Amplifying an Active Band-Pass Filter Ouput

Discussion in 'The Projects Forum' started by Chardi, May 14, 2016.

  1. Chardi

    Thread Starter New Member

    Oct 3, 2015
    28
    1
    After a day of research and troubleshooting, I'm once again resorting to you guys' knowledge to provide some help for my project, so thank you all in advance!

    I have an active BPF which I have designed and made into PCB (it works very well) which outputs a signal of around 500mV with my pre-amp circuit and all; however, I wish to amplify it to a 5V (2.5Vpp) signal.

    My problem is, connecting another op-amp and amplifying the signal does not work at all. There's no signal coming out of the output pin. Any idea what is my issue?

    Here is a schematic of my band-pass filter:
    Capture.PNG

    The output of UA4 is the final output of my band-pass filter!

    *Yes, I have doubled each filter (LPF & HPF) because it resulted in a cleaner, more precise frequency response for some unknown reason

    Thanks again!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    You are using a single supply voltage and the amp is biased at 0V so it can't amplify any negative going signals.
    You need to bias the amps at 1/2 the supply voltage.
    A resistive divider can be used for that purpose.
    Connect the divider output in place of the ground connection for R2 and R4.

    Also 33MΩ is way to large a resistor value for R2 and R4 as it will cause a large offset from the LM324's bias current.
    Try to keep the value no more than a Megohm.
    This will mean the associated resistors and capacitors will need to be changed proportionally also to keep the frequency response the same.

    Of course doubling the filter stages will increase the sharpness of the rolloff.
    But if you need that, then the values should be optimized for a proper 4-pole filter, which connecting two 2-poles in series does not.
     
  3. Chardi

    Thread Starter New Member

    Oct 3, 2015
    28
    1
    From what I can get with this, I connect the 1/2 supply voltage from the voltage divider to R2 and R4 (so between the two resistors), which will allow amplification of negative signals. I'll try this on breadboard with my current design and see if it helps!

    I'll also try to play with the component values to lessen the resistance of R2 and R4, but that's less important unless it plays a role in my current issue (as well as the doubling of my filters) though I thank you for the advices!

    I will be back with some updates if anything happens!
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The voltage drop across 33MΩ from the LM324 80nA nominal input bias current will give an offset voltage of 1.32V.
    That may not be a problem in your circuit as long as you AC couple the output signal.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    What are your highpass and lowpass corner frequencies?
    What is the DC voltage on the U4A output pin when the XFG1 AC output voltage is 0 V?

    ak
     
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