Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

A pnp stage with resistive divider biasing b thevenin

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(a) PNP stage with resistive divider biasing, (b) Thevenin equivalent of divider and . Adding the voltage drop across and to yields (5.97) that is, (5.98) (5.99) It follows that (5.100) As in Example 5.9, some iteration between and may be necessary. Equation (5.100) indicates that if is significant, then the transistor bias heavily depends on . On the other hand, if , we equate the voltage drop across to , thereby obtaining the collector current: (5.101) (5.102) Note that this result is identical to Eq. (5.30). Exercise What is the maximum value of is must remain in soft saturation? Example 5.17 Assuming a negligible base current, calculate the collector current and voltage of in the
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 199 (1) Sec. 5.2 Operating Point Analysis and Design 199 circuit of Fig. 5.26. What is the maximum allowable value of for to operate in the forward active region? R I C Y R X C Q 1 V CC 1 R 2 I B I 1 V EB R E V RE Figure 5.26 PNP stage with degeneration resistor. Solution With , we have . Adding to the emitter-base voltage and the drop across , we obtain (5.103) and hence (5.104) Using , we can compute a new value for and iterate if necessary. Also, with , we can verify the assumption . In arriving at (5.104), we have written a KVL from to ground, Eq. (5.103). But a more straightforward approach is to recognize that the voltage drop across is equal to , i.e., (5.105) which yields the same result as in (5.104). The maximum allowable value of is obtained by equating the base and collector voltages: (5.106) (5.107) It follows that (5.108)
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 200 (1) 200 Chap. 5 Bipolar Amplifiers Exercise Repeat the above example if . Example 5.18 Determine the collector current and voltage of in the self-biased circuit of Fig. 5.27. R Y X C Q 1 V CC V EB R B I C I B Figure 5.27 Self-biased pnp stage. Solution We must write a KVL from through the emitter-base junction of , , and to ground. Since and hence , carries a current approximately equal to , creating . Moreover, , yielding (5.109) (5.110) (5.111) Thus, (5.112) a result similar to Eq. (5.78). As usual, we begin with a guess for , compute , and deter- mine a new value for , etc. Note that, since the base is higher than the collector voltage, always remains in the active mode. Exercise How far is from saturation?
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 201 (1) Sec. 5.3 Bipolar Amplifier Topologies 201 5.3 Bipolar Amplifier Topologies Following our detailed study of biasing, we can now delve into different amplifier topologies and examine their small-signal properties. Since the bipolar transistor contains three terminals, we may surmise that three possibilities exist for applying the input signal to the device, as conceptually illustrated in Figs. 5.28(a)- (c). Similarly, the output signal can be sensed from any of the terminals (with respect to ground) [Figs. 5.28(d)-(f)], leading to nine possible combinations of input and output networks and hence nine amplifier topologies.
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