Obtaining Maximum Feedback and Desired Phase Margin

R. Mitchell (UK)


Feedback Systems, Bode diagrams, Asymptotic analysis.


In his classic book, Network Analysis and Feedback Amplifier Design[1], Bode describes how to obtain using asymptotic analysis, for a specified amount of bandwidth, gain margin and phase margin, the `maximum obtainable feedback'. This means, how to get the largest possible low frequency loop gain for a single loop amplifier. One problem with the method is that, particularly when specifying too high a bandwidth, it is not possible to achieve the specified phase margin, with the consequence that the system step response can be too oscillatory. This paper considers ways in which the phase margin can be achieved, whilst still achieving the maximum obtainable feedback over a large bandwidth.

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