operating as a source-follower. To fully turn on the N-MOSFET you would have to supply the gate with a voltage equal to 5V plus the gate turn-on voltage (≈5V). Thus if you applied 10V to the N-MOSFETs' gates to turn them on, then you output would be the same as if you used P-MOSFETs.

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