Active-device capacitances

The charge-control capacitance ∂Q/∂V looking into a device port is not in general equal to the intrinsic small-signal capacitance found from the low-frequency admittance ratios up to 7:1 occur in practice. This paper explains why the discrepancy occurs, and derives the condition for the discrepancy to be zero. Finally, it is shown that charge-control theory does correctly predict the gain-bandwidth product of an active device.