Question 7.SP.13: For the JFET drain characteristics of Fig. 4-2(a), take vDS ......

For small variations about a Q point, the chain rule gives
v_{d s} = \Delta v_{D S} \approx d v_{D S} = {\frac{\partial v_{D S}}{\partial v_{G S}}}\bigg|_{Q} v_{g s} + {\frac{\partial v_{D S}}{\partial i_{D}}}\bigg|_{Q}\bigg. i_d                    (1)

Now we may define
\left.{\frac{\partial v_{D S}}{\partial v_{G S}}}\right|_{Q} = \mu \qquad {\mathrm{and}} \qquad \left.{\frac{\partial v_{D S}}{\partial i_{D}}}\right|_{Q} = r_{ds}

If the JFET operates in the pinchoff region, then gate current is negligible and (1) is satisfied by the equivalent circuit of Fig. 7-1(b).