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I am attempting to decipher the datasheet for TI's LM6171 op-amp.

Here is the open loop frequency response from the datasheet:

Open loop frequency response for LM6171

How can I tell which curve is the gain and which curve is the phase? They're not labeled. I would guess that the top curve at 10 kHz is the gain and the bottom is the phase, but I don't know how to confirm this.

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For the main part of the open loop response a typical op-amp acts like an integrator and therefore shifts the phase angle by about 90 degrees. The curve that looks most like 90 degrees is the phase response. Here's the response for an OPA192: -

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Taken from this blog. And below is a generic op-amp's bode plot: -

enter image description here

Taken from here and below an op77: -

enter image description here

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  • \$\begingroup\$ Okay, perfect. I'll wait a bit and then accept your answer for being the fastest gun. \$\endgroup\$ – Greg d'Eon Mar 24 '15 at 17:49
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This is a dominant-pole compensated op amp, so the gain curve starts out with high gain at low frequency and rolls off with a slope of -20dB/decade. The phase starts off at low frequency with a -90 degree shift and stays relatively constant until the other poles (due to transistor Ft etc.) start to influence it. So the top curve at 10kHz is the gain.

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There are a couple clues you can look at to determine which line is gain and which is phase. To get near-ideal performance, the open loop gain should be very high -- sometimes as high as 120 dB (1,000,000x). The gain wouldn't start out at 20 dB (10x). Meanwhile, when the phase response is flat, it should be sitting at a multiple of 90 degrees. Phase doesn't change at a constant rate over four orders of magnitude, either. Magnitude, however, can, and on the gain axis the top line shows a slope of ~20 dB/decade, which is the sort of number you'd expect to see from a magnitude.

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