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21_Lecutre

# 21_Lecutre - Lecture 21 EE 114 Lecture 21 Feedback and Port...

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EE 114 Lecture 21 R. Dutton, B. Murmann 1 Lecture 21 Feedback and Port Impedances R. Dutton, B. Murmann Stanford University EE114 (HO #25) R. Dutton, B. Murmann 2 EE114 (HO #25) Using Feedback to Modify Port Impedances Feedback can be used to increase/decrease port impedances In fact, we have already seen an example of such behavior (Lecture 19, slide 7…implicitly) Closed loop bandwidth of feedback amplifier can be (1+T) higher than open loop bandwidth This means the impedance seen by the load capacitor must drop in the presence of feedback We can calculate the port impedances of arbitrary feedback circuits using "Blackman's Impedance Formula" Based on loop gain calculations (RR…doesn’t have to be T) Extremely useful and easy to use

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EE 114 Lecture 21 R. Dutton, B. Murmann 3 EE114 (HO #25) Blackman's Impedance Formula 1. First, find port impedance with feedback loop broken E.g. set g m =0 (k=0 in model; can also be a v --see examples) 2. Calculate loop gain in circuit with port under consideration shorted 3. Calculate loop gain in circuit with port under consideration open Note : In many cases, either the "shorted" or "open" loop gain is zero Z port = Z port k = 0 ( ) " 1 + # port shorted ( ) 1 + # port open ( ) First, let’s quickly review a couple of aspects of the RR methodology * * “k” is the terminology used if Gray et al, Ch.8, p. 601-3 R. Dutton, B. Murmann
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21_Lecutre - Lecture 21 EE 114 Lecture 21 Feedback and Port...

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