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How to bias a PMOS mos cap?

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szekit

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how to bias a mos cap? let's say a PMOS mos cap? does it have to be off (accumulation)?

what if i use it as a supply decoupling cap (like at the gate of current mirror)? it is in "on" state as i have seen in many circuits before. ~confused~
 

moscap

yes, accumulation is the best, else your capacitance value changes greatly as you move in and out of inversion.
 

Re: how to bias mos cap

electronrancher said:
yes, accumulation is the best, else your capacitance value changes greatly as you move in and out of inversion.
Click to expand...

This logic doesn't seem good. The same can happen with accumulation too. More over from designer point of view the point of inflection (the point at which capacitance starts changing) is more well defined in the case of inversion than that of accumulation.

Also it is clearly mentioned in "CMOS circuit design, layout and simulation by Baker, Lee, Boyce" that MOSFET in accumulation is a bad capacitor and in inversion is a good capacitor. Reason: Series resistance of capacitor in accumualtion is large. So bias the MOSFET in inversion with (VGS-VT)>100mV for proper operation of MOSCAP.
 

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