Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

We conclude our study of the ce stage with a brief

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We conclude our study of the CE stage with a brief look at the more general case depicted in Fig. 5.58(a), where the input signal source exhibits a finite resistance and the output is tied to a load . The biasing remains identical to that of Fig. 5.56(a), but and lower the voltage gain . The simplified ac circuit of Fig. 5.58(b) reveals is attenuated by the voltage division between and the impedance seen at node , , i.e., Q 1 V CC R R C 1 R 2 in v Q 1 R C out v R E R E (a) (b) R S C 1 C 2 R L out v R L R R 1 2 in v R S X Q 1 R C out v R E (b) R L R R 1 2 in v R S X Figure 5.58 (a) General CE stage, (b) simplified circuit, (c) Thevenin model of input network. (5.246) The voltage gain from to the output is given by (5.247) (5.248)
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 228 (1) 228 Chap. 5 Bipolar Amplifiers As expected, lower values of and reduce the gain. The above computation views the input network as a voltage divider. Alternatively, we can uti- lize a Thevenin equivalent to include the effect of , , and on the voltage gain. Illustrated in Fig. 5.58(c), the idea is to replace , and with and : (5.249) (5.250) The resulting circuit resembles that in Fig. 5.43(a) and follows Eq. (5.185): (5.251) where the second fraction on the right accounts for the voltage attenuation given by Eq. (5.249). The reader is encouraged to prove that (5.248) and (5.251) are identical. The two approaches described above exemplify analysis techniques used to solve circuits and gain insight. Neither requires drawing the small-signal model of the transistor because the reduced circuits can be “mapped” into known topologies. Figure 5.59 summarizes the concepts studied in this section. R C g m A v = R C Gain Headroom R in R out R C g m A v = r O R C ( r O ) A v R , in R E R out R R C 1 R 2 R E C 2 C 1 Q 1 R R C 2 R 1 Figure 5.59 Summary of concepts studied thus far. 5.3.2 Common-Base Topology Following our extensive study of the CE stage, we now turn our attention to the “common-base“ (CB) topology. Nearly all of the concepts described for the CE configuration apply here as well. We therefore follow the same train of thought, but at a slightly faster pace. Given the amplification capabilities of the CE stage, the reader may wonder why we study other amplifier topologies. As we will see, other configurations provide different circuit proper- ties that are preferable to those of the CE stage in some applications. The reader is encouraged to review Examples 5.2-5.4, their resulting rules illustrated in Fig. 5.7, and the possible topologies in Fig. 5.28 before proceeding further.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 229 (1) Sec. 5.3 Bipolar Amplifier Topologies 229 Figure 5.60 shows the CB stage. The input is applied to the emitter and the output is sensed at the collector. Biased at a proper voltage, the base acts as ac ground and hence as a node “common” to the input and output ports. As with the CE stage, we first study the core and subsequently add the biasing elements.
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