Common-Source Configuration The resisti vely loaded common-source (CS) amplifier configuration is shown in Fig. 3.12a usingann-channel MOS transistor.The correspon dingsmall-signal equivalent circuit is shown in Fig.3.12b.A sin the case of the bipolar transistor,the MOS transistor is cut offfor Vi =0and thus Id =0 and Vo = VDD.AsVi is increased beyond the threshold voltage Vt, nonzero drain current flows and the transistor operates in the active region (which is often called saturation for MOS transistors) when Vo >VGS −Vt. The large-signal model of Fig. 1.30 can then be