ho24.l21_fb3_updated

ho24.l21_fb3_updated - Lecture 21 Feedback and Port...

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1 Lecture 21 Feedback and Port Impedances (updated Slides 6, 10, 11) R. Dutton, B. Murmann Stanford University R. Dutton, B. Murmann 1 EE114 (HO #24) 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 In fact, we have already seen an example of such behavior (Lecture 19, slide 7) • 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" R. Dutton, B. Murmann 2 EE114 (HO #24) 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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2 Blackman's Impedance Formula Z port = Z port k = 0 () 1 + ℜ port shorted ( ) 1 +ℜ port open 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 R. Dutton, B. Murmann 3 EE114 (HO #24) In many cases, either the "shorted" or "open" loop gain is zero First, let’s quickly review a couple of aspects of the RR methodology s ic + - s oc + - ks ic + - s in + - s out + - Reminder about RR Formulation
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This note was uploaded on 11/09/2009 for the course EE 114 at Stanford.

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ho24.l21_fb3_updated - Lecture 21 Feedback and Port...

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