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Reg:Gain effect on Opamp bandwidth

KISHORE TERLAPU
Prodigy 30 points
Other Parts Discussed in Thread: THS4131

Dear Sir/Madam,

I am using THS4131 high speed differential opamp in my design for differential outputs.

I have seen the open loop response of the opamp it was very high (34MHz) and it is acceptable.

After that I will connect the circuit according to http://www.ti.com/lit/an/slyt343/slyt343.pdf figure3 it's -3dB bandwidth inBode plot is 10.48kHz.For the same circuit I will change the unity gain to gain 4.Then it's bandwidth at -3dB in bode plot is 3.18kHz. I don't know how it's happened.Is bandwidth of opamp is depends on gain. Here I am also attaching the gain 4 differential amplifier circuit diagram.

 

Please tell me the things which cause this effect. Please reply to this question inorder to overcome this type of problem in  future.

Thanks&Regards,

T.Kishore

 

  • 0
    TI__Genius 13935 points

    Hello T.Kishore,

     

    I am not sure what you are asking.  The document you referred to is an app note on filters.  Filters shape the frequency response the way they are designed.  The amplifier gain bandwidth product is usually far below the filter response frequency, so the amplifier gain should not make that much difference.

    What frequency response do you want?    You may want to download the TINA program and simulate your circuit to see how the gain impacts the frequency response.  http://www.ti.com/tool/tina-ti

    In general, the gain will impact the amplifier andwidth with a (Gain x Bandwidth ~= constant) manner.  But when using an amplifier in a filter you will want the filter frequency to be much lower than the overall bandwidth.  That is the case for this amplifier and audio frequency filters. 

     

    Regards,

    Loren

     

  • 0
    TI__Genius 13935 points

    Hello T.Kishore,

    Also, I do not understand how you changed the amplifier gain without having to redesign the filter.   I don't think you can change components in Figure 3 without a complete redesign.

    Sallen Key filters have gain independent of the filter response, but most other filters do not.

     

    Loren