ABSTRACT
This chapter described the favorite semiconductor devices employed in amplification, the bipolar junction transistor (BJT), and the metal oxide semiconductor (MOS) transistor. It also presents transistor linear models for operation with small signals at low frequencies. The BJT alternating current (AC) models represent both the PNP and the NPN types. In the same way, the MOS field‐effect transistor AC model applies for the n-channel as well as the p‐channel devices. Since they consist of linear networks, their validity is conditioned to small‐signal operation, which means that the AC component amplitude of each signal should be a small fraction of its DC component or its quiescent value, i.e. not more than 5 or 10%. Otherwise, the precision of the analysis may be significantly affected. It is necessary to emphasize that the transistor small‐signal models represent relationships only between the AC components of the terminal voltage and current signals and thus should replace the amplifying device only in the so‐called AC‐equivalent network.
