whether the bottom plate of a MOSCAP must be connected to gnd or VCC?

[fightshan]

In actual IC manufacturing,the two plates of a MOSCAP is not symmetrical.generally,the bottom plate should be connected to gnd(the lowest voltage in a chip,NMOSCAP) or VCC(the highest voltage in a chip,PMOSCAP).as shown in picture1,G is the top plate and B/S/D is the bottom plate.

however,in the circuit of a integrator ,we need a cap which is connected between a input and the output as a feedback capacitor ,as shown in picture2 C1 and C2,so ,my question is : can i use the MOSCAP as the feedback cap?

 

[erikl]

can i use the MOSCAP as the feedback cap?

Depends, if you can get along with, or if you can avoid the C/V dependency, s. the following figure: View attachment PMOScap.pdf. The cap./area values given are for a 3.3V oxide thickness.

If you operate the PMOS in inversion mode (D,S,B+ , G- , normal PMOS operation), you need at least Vg=-Vth as difference voltage to make profit of the high cap./area ratio.

In accumulation mode (D,S,B- , G+) you can achieve even a little bit better cap./area ratio, but you need a pos. difference voltage of Vg >≈ 2|Vth| to make profit of it.