Op Amp Feedback Biasing Voltage

Discussion in 'General Electronics Chat' started by alt234, Mar 12, 2009.

  1. alt234

    Thread Starter New Member

    Mar 12, 2009
    3
    0
    I have a question that has been annoying me for quite some time

    [​IMG]

    In A) The capacitor C2 is connected to ground and in B) it's connected to Vref.
    Can anyone explain how the difference between the two affects the circuit and if one method is preferable to the other.

    Thanking you for your time and trouble.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    In the case where a capacitor is present in the feedback circuit (your first two circuit examples), the opamp should work the same in either instance. Remember that the capacitor blocks DC so the opamp's negative terminal will alway return to the same voltage as the voltage present at the opamp's positive input in the presence of a DC input signal.

    The AC signal gain as a function of frequency will be the same in either case.

    hgmjr
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The circuit in figure 3 does not have capacitor coupling for R3 so the opamp amplifies its own input DC offset voltage.
     
  4. italo

    New Member

    Nov 20, 2005
    205
    1
    Circuit A will saturate at power on same foer circuit B Circuit C is stabe and the gain can be calculated by R2/R3. capacitors isolate DC inputs in my books
     
  5. alt234

    Thread Starter New Member

    Mar 12, 2009
    3
    0
    Thank you for your answers. If I may extend:

    If I obtain my bias voltage (Vref) from a single power supply - 9V battery and a voltage divider, two (R4 and R5) resistors then to my mind the a.c. gain in circuit A will be greater than circuit B. This is because the a.c. path to ground in circuit B is through the lower resistor (R5) of the voltage divider.

    The a.c. gain in circuit A will be 1+ the impedance (read resistance) of R2 divided by the impedance (resistance and reactance) of R3 and C2.
    In circuit B it will be 1+ the impedance (read resistance) of R2 divided by the impedance (resistance and reactance) of R3 plus R5 and C2.

    Am I correct in my thinking or, as Mr Bob Dylan said, just blowin in the wind.

    I can see that in a dual power supply this problem doesn't arise as there is no voltage divider and the internal resistance of the battery has little effect.

    My question originally comes from looking at various guitar effects circuits which use a single power supply, some connect C2 to ground and some to Vref - I could never work out why, I thought there must be a reason as circuit designers must know more than I do.

    Again, thanking you for your time and trouble.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    896
    Vref is a low impedance because it is bypassed to ground with a capacitor. Then it is the same as ground.
     
  7. alt234

    Thread Starter New Member

    Mar 12, 2009
    3
    0
    Thank you audioguru !!! Actually, I feel quite embarrased - the answer has been staring me in the face all along - just needed somebody with a sharp stick to give me a prod. Now I can get a good nights sleep
     
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