CHAPTER 5. TRANSISTOR CIRCUITS90b)hfeiBhieiBRBisvBB'BRcvcCa)isvsRsvBiBBRBRcCEicvcVccVccSGFigure 5.8: a) Basic CE ampliﬁer and b) AC equivalent circuit drawn using an ideal transistorsymbol with hie shown explicitly.The minus sign indicates that the voltage signal at the collector is 180o out of phase withthe signal at the base.The input impedance to this ampliﬁer circuit is just the parallel combination of RB andhie, and since hie is usually much smaller than RB, the input impedance generally reduces tojust the input impedance of the transistor itself, namely, hie. The circuit output impedanceis the collector resistance RC.The high-frequency operation of the common emitter ampliﬁer is limited by the parasiticcapacitance between the collector and base. This capacitance provides a path by whichthe large and inverted signal at the collector drives a feedback current into the base. Thebase-to-collector voltage gain of this ampliﬁer looks like a low-pass ﬁlter.5.2.4CE Ampliﬁer with Emitter ResistorThe CE ampliﬁer is often constructed with an emitter resistor RE as shown in ﬁgure 5.9.This resistor provides a form of negative feedback that can be used to stabilize both the DCoperating point and the AC gain. It can be shown that the voltage transfer function acrossthe transistor isvCvB=−A =−hfeRChie + hfeRE.(5.22)If hfeREhie, the gain becomes independent of the hybrid parameters:vCvB=−A =−RCRE.(5.23)Because it is unaﬀected by variations in the hybrid parameters, this result is valid even fora large-amplitude signal.The input resistance can be shown to be Rin = hfeRE.Since hie is small, it can usuallybe neglected whenever the emitter impedance is more than a few hundred ohms.