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AC coupled (HPF) non-inverting amplifier circuit

hua liao
Prodigy 50 points

hello;

in this chapter, how this 1.557 comes out ?

thanks !

  • 0
    Guru 48395 points

    The general equation for repeated real high pass poles is 

    F-3dB (overall) =F-3dB (stage)/(square root((2^(1/n))-1)

    If you put n=2 into this, it will solve to 1.5538. So that 1.557 should have been 1.554 but you get the idea. 

    You are essentially setting each HP 1st order stage at your target overall divided by 1.554 to get the combination at the target. 

    Just cascading series RC stages won't do this as they would need to be isolated by amplifiers - not sure the context where you got this. 

  • 0 in reply to Michael Steffes
    Guru 140200 points

    Good morning Michael,

    seems to come from this document:

    CircuitCookbook-OpAmps.pdf

    See page 105.

    Wish you a happy Thanksgiving :-)

    Kai

  • 0 in reply to kai klaas69
    Guru 48395 points

    I will take a look, happy thanksgiving to you as well - 

    Here is a better view of that repeated HP pole equation where "n" is the number of poles, 2 in this case, 

    The example on page 105 does isolate the 2 single order HP poles, but it does not seem they are equal in this case. But the topology is ok to get two real HP poles. 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Going just a little further with that example on page 105, 

    Aiming for a HP at 16Hz, that places each repeated HP pole target at 10.3Hz

    I understand the standard value argument, the topology suggests using cap at a 2X ratio, 

    Staying with the 15uF on the inverting side, the input R (E96 steps) would be 1.03kohm exact, 1.02kohm E96

    Then, make the non-inverting input C 1/2 this value or 7.5uF - then the parallel combination of those 2 R's needs to be 2.04kohm or double that for each value 4.08kohm exact, or 4.02kohm E96. 

    Then using standard RC, the HP on the inverting side is at 10.4Hz and on the non-inverting side at 10.6Hz - this will get you closer to a 16Hz resulting HP. 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    And here are those changes, the midband gain is 19.92dB, so -3.02dB down is 16.9dB spans, here those are nominally 16.18Hz to 1.05Mhz, little closer on that 16Hz target than the 15.6Hz on page 107

     And the TINA V7 file, 

    TLV9062 2pole HP.TSC

  • 0 in reply to Michael Steffes
    Prodigy 50 points

    hello:

    many thanks for your detail explanation !!