# Example 554 determine the voltage gain and io

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Example 5.54 Determine the voltage gain and I/O impedances of the topology shown in Fig. 5.103(a). Assume and equal ’s for and transistors. R C out v R E Q R out v R Q Q 1 R B2 V CC 3 Q 2 R B1 in v R eq2 R eq1 eq2 E R eq1 in v R C (a) (b) 3 Figure 5.103 (a) Example of CE stage, (b) simplified circuit. Solution We identify the stage as a CE amplifier with emitter degeneration and a composite collector load. As the first step, we represent the role of and by the impedances that they create at their emitter. Since denotes the impedance seen looking into the emitter of with a base resistance of , we have from Fig. 5.75 (5.360)

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 259 (1) Sec. 5.5 Chapter Summary 259 Similarly, (5.361) leading to the simplified circuit shown in Fig. 5.103(b). It follows that (5.362) (5.363) Also, (5.364) (5.365) and (5.366) (5.367) Exercise What happens if ? 5.5 Chapter Summary In addition to gain, the input and output impedances of amplifiers determine the ease with which various stages can be cascaded. Voltage amplifiers must ideally provide a high input impedance (so that they can sense a voltage without disturbing the node) and a low output impedance (so that they can drive a load without reduction in gain). The impedances seen looking into the base, collector, and emitter of a bipolar transistor are equal to (with emitter grounded), (with emitter grounded), and (with base grounded), respectively. In order to obtain the required small-signal bipolar device parameters such as , , and , the transistor must be “biased,” i.e., carry a certain collector current and operate in the active region. Signals simply perturb these conditions. Biasing techniques establish the required base-emitter and base-collector voltages while pro- viding the base current.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 260 (1) 260 Chap. 5 Bipolar Amplifiers With a single bipolar transistor, only three amplifier topologies are possible: common-emitter and common-base stages and emitter followers. The CE stage provides a moderate voltage gain, a moderate input impedance, and a moderate output impedance. Emitter degeneration improves the linearity but lowers the voltage gain. Emitter degeneration raises the output impedance of CE stages considerably. The CB stage provides a moderate voltage gain, a low input impedance, and a moderate output impedance. The voltage gain expressions for CE and CB stages are similar but for a sign. The emitter follower provides a voltage gain less than unity, a high input impedance, and a low output impedance, serving as a good voltage buffer. Problems 1. An antenna can be modeled as a Thevenin equivalent having a sinusoidal voltage source and an output resistance . Determine the average power delivered to a load resistance and plot the result as a function of .

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