High-frequency equivalent circuits, of the hybrid-π form, for common-emitter and common-base operations are given the different, admittances are not simple RC parallel combinations. Particularly, the short-circuit common-base input admittance of the ’ intrinsic transistor ’. arrived at theoretically and confirmed by experiment in this paper, is substantially different from the usual representation of a ge-ce parallel combination. All the constants of the ’ intrinsic transistor ’ are expressed as simple functions of one or more of just three physical parameters of the transistor, viz. the base region lifetime, minority currier transit time in the base and a dimensionless base-width modulation parameter. λ. apart from the quiescent currents in the three leads of the transistor. The representation affords a simple means of experimentally determining these physical parameters. Though some approximations ere implied in the analysis in that it is one-dimensional and restricted to minority currier behaviour in t...