Consider the BiCMOS amplifier shown in Fig. P8.97. The BJT has VBE = 0.7 V and = 200

Chapter 8, Problem 8.97

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Consider the BiCMOS amplifier shown in Fig. P8.97. The BJT has VBE = 0.7 V and = 200. The MOSFET has Vt = 1 V and kn = 2 mA/V2 . Neglect the Early effect in both devices. Rin C1 Vsig vi 5 V vo 1 k 500 k 6.8 k RG 10 M C2 Q2 Q1 3 k Figure P8.97 (a) Consider the dc bias circuit. Neglect the base current in Q2 in determining the current in Q1. Find the dc bias currents in Q1 and Q2 and show that they are approximately 100 A and 1 mA, respectively. (b) Evaluate the small-signal parameters of Q1 and Q2 at their bias points. (c) Determine the voltage gain Av = vo/vi . For this purpose you can neglect RG. (d) Noting that RG is connected between the input node where the voltage is vi and the output node where the voltage is Av vi , find Rin and hence the overall voltage gain vo/vsig. (e) To considerably reduce the effect of RG on Rin and hence on Gv , consider the effect of adding another 10 -M resistor in series with the existing one and placing a large bypass capacitor between their joint node and ground. What will Rin and Gv become? 8.9