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Blackman's theorem

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Blackman's theorem is a general procedure for calculating the change in an impedance due to feedback in a circuit. It was published by Ralph Beebe Blackman in 1943,[1] was connected to signal-flow analysis by John Choma, and was made popular in the extra element theorem by R. D. Middlebrook and the asymptotic gain model of Solomon Rosenstark.[2][3][4][5] Blackman's approach leads to the formula for the impedance Z between two selected terminals of a negative feedback amplifier as Blackman's formula:

where ZD = impedance with the feedback disabled, TSC = loop transmission with a small-signal short across the selected terminal pair, and TOC = loop transmission with an open circuit across the terminal pair.[6] The loop transmission also is referred to as the return ratio.[7][8] Blackman's formula can be compared with Middlebrook's result for the input impedance Zin of a circuit based upon the extra-element theorem:[4][9][10]

where:

is the impedance of the extra element; is the input impedance with removed (or made infinite); is the impedance seen by the extra element with the input shorted (or made zero); is the impedance seen by the extra element with the input open (or made infinite).

Blackman's formula also can be compared with Choma's signal-flow result:[11]

where is the value of under the condition that a selected parameter P is set to zero, return ratio is evaluated with zero excitation and is for the case of short-circuited source resistance. As with the extra-element result, differences are in the perspective leading to the formula.[10]

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See also

Further reading

  • Eugene Paperno (September 2012). "Extending Blackman's formula to feedback networks with multiple dependent sources" (PDF). IEEE Transactions on Circuits and Systems II: Express Briefs. 59 (10): 658–662. CiteSeerX 10.1.1.695.4656. doi:10.1109/TCSII.2012.2213355. S2CID 8760900.
  • Rahul Sarpeshkar (2010). "§10.7 Driving-point transistor impedances with Blackman's formula". Ultra Low Power Bioelectronics: Fundamentals, Biomedical Applications, and Bio-Inspired Systems. Cambridge University Press. pp. 258 ff. ISBN 9781139485234.
  • Amaldo D'Amico; Christian Falconi; Gianluca Giustolisi; Gaetano Palumbo (April 2007). "Resistance of feedback amplifiers: A novel representation" (PDF). IEEE Transactions on Circuits and Systems – II Express Briefs. 54 (4).
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References

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