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VCVS gain control... help needed

Discussion in 'General Electronics Discussion' started by zealer, Dec 25, 2011.

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  1. zealer

    zealer

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    Dec 25, 2011
    Hello everyone.

    I am trying to find out why I cannot use the neg feedback voltage divider on the famous VCVS topology (Ra and Rb from the fig below) to simply control the gain of a bandpass filter?
    [​IMG]

    Using LTSpice, any attempt to change the gain using the above method fails. Results do not meet expectations at all and frequency response gets worse. According to a lot of info sources,
    Code:
    "[I]A definite advantage is the ability to precisely set the gain of the filter with resistors Ra and Rb, without upsetting the center frequency.[/I]"
    Now what am I missing here? Using the BP filter below, that has a center freq 4.5 kHz, I can control pretty much nothing with Ra and Rb... it just gets worse.
    [​IMG]

    If I set Ra = Rb, I get a unity gain at the filter output. What must I do to get 2x gain? Standard non-inverting op amp negative feedback formulas don't work :/

    Thank you in advance,
    Simeon
     
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