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Bandwidth Rating of Op-Amp.

  1. Feb 13, 2014 #1
    Hello, I have a question that I thought some of you may know an answer to.

    I was recently testing an op amp is several closed-loop feedback configurations. Primarily inverting, and non-inverting.

    When testing the output voltage of an invertin amplifier, I found that the output signal became attenuated at around 1.6MHz. The op-amp's data sheet says the bandwidth is 1.5MHz so this makes sense.

    However, when I did the similar test with a non-inverting amplifier, I could only get up to 29KHz before bad deformation occurred. Why is this? I thought the op-amp was rated at 1.5MHz?

    Does it have something to do with how many capacitors within the op-amp are being used in a non-inverting configuration?

    I haven't been able to find a good answer so I will appreciate any help!
  2. jcsd
  3. Feb 13, 2014 #2


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    Staff: Mentor

    What gains were you testing at? Can you share the schematics of your test configurations?
  4. Feb 14, 2014 #3


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    Gold Member

    The closed loop gain capabilities are a function of the input frequency. If I am not mistaken what would be supplied to you on the specification sheets are the gain bandwidth product of the chip. This can be used, along with your resistors (see 'feedback factor') to find the gain for varying input frequency. The derivation is a little cumbersome, especially in the case of the inverting amplifier, but the end result is that closed loop gain for both configurations only differs by 1 for the same feedback factor. So, for different instances of the same chip, it is odd I believe that you have received such a dramatic bandwidth decrease. As berkeman said it would be a good idea to post how the circuits are connected, I expect these feedback factors are wildly different. The feedback factor is calculated as one divided the non-inverting gain for both an inverting and non inverting amplifier arrangement. This is an oddity but not a unique one in electronics, many things that are common to both configurations (inverting and non inverting) are determined by the non-inverting configuration only. It appears not only here in frequency response but also in noise calculations and beyond.

    Hopefully I didn't lead you astray if I did I am sure someone will set us both straight
  5. Feb 14, 2014 #4
    The LM741 op-amps were powered with +/ 15V. The gain for the inverting amplifier was -3 and the non-inverting was 4.

    I've posted images below of the circuits.

    For the inverting op-amp, Rin = 3.3K ohm and Rf = 10K ohm.

    Here is a screen shot of the output being shifted/deformed at 1.75 MHz which is outside the bandwidth of the op-amp. Refer to the attached screenshots from the oscilloscope.

    The non-inverting op amp has Rg = 3.3K ohm and Rf = 10K ohm. The non-inverting output became deformed at 29Khz. The op-amp is rated at 1.5Mhz. Why is the output signal being distorted this much? Refer to the second attached screenshot from the oscillscope featuring the triangle wave.



    Attached Files:

  6. Feb 14, 2014 #5


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    Staff: Mentor

    Strange. If you use smaller signals, like around 1V max output voltage swing, what do you get for the bandwidths?
  7. Feb 14, 2014 #6
    LM741 has a slew rate around 0.5V/us So the Fmax without distortion for 6V peak output is equal to
    Fmax = (0.5V/us)/( 2 * pi * 6V) = 13.4KHz
    So your op amp has slightly better SR then 0.5V/us.
    Also the small signal bandwidth should be equal to.
    fc = 1.5MHz/gain = 1.5MHz/4 = 375kHz
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