Transimpedance Amplifier Bandwidth Calculation

Discussion in 'General Electronics Chat' started by dannybeckett, Apr 17, 2013.

  1. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Hi guys -

    I'm trying to calculate the bandwidth of my transimpedance amplifier. I was assuming that you can simply divide the GBWP by the feedback resistor you incorporate (and hence the transimpedance gain) to find your GBWP limit, but I don't think this is the way to do it. I would appreciate a more knowledgable insight into this calculation!

    Thanks a lot
     
  2. crutschow

    Expert

    Mar 14, 2008
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    You calculate the amplifier circuit bandwidth by calculating the gain from the non-inverting (+) input to the output and dividing the GBWP by that gain. Thus if you have an inverting gain of -1, the gain from the non-inverting input would be 2 so you divide the GBWP by 2 to get the circuit bandwidth. This assumes there are no other frequency determining components in the op amp circuit.
     
    Last edited: Apr 17, 2013
  3. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    I thought this was the case too crutschow, but transimpedance amplifiers are not governed by the GBWP in the same way normal amplifiers are -

    Page 11: http://www.ti.com/lit/ds/symlink/opa846.pdf

    "Adding the common-mode and differential-mode input capacitance (1.8 + 2.0)pF to the 50pF diode source capacitance of Figure 3, with a 10kΩ transimpedance gain using the 1750MHz GBP for the OPA846, requires a feedback pole set to 16.1MHz. This requires a 1pF total feedback capacitance. Typical surface-mount resistors have 0.2pF parasitic capacitance leaving a required extrinsic 0.8pF value, as shown in Figure 3. Equation 2 gives the approximate –3dB bandwidth, if CF is set using Equation 1."

    Equation 2:
    [​IMG]

    This theory is backed up at the beginning of the OPA656 datasheet:
    http://www.ti.com/lit/ds/symlink/opa656.pdf

    "Broad transimpedance bandwidths are achievable given the OPA656’s high 230MHz gain bandwidth product. As shown below, a –3dB bandwidth of 1MHz is provided even for a high 1MΩ transimpedance gain from a 47pF source capacitance."

    [​IMG]

    If transimpedance amplifiers worked as normal voltage amps do, a 1MΩ gain would result in a bandwidth of 230Hz, using this specific op amp.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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  5. crutschow

    Expert

    Mar 14, 2008
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    Well, I did give myself an out with my statement "This assumes there are no other frequency determining components in the op amp circuit." :rolleyes: Obviously the various input stray capacitances do affect the bandwidth.
     
  6. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
    163
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    Ron, thanks for those links - I'm studying them as we speak
     
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