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Fundamentals of Microelectronics, 2nd Edition by Behzad Razavi

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Diode Models and Circuits

Having studied the physics of diodes in Chapter 2, we now rise to the next level of abstraction and deal with diodes as circuit elements, ultimately arriving at interesting and real-life applications. This chapter also prepares us for understanding transistors as circuit elements in subsequent chapters. We proceed as follows:

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   3.1 IDEAL DIODE

   3.1.1 Initial Thoughts

In order to appreciate the need for diodes, let us briefly study the design of a cellphone charger. The charger converts the line ac voltage at 110 V1 and 60 Hz2 to a dc voltage of 3.5 V. As shown in Fig. 3.1(a), this is accomplished by first stepping down the ac voltage by means of a transformer to about 4 V and subsequently converting the ac voltage to a dc quantity.3 The same principle applies to adaptors that power other electronic devices.

How does the black box in Fig. 3.1(a) perform this conversion? As depicted in Fig. 3.1(b), the output of the transformer exhibits a zero dc content because the negative and positive half cycles enclose equal areas, leading to a zero average. Now suppose this waveform is applied to a mysterious device that passes the positive half cycles but blocks the negative ones. The result displays a positive average and some ac components, which can be removed ...

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