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OPA656: Small Signal Bandwidth and its relationship to the Gain-Bandwidth Product

Stephen Pflanze
Prodigy 40 points
Part Number: OPA656

In this spec sheet:
http://www.ti.com/lit/ds/symlink/opa656.pdf

I see on page 5 a Gain-Bandwidth Product spec of 230 MHz.. There is a test condition for this of G>+10V/V. I interpret this to mean the gain bandwidth product is measured at a frequency where the open loop gain is greater than 10.

Just above this I see in the Small-Signal Bandwidth of 500MHz that is measured under the test condition G = +1 V/V. I interpret this to mean a 500MHz bandwidth with feedback configured to be unity gain (a voltage follower with output connected to the inverting input). But this 500MHz is more than double the Gain-Bandwidth Product. How can this Small-Signal Bandwidth exceed the Gain-Bandwidth Product? The op amp model I am familiar with is that the highest bandwidth possible is the Gain Bandwidth Product divided by the closed loop gain.

  • 0
    TI__Expert 8070 points
    Hi Stephen,

    Many of our high speed amplifiers are compensated in such a way that the phase margin of the amplifier starts to reduce as it tends towards unity gain. The amplifier is still stable in this configuration, but it will cause some peaking in the frequency response, which then extends the bandwidth. To calculate gain bandwidth product, we configure the device in a gain of 10 V/V and then project the intersection of the amplifier's 20 dB/decade roll-off and see at what frequency it crosses 0dB of gain.

    The calculation of gain vs. bandwidth will apply to the gain bandwidth product of the OPA656 for all higher gain values, but we get a slight extension at low gains due to the low phase margin caused by the effect of a second poll in the amplifiers Aol. You typically don't see this behavior in lower speed amplifiers because they are compensated in such a way that the second poll occurs well below 0 dB.

    Regards,