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    OP Amp circuit tweak to reduce gain and increase speed?

    Afternoon All,

    I have been tinkering with the circuit below to amplify output from a Piezo disk but need to make some tweaks - my knowledge is not up to speed with OP Amps so wondering if I can swing this one past you good folk for thoughts?

    Basically the circuit has too much gain for my application and I am struggling to get the control I need from the POT as its all down one end - I guess the first question is what is the gain of this circuit and which resistors would I change to which values to say halve the gain?

    The next question is speed, I think this circuit is designed for audio out so it has some capacitors which are smoothing the output signal I guess - if I am more interested in speed (ie I just want to see the output as quickly as possible after the input) can I just remove these to achieve this end or is there a better way? Also, any idea how I would know the time delay between input and output?

    Thanks for any thoughts

    Piezo.png

    •   Alt8th June 2013, 17:26

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    Re: OP Amp circuit tweak to reduce gain and increase speed?

    well, your first question requires calculations (small though) so I would leave that for yourself to do. For the second question, you are getting it wrong, at output you see the filter, which smooths the signal, that doesnot mean its decreasing the speed of the input, its just removing the high frequency ripples. A low pass filter to block high frequency components.



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    Re: OP Amp circuit tweak to reduce gain and increase speed?

    There is no HP filtering in the signal path so you are getting the maximum bandwidth the op amps can provide. The capacitors shown are just to filter the supply voltage and the pseudo ground provided by R2 and R3, and to block the DC voltage at the output.

    If the gain pot is at its minimum value then the gain should be essentially zero.
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    •   Alt8th June 2013, 20:09

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    Re: OP Amp circuit tweak to reduce gain and increase speed?

    Thanks folks,

    Sounds like the capacitors will not produce a time lag between input and output then

    On the gain question, may I ask which 2 resistors in the circuit control the gain?

    Thanks



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    Re: OP Amp circuit tweak to reduce gain and increase speed?

    The gain is basically set by the ratio PT1/R5. Some restrictions should be considered:

    - According to the limited OP gain-bandwidth product (about 2 MHz for TLC272) a higher gain will involve a lower bandwidth.
    - The overall DC-coupling generates an output offset of input offset times gain. AC coupling might be reasonable.



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    Re: OP Amp circuit tweak to reduce gain and increase speed?

    The first opamp is just a "unity gain follower", so gain is 1, and you get maximum bandwidth from the first opamp (IC1A), hence lowest delay.

    The second opamp is just an inverting opamp, and it suprises me that you have too much of output signal. When the potentiometer is zero ohms (that is the sliding contact is at position A), the gain would be zero. If you have problems with controlling the gain, reduce the value of the potentiometer (for example go from 1 MOhm to 10K). The gain of the second stage = -Rpotentiometer/R5. Increasing R5 may result in reduced bandwidth.

    There is no low pass filtering (smoothing) in the signal path. C3 is just for smoothing the virtual ground to enable single supply (instead of pos. and neg. supply).

    Depending on the disk, and how you mechanically excites it, the output voltage of the disk can be well above the supply voltage and you may overdrive the opamps resulting in increased delay due to saturation issues. The first opamp may also overdrive the second one, resulting in unpredictable results.

    Try to know your input signal to make sure that this is the right circuit for your task. Input filtering and clipping may be required depending on your application.



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    Re: OP Amp circuit tweak to reduce gain and increase speed?

    Thanks for taking the time to explain folks, much appreciated - based on what you have said I'm going to try and alter the circuit I have used so far to see if I can get the output I am looking for.

    I will report back what I find - it sounds like I may have to consider a different circuit but lets see



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