now that the important parameters of an ac equivalent circuit have been introduced and discussed, a model for the JFET transistor in the ac domain can be constructed. The control of I d by V gs is included as a current source gmVgs connected from drain to source as shown in Fig. 8.8 . The current source has its arrow pointing from drain to source to establish a 180° phase shift between output and input voltages as will occur in actual operation.