What is the difference between these two expressions:
C=Q/V
C=dQ/dV
I encountered the second one while studying MOSFET.
C=Q/V
C=dQ/dV
I encountered the second one while studying MOSFET.
The second version is for a small change in charge for a small change in voltage (which still equals capacitance over that small range). They use that for semiconductor measurements where the incremental capacitance can change with DC bias. An extreme example of this is the varactor diode.What is the difference between these two expressions:
C=Q/V
C=dQ/dV
I encountered the second one while studying MOSFET.
The C=Q/V is the true definition of capacitance. The other relation is used when there is a nonlinear capacitance. In such cases, the effective capacitance for small AC signals (operating around a DC condition) can be related to dQ/dV.In cases where the charge distribution is a spatially dependent, like in the case of the inversion layer under SiO2, the capacitance will vary from point to point; then,unlike parallel plate capacitor, average is different from instantaneous.
Is this reasoning correct?
The two are interchangeable and the second is not in error. Both equations define capacitance as the amount of charge moved (or stored) for a given change in voltage. In the first equation it is implied that the voltage change is from 0V to the measured voltage or V = dV and that the charge is the change in charge caused by going from 0V to V or Q = dQ.These two expressions are not interchangeable. The former is the fundamental definition of capacitance. Thus the second expression seems to be in error (in the general sense).
.................
I believe you've got it.So, the thing I get is that capacitance can depend upon the voltage(that's the non-linear capacitor). Then it makes sense to calculate the 'small signal capacitance' around a bias point, where we are linearizing the capacitance around that point, like we do in case of BJT/MOS small signal model.
Thus if a small signal comes, it will feel the capacitance given by (dQ/dV) and not the absolute Q/V.
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