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OPA355: gain bandwidth product (OPA355)

vemireddi Nagamani
Prodigy 10 points

Part Number: OPA355
Other Parts Discussed in Thread: OPA827, OPA847

Hello Team,

In the OPA355 datasheet, multiple bandwidth-related parameters are specified, which is causing some confusion in deciding which parameter should be used for a given application.

For OPA355, the datasheet mentions:

  • Unity-gain bandwidth ≈ 450 MHz

  • Gain-bandwidth product ≈ 200 MHz at gain = 10

  • Small-signal −3 dB bandwidth values specified at different closed-loop gains

Since these values are not identical, it is unclear which gain bandwidth product specification should be considered during design.

My questions are:

  1. For a specific closed-loop gain, should the small-signal bandwidth specified at that gain be preferred over unity-gain bandwidth or GBW-based estimation?

  2. In what type of applications should each parameter be used?

    • Unity-gain bandwidth

    • Gain-bandwidth product specified at a particular gain

    • Small-signal bandwidth at a given gain

  • 0
    TI__Mastermind 31915 points

    Hello Vemireddi,

       Thank you for reaching out. We specify various bandwidths in the datasheet in order to make it easier to determine which amplifier would work best depending on the use-case. As you mentioned, the value you would need to look at depends on your design. 

       For example the OPA355,

    • Unity-Gain bandwidth: This is the value of the closed-loop bandwidth at unity gain (Gain = 1V/V)

    • Gain Bandwidth Product: This is an estimated value of how bandwidth might change over various gains. The equation is as follows: GBW = Gain (V/V) * Closed loop Bandwidth. As you pointed out, it is specified at 10V/V, this means this equation is only valid for that gain. However, if you look at the lower-bandwidth amplifiers it will have a much wider range usually for all gains or it will specify at >= a gain. The reason is that when you get to the higher speed amplifiers, their AOL at higher frequencies can be within the range of the device and will have complex poles/zeros compared to a lower speed amplifier with usually a single pole AOL and maybe have some affects from a second pole later in higher frequencies that might affect the closed loop bandwidth. 
      • For example the OPA847 a high speed amplifier vs the OPA827 a lower speed amplifier:
      •             

      • Therefore, in high speed case, if you expect to have a gain that is not unity gain (1V/V). We would suggest looking at the frequency response curves that are after the electrical characteristic tables. For the OPA355, this would be on page 10 and 13. We will have even more information at this figures showing over power supply or over capacitive or resistive load. I would recommend looking at this figures over the value you see in the electrical characteristics table. 

    • Small Signal and Large Signal Bandwidth: This value is showing the closed-loop bandwidth differences between if you have an small signal input to the amplifier versus a large signal input. For small signal, this is usually referring to 100mV or less (sometimes maybe up to 500mV or less). The amplifier with small input signals will have higher closed loop bandwidth compared to when you have a larger input signal. This will depend what your input signal range will be for your application. If you expect larger than 100s of mV, we would suggest looking at the amplifier large signal bandwidth specifications. 

       For more detailed information on all the above terms, I recommend looking at this TI precision lab video series on op amp bandwidth theory: https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html#transcript-tab. 

    Thank you,

    Sima