5.2.2 Approximate AC Model

Chapter 5.2.2 Approximate AC Model

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 5. TRANSISTOR CIRCUITS89c)b)a)IcRcVEREVcVccVB~VERB2RB1I2I1IR = Ic + IB ~ IcIc = hFE IBVcIBVB~0RcRBVccIc = hFE IBVcVB~0VccIBRcRFFigure 5.7: Bias circuits for the common emitter amplifier.A further improvement can be made by introducing a second base-bias resistor as shownin figure 5.7c. The bias voltage is determined almost entirely by the two bias resistors. Thesebiasing methods can also be used for the common collector and common base configurations.5.2.2Approximate AC ModelThe circuit shown in figure 5.8a is the basic common emitter amplifier using the simplestbiasing method. Because it is constant, the power supply voltage VCC is an AC groundindistinguishable from the normal ground of the circuit.We can therefore relocate theupper end of RB and RC to the common ground line as shown in figure 5.8b. The transistorsymbol is ideal and hie is shown explicitly as the input impedance and hence iS = iB.5.2.3The Basic CE AmplifierWe can now use the AC equivalent circuit to calculate the AC voltage gain between the baseand collector. The base voltage is developed across the input resistor hie and vB = hieiB.The collector voltage can be similarity expressed as the voltage drop across the resistor RC:0− vC = RChfeiB. Eliminating iB, we can write the amplifier voltage transfer functionbetween the base and collector asvCvB=−hfeRChie.(5.21)