How can I measure gate capacitance?
The gate capacitance of a MOSFET is a more complicated topic than a lot of people realize. It depends very strongly on the operating conditions of the device. This makes sense &mdash; the capacitance we're talking about has the gate itself as one plate, which is a fixed physical structure, but the other "plate" is not just the source, drain and substrate structures nearby, but also the charge carriers flowing in the source-to-drain channel, and their concentration varies considerably.

To get some insight into this, look at Figure 6 in the IRF530N datasheet (reproduced below), which shows the gate charge as a function of gate-source voltage. The definition of capacitance is \$\frac{\Delta charge}{\Delta voltage}\$, so given how this chart is laid out, the effective gate capacitance is the inverse of the slope of the curve at any given point.



The \$C_{ISS}\$ value is measured at \$V_{GS}\$ = 0V, so it corresponds to the slope at the lower-left corner of the graph. But note how the graph flattens out near the threshold voltage &mdash; this reduced slope indicates a much greater effective capacitance (roughly 10&times;) at that operating point. And more to the point, this is exactly the point at which your current regulator circuit is operating.

So, to fully characterize the load capacitance your opamp is seeing, you need to test the MOSFET in the manner shown in Figure 13, with suitable bias voltages on the gate and drain.