Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

Exceptions are described in br wileyrazavi

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Exceptions are described in Chapter 12. 175
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 176 (1) 176 Chap. 5 Bipolar Amplifiers 5.1.1 Input and Output Impedances In addition to the above parameters, the input and output (I/O) impedances of an amplifier play a critical role in its capability to interface with preceding and following stages. To understand this concept, let us first determine the I/O impedances of an ideal voltage amplifier. At the input, the circuit must operate as a voltmeter, i.e., sense a voltage without disturbing (loading) the preceding stage. The ideal input impedance is therefore infinite. At the output, the circuit must behave as a voltage source, i.e., deliver a constant signal level to any load impedance. Thus, the ideal output impedance is equal to zero. In reality, the I/O impedances of a voltage amplifier may considerably depart from the ideal values, requiring attention to the interface with other stages. The following example illustrates the issue. Example 5.1 An amplifier with a voltage gain of 10 senses a signal generated by a microphone and applies the amplified output to a speaker [Fig. 5.1(a)]. Assume the microphone can be modeled with a voltage source having a 10-mV peak-to-peak signal and a series resistance of 200 . Also assume the speaker can be represented by an 8- resistor. Microphone Amplifier Speaker A v = 10 v m 200 R m 10 mV 8 R in v m 200 R m 1 v v R amp amp 8 R L out v (c) (a) (b) Figure 5.1 (a) Simple audio system, (b) signal loss due to amplifier input impedance, (c) signal loss due to amplifier output impedance. (a) Determine the signal level sensed by the amplifier if the circuit has an input impedance of 2 k or 500 . (b) Determine the signal level delivered to the speaker if the circuit has an output impedance of 10 or 2 . Solution (a) Figure 5.1(b) shows the interface between the microphone and the amplifier. The voltage
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 177 (1) Sec. 5.1 General Considerations 177 sensed by the amplifier is therefore given by (5.1) For k , (5.2) only 9 less than the microphone signal level. On the other hand, for , (5.3) i.e., nearly 30 loss. It is therefore desirable to maximize the input impedance in this case. (b) Drawing the interface between the amplifier and the speaker as in Fig. 5.1(c), we have (5.4) For , (5.5) a substantial attenuation. For , (5.6) Thus, the output impedance of the amplifier must be minimized. Exercise If the signal delivered to the speaker is equal to , find the ratio of and . The importance of I/O impedances encourages us to carefully prescribe the method of mea- suring them. As with the impedance of two-terminal devices such as resistors and capacitors, the input (output) impedance is measured between the input (output) nodes of the circuit while all independent sources in the circuit are set to zero. Illustrated in Fig. 5.2, the method involves ap- plying a voltage source to the two nodes (also called “port”) of interest, measuring the resulting current, and defining as the impedance. Also shown are arrows to denote “looking into”
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