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
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This note was uploaded on 11/02/2011 for the course EE 114 taught by Professor Murmann during the Fall '08 term at Stanford.

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

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