Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

Cb stage with biasing having learned the small signal

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CB Stage with Biasing Having learned the small-signal properties of the CB core, we now extend our analysis to the circuit including biasing. An example proves instructive at this point.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 240 (1) 240 Chap. 5 Bipolar Amplifiers Example 5.40 The student in Example 5.31 decides to incorporate ac coupling at the input of a CB stage to ensure the bias is not affected by the signal source, drawing the design as shown in Fig. 5.77. Explain why this circuit does not work. Q 1 R C V CC out C 1 in V V V b Figure 5.77 CB stage lacking bias current. Solution Unfortunately, the design provides no dc path for the emitter current of , forcing a zero bias current and hence a zero transconductance. The situation is similar to the CE counterpart in Example 5.5, where no base current can be supported. Exercise In what region does operate if ? Example 5.41 Somewhat embarrassed, the student quickly connects the emitter to ground so that and a reasonable collector current can be established (Fig. 5.78). Explain why “haste makes waste.” Q 1 R C V CC out C 1 in V b v v Figure 5.78 CB stage with emitter shorted to ground. Solution As with Example 5.6, the student has shorted the signal to ac ground. That is, the emitter voltage is equal to zero regardless of the value of , yielding .
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 241 (1) Sec. 5.3 Bipolar Amplifier Topologies 241 Exercise Does the circuit operate better if is raised? The above examples imply that the emitter can remain neither open nor shorted to ground, thereby requiring some bias element. Shown in Fig. 5.79(a) is an example, where provides a path for the bias current at the cost of lowering the input impedance. We recognize that now consists of two parallel components: (1) , seen looking “up” into the emitter (with the base at ac ground) and (2) , seen looking “down.” Thus, Q 1 R C V CC out C 1 in V b R E g m 1 R E Q 1 R C V CC out C 1 in V b R E R S X in R v v v v (a) (b) Figure 5.79 (a) CB stage with biasing, (b) inclusion of source resistance. (5.295) As with the input biasing network in the CE stage (Fig. 5.58), the reduction in manifests itself if the source voltage exhibits a finite output resistance. Depicted in Fig. 5.79(b), such a circuit attenuates the signal, lowering the overall voltage gain. Following the analysis illustrated in Fig. 5.67, we can write (5.296) (5.297) (5.298) Since , (5.299) As usual, we have preferred solution by inspection over drawing the small-signal equivalent. The reader may see a contradiction in our thoughts: on the one hand, we view the low input impedance of the CB stage a useful property; on the other hand, we consider the reduction of the input impedance due to undesirable . To resolve this apparent contradiction, we must distinguish between the two components and , noting that the latter shunts the input
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 242 (1) 242 Chap. 5 Bipolar Amplifiers Q 1 R C V CC out C 1 in V b R E R S v v i in i 2 i 1 Figure 5.80 Small-signal input current components in a CB stage.
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