Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

A a diode operating as a rectifier b complete

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(a) A diode operating as a rectifier, (b) complete rectifier, (c) input and output waveforms, (d) input/output characteristic. (3.7) (3.8) Thus, the average is proportional to , an expected result because a larger input amplitude yields a greater area under the rectified half cycles. The above observation reveals that the average value of a rectified output can serve as a mea- sure of the “strength” (amplitude) of the input. That is, a rectifier can operate as a “signal strength indicator.” For example, since cellphones receive varying levels of signal depending on the user’s location and environment, they require an indicator to determine how much the signal must be amplified. Example 3.9 A cellphone receives a 1.8-GHz signal with a peak amplitude ranging from 2 V to 10 mV. If the signal is applied to a rectifier, what is the corresponding range of the output average? Solution The rectified output exhibits an average value ranging from 2 V to 10 mV/ mV. Exercise Do the above results change if a 1- resistor is placed in series with the diode?
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 70 (1) 70 Chap. 3 Diode Models and Circuits In our effort toward understanding the role of diodes, we examine another circuit that will eventually (in Section 3.5.3) lead to some important applications. First, consider the topology in Fig. 3.11(a), where a 1-V battery is placed in series with an ideal diode. How does this circuit behave? If , the cathode voltage is higher than the anode voltage, placing in reverse bias. Even if is slightly greater than zero, e.g., equal to 0.9 V, the anode is not positive enough to forward bias . Thus, must approach +1 V for to turn on. Shown in Fig. 3.11(a), the I/V characteristic of the diode-battery combination resembles that of a diode, but shifted to the right by 1 V. V I 1 D V I (a) 1 1 V B 1 1 1 V +1 V 1 D V V R 1 out V in out V in 1 t t in V out V V 1 D V B 1 V in R 1 out V t out V in V + V p V p t V p + 1 V V out V in 1 +1 V +1 V (c) (b) Figure 3.11 (a) Diode-battery circuit, (b) resistor-diode circuit, (c) addition of series battery to (b). Now, let us examine the circuit in Fig. 3.11(b). Here, for , remains off, yielding . For , acts a short, and . The circuit therefore does not allow the output to exceed zero, as illustrated in the output waveform and the input/output characteristic. But suppose we seek a circuit that must not allow the output to exceed V (rather than zero). How should the circuit of Fig. 3.11(b) be modified? In this case, must turn on only when approaches +1 V, implying that a 1-V battery must be inserted in series with the diode. Depicted in Fig. 3.11(c), the modification indeed guarantees V for any input level. We say the circuit “clips” or “limits” at +1 V. “Limiters” prove useful in many applications and are described in Section 3.5.3.
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