Blackman's Impedance Formula Explained

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Blackman's Impedance formula (Blackman's Theorem) is a return-ratio based method that allows us to easily calculate the input and output impedance of a complex feedback circuit. In this video I go over the case of the noninverting amplifier.
Hope you found this video helpful, please post in the comments below anything I can do to improve future videos, or suggestions you have for future videos.

Опубликовано:

9 дек 2018

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Комментарии : 14

@FH_Forge 5 лет назад

thank you so much for this video and clear explanation! You explained it very well

@federicosirica3696 4 года назад

Thank you. Simple and clear explanation.

@navidrad1680 2 года назад

r0_out with a=0 should be seen from the spesific Output-port into the resistor network. Hence from your Vout r0_out= ro||(Rf +(rd|| R1))~ ro= 50-150 [ Ω]. Therefor Ro~ ro/(T)~ 0 [Ω] ; and this makes sense, since a voltage generator has ideally zero impedence. thank you for your effort and explanation.

@shubham1999 5 лет назад

Sir, thanks for uploading useful videos regularly now. Can I ask you what are Quasi Fermi level?

@kkjj4937 3 года назад

thank you so much

@buttercup7717 Год назад

Is there any example where you perform blackman analysis of more complex circuits like a current mirror ( Wilsons) or OTAs. It will help in better understanding! Thanks a lot for the quality contents!

@jjw7503 3 года назад

Thank you for the video! I'd like to ask that if the feedback is a complex impedance, is the expression still the same?

@JordanEdmundsEECS 3 года назад

Good question, I believe so, yes.

@francorenatocampanavalderr2109 2 года назад

Yes, You can use the Blackman theorem for complex impedances

@bhongusa Год назад

The result is wrong. If Ro = 0, both Rout and Rout(A->0) = 0. Basically, the op amp has no output resistance, and so you see a short circuit to ground when looking into its output. That zero resistance is in parallel with R1+RF, which is still zero. To convince you of this, look at the result if Ro != 0. When A->0, the output resistance is Rout(A->0) = Ro || (R1+RF), and the return ratios are RR_sc = 0 and RR_oc = A*R1/(R1+RF+Ro). Putting everything together, Blackman’s Formula gives Rout = Ro*(R1+RF)/[(1+A)R1+RF+Ro], which you can also verify with a direct nodal analysis of the full feedback circuit. Setting Ro=0 zeroes out the output resistance of the entire circuit.

@vdesmedt2 Год назад

Hi Brian, I completely agree: both an independent and depedent voltage source have a zero output impedance, so if A approaches zero, this is just connection to ground (with an Ro of the amplifier in series), rather than an open circuit.

@jangjay 2 года назад

Thanks for the video. Btw, I cannot imagine a case that would make Rshort not being 0. Any examples?

@arnold5328 4 года назад

This seems wrong. R0 doesn't show up in the Endresult. If r0 is zero, your output impedance is zero.

@navidrad1680 2 года назад

I agree with you; He forgot to take ro=50-200 [Ω] in consideration For this output-voltage topology Ro should be ro/(T)~ 0 [Ω] ; and this makes sense, since a voltage generator has ideally zero impedance.