Why a Buffer stage in between amplifier and LPF?

Discussion in 'General Electronics Chat' started by manjuarun, Apr 19, 2010.

  1. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    What is the use of a buffer stage between an amplifier stage and an LPF stage? My understanding was that its for impedance matching.Or is it for holding the signal for more processing time from amplifier to LPF stage? Is the buffer is same as buffer amplifier, or they are two different circuits? If it is a buffer amplifier how do we choose the gain? Can any body of you please clear my doubts?

    Many Thanks
    Manju
     
  2. kingdano

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    Apr 14, 2010
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    Do you have a specific circuit in mind? Or are these general design questions?
     
  3. manjuarun

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    Mar 8, 2010
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    I want to design a simple signal processing circuit consists of an amplifier and a filter circuit.I got a review comment like just add one buffer between this. But I dont really know that why it is needed? or what will it do with my signal processing.I would like to get a clear understanding of buffer and its purpose,then only I can decide that whether a buffer should be included in my ckt or not.

    Thanks
    Manju
     
  4. t06afre

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    you should post your schematics
     
  5. rjenkins

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    A buffer is, generically, a low or zero gain amplifier for impedance conversion or simply to isolate the previous stage from any loading effects of the next stage.

    As t06afre says, you need to supply more info to get a better reply.
     
  6. t_n_k

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    Zero gain would mean zero signal transmission - gain should be greater than zero.
     
  7. rjenkins

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    Terminology - zero gain as in zero increase of signal level.
     
  8. t06afre

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    Then we talk about gain we often use dB. And a gain of zero dB is equal to a gain of one. So zero gain is more an expression or figure of speech there the dB has been left out
     
  9. manjuarun

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    Mar 8, 2010
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    Thanks rjenkins..

    If buffer is a zero gain amplifier, does it have inverting and non inverting type like normal opamp amplifiers? does it have differential type? How do we ensure the empedance matching?

    Thanks
    Manju
     
  10. rjenkins

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    That's all down to the overall circuit and frequencies involved, which you have not yet provided info on.

    For something like a low frequency sensor signal, it could be an opamp.

    For an RF stage, it would likely be a transistor.

    For matching within a circuit, it's often enough to know that the output impedance of one stage is lower than the input impedance of the next, but again details of the application are required.
     
  11. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    for giving a schematic diagram , its not yet ready. Its only in my mind. But I can give you the details. A differential amplifier with a gain factor of 10 and a Low pass filter.The cut off frequency is 50 Hz. To isolate the amplifier stage from the LPF I can use a buffer right? And also for impedance matching. The LPF, I would like to take is of butterworth type which has flat pass band response(It is the rough idea, need to think on this).

    Ok..Now would u pls give me suggestions ?Can I use an opamp type buffer for my application? What are the other factors I need to study before going for a design?

    Thanks
    Manju
     
  12. manjuarun

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    Mar 8, 2010
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    Please do bear with me If I'm asking irrelevent question.I am just a beginner on this.

    Ok..I will reframe my doubt like this

    A buffer is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. For my application first circuit is an opamp based differential amplifier which has high input impedance and low output impedance. The second stage is an opamp based butterworth low pass filter which also has high input impedance and low output impedance. Then how do a buffer will server here?

    Thanks
    Manju
     
  13. rjenkins

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    From what you describe, it does not sound like a buffer should be needed.
     
  14. t06afre

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    I agree with Robert. But if your teacher advice you to put in a buffer just do it. No point in agruing to much with your teacher ;). It will not do any bad in your circuit, and it is not wrong to put it in. But from the information you have given, not needed.
    Here is a link to a good ebook http://focus.ti.com/lit/an/slod006b/slod006b.pdf See section A.3.17 Buffer ​
     
  15. Darren Holdstock

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    Feb 10, 2009
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    I agree with the other respondents, if the differential amp is constructed around an op-amp then the output impedance should be low enough not to need buffering when driving an active filter input. However, if it's a 2 transistor differential amplifier circuit (AKA long-tailed pair) then a buffer might be necessary.

    The input impedance of an active filter can be complex (depending on architecture), and when plotted against frequency can have the same shape as the transfer function characteristic. I've made sneaky use of this for equalisers based on the old 1970s Williamson graphic EQ circuit - where the original had passive LRC networks, I used the inputs of gyrator and Sallen and Key circuits (like Figure 5 here). Works a treat if it's done right.
     
  16. kingdano

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    Apr 14, 2010
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    Is that your design on the website?
     
  17. chetan mehra

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    Aug 18, 2010
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    See there is a main difference between buffer gate and buffer.a buffer gate is used to enhance the weak signals because every gate is a signal amplifier,in this configuration two not gates are connected in series so that the current sinking or sourcing capabilities of the resulting gate would increase.
    whereas the buffer circuit is used to isolate one circuit to another,it means the changes which occurs in one circuit will not affect the other.(it may be made up of transistors or may not be.)
     
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