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Lecture 9 - Lecture 9 Introduction to electronic analog...

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Lecture 9: Introduction to electronic analog circuits 361-1-3661 1 6.3. Effect of negative feedback on the input and output impedances To determine the effect of negative feedback on the input and output impedances let us first determine the effect of feedback on the impedance at an arbitrary port x [see Fig. 1(a)]. (This impedance will be equal to the circuit input or output impedances if the port is chosen at the circuit input or at the circuit load.) To find the impedance R x at port x , we first connect a voltage test source v t to the port and obtain v t as a function of the circuit return ratio. Since the port x is short-circuited when the return ratio is calculated, we call it as RR SC : v SC t SC t v v v G RR s v RR s v G s s s ) 1 ( + = - = + = ε ε ε ε ε , (1) where G v is the input transmission from the voltage test source to the control terminals of the dependent source. We second connect a current test source i t = v t / R x [see Fig. 1(b)] to the port and obtain i t as a function of the circuit return ratio. Since the port is open-circuited when this return ratio is calculated, we call it as RR OC : i OC t OC t i i i G RR s i RR s i G s s s ) 1 ( + = - = + = ε ε ε ε ε , (2) where G i is the input transmission from the current test source to the control terminals of the dependent source. Note that s ε in (1) and (2) have the same values, because i t and v t in Figs. 1(a) and (b) are the same. From (1) and (2), R x can be expressed as follows: OC SC v i t t x RR RR G G i v R + + = = 1 1 . (3) To find the ratio of the transmissions G i and G v in (3), we suppress in Fig. 1(c) the dependent source and connect to port x a current source
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