Derive the rule that states: The drain–gate capacitance sees an effective open-circuit resistance…

Derive the rule that states: The drain–gate capacitance sees an effective open-circuit resistance given mnemonically by “rleft + rright + gm,effr left
r
right,” where gm,e
ff
is the ratio of short-circuit drain current to the gate voltage (not gateto-source voltage). In this case, neither a test voltage nor a test current is a perfect choice. However, fewer algebraic steps are needed if a test current is used.