Three stage amplifier

poles are located as s=+/-1 which is corresponds to an unstable system

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promach said:

As in section 2.2.3 of View attachment 144690 , I have few questions:

1) Why assume "third order Butterworth frequency response with unity gain frequency" ?

Choice for zero peaking response (Butterworth) needs adequate GBW for pole selected. Otherwise there will be peaking, so the assumption may be false. (later he says 10% due to phase margin is the evidence of this false assumption. My tools tell me

1st stage (n=1) Q=0.5 must have GBW=50x breakpoint and
2nd stage (n=2) Q=1 then GBW=100x breakpoint to get
net (n=3 order) Q = 1⁄√2

... use Butterworth for flattest passband frequency response) but is underdamped Q = ​1⁄√2.)
- choose Bessel for maximally flat group delay, since Q is lower. 0.5, 0.69 still needs GBW = 100x breakpoint as 2 decades still has phase shift below breakpoint of open loop.

2) Why negative DC feedback ?

see https://en.wikipedia.org/wiki/Butterworth_filter#Transfer_function .

3) How do we obtain expression (2.2) from (2.1) ?

see above again... poorly defined

4) How do we obtain the expression of open loop undamped natural frequency response, ωno as (sqrt(2) * ωc)?



5) Could anyone elaborate more on "a peaking around 10 % of the final value will be found in the closed loop transient response due to the phase margin value." ?


I think it is due to inadequate open loop GBW

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