# Example 129 with a current gain of unity the topology

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Example 12.9 With a current gain of unity, the topology of Fig. 12.17(d) hardly appears better than a piece of wire. What is the advantage of this circuit? Solution The important property of this circuit lies in its input impedance. Suppose the current source serving as the input suffers from a large parasitic capacitance, . If applied directly to a resistor [Fig. 12.18(a)], the current would be wasted through at high frequencies, exhibiting a C R P D in out V i M 1 R D V DD V b V out C P in i I 1 (a) (b) Figure 12.18 -dB bandwidth of only . On the other hand, the use of a CG stage [Fig. 12.18(b)] moves the input pole to , a much higher frequency. Exercise Determine the transfer function for each of the above circuits. Recall from Chapter 7 that the gain of the source follower is equal to unity in this case.

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 618 (1) 618 Chap. 12 Feedback 12.4 Sense and Return Techniques Recall from Section 12.1 that a feedback system includes means of sensing the output and “re- turning” the feedback signal to the input. In this section, we study such means so as to recognize them easily in a complex feedback circuit. How do we measure the voltage across a port? We place a voltmeter in parallel with the port, and require that the voltmeter have a high input impedance so that it does not disturb the circuit [Fig. 12.19(a)]. By the same token, a feedback circuit sensing an output voltage must appear in (a) V out Voltmeter Feedforward System Feedback out V A R 1 R 2 1 out V V F in V (c) (b) Network Figure 12.19 (a) Sensing a voltage by a voltmeter, (b) sensing the output voltage by the feedback network, (d) example of implementation. parallel with the output and, ideally, exhibit an infinite impedance [Fig. 12.19(b)]. Shown in Fig. 12.19(c) is an example, where the resistive divider consisting of and senses the output voltage and generates the feedback signal, . To approach the ideal case, must be very large so that does not “feel” the effect of the resistive divider. How do we measure the current flowing through a wire? We break the wire and place a current meter in series with the wire [Fig. 12.20(a)]. The current meter in fact consists of a R I out Current Meter R I out L L Feedforward System Feedback r Voltmeter 0 I out (c) (a) (b) M 1 R S I out V F (d) Network Figure 12.20 (a) Sensing a current by a current meter, (b) actual realization of current meter, (c) sensing the output current by the feedback network, (d) example of implementation. small resistor, so that it does not disturb the circuit, and a voltmeter that measures the voltage drop across the resistor [Fig. 12.20(b)]. Thus, a feedback circuit sensing an output current must appear in series with the output and, ideally, exhibit a zero impedance [Fig. 12.20(c)]. Depicted in Fig. 12.20(d) is an implementation of this concept. A resistor placed in series with the source
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 619 (1) Sec. 12.4 Sense and Return Techniques 619 of senses the output current, generating a proportional feedback voltage, . Ideally, is so small ( ) that the operation of remains unaffected.

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