1
Lecture 21
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db
k
d P
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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
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Based on loop gain calculations (RR…doesn’t have to be T)
–
Extremely useful and easy to use

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