Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

We next study the more robust biasing scheme of fig

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We next study the more robust biasing scheme of Fig. 5.19, repeated in Fig. 5.56(a) along with an input coupling capacitor. The bias point is determined by opening and following Eqs. (5.52) and (5.53). With the collector current known, the small-signal parameters of can be computed. We also construct the simplified ac circuit shown in Fig. 5.56(b), noting that the voltage gain is not affected by and and remains equal to (5.225) where Early effect is neglected. On the other hand, the input impedance is lowered to: (5.226) whereas the output impedance remains equal to if . As explained in Section 5.2.3, the use of emitter degeneration can effectively stabilize the bias point despite variations in and . However, as evident from (5.225), degeneration also lowers
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 225 (1) Sec. 5.3 Bipolar Amplifier Topologies 225 Q 1 V CC R R C 1 R 2 C 1 in v Q 1 R R C out v in v 2 R 1 R E R E (a) (b) Figure 5.56 (a) Degenerated stage with capacitive coupling, (b) simplified circuit. the gain. Is it possible to apply degeneration to biasing but not to the signal? Illustrated in Fig. 5.57 is such a topology, where is large enough to act as a short circuit for signal frequencies of interest. We can therefore write Q 1 V CC R R C 1 R 2 C 1 in v R E C 2 Figure 5.57 Use of capacitor to eliminate degeneration. (5.227) and (5.228) (5.229) Example 5.34 Design the stage of Fig. 5.57 to satisfy the following conditions: mA, voltage drop across mV, voltage in the audio frequency range (20 Hz to 20 kHz), input . Assume , , and V. Solution With , the value of is equal to 400 . For the voltage gain to remain unaffected by degeneration, the maximum impedance of must be much smaller than . Occurring at 20 Hz, the maximum impedance must remain below roughly : (5.230) A common mistake here is to make the impedance of much less than .
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 226 (1) 226 Chap. 5 Bipolar Amplifiers Thus, (5.231) (This value is unrealistically large, requiring modification of the design.) We also have (5.232) obtaining (5.233) Since the voltage across is equal to 400 mV and mV, we have V. Also, with a base current of 10 A, the current flowing through and must exceed 100 A to lower sensitivity to : (5.234) and hence (5.235) Under this condition, (5.236) yielding (5.237) (5.238) We must now check to verify that this choice of and satisfies the condition . That is, (5.239) (5.240) Unfortunately, and lower the input impedance excessively. To remedy the problem, we can allow a smaller current through and than , at the cost of creating more sensitivity to . For example, if this current is set to A and we still neglect in the calculation of , (5.241) and (5.242)
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 227 (1) Sec. 5.3 Bipolar Amplifier Topologies 227 Consequently, (5.243) (5.244) giving (5.245) Exercise Redesign the above stage for a gain of 10 and compare the results.
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