I would like to know how to use the THS4022 with a gain of less than 1. I have read the applications note "Using a Decompensated Op Amp for Improved Performance" (SLYT015 - May 2000) and if I understand correctly, when running at unity gain the device doesn’t have sufficient phase margin at the cross-over frequency (0 dB). However if running with a higher gain, e.g. greater than 10, phase margin is measured where the 20dB mark crosses the open-loop gain. There is quite a bit of margin there, but suppose I want to run with gain of less than one, how is the phase margin measured? At this gain, the phase margin is clearly worse than when the gain is 0 dB. Do I use the phase at this new point, or do I use the phase at the 0dB cross-over, or something else? How hard is it to stabilize an amplifier running with gain less than 0 dB?
When we talk about gain for purposes of stability, we really mean noise gain which is the gain from the “+” input of the op amp to the output with the other signal sources disabled/grounded. In simple terms, add 1 to the absolute value of the inverting gain to get the noise gain. Thus a signal gain of -1V/V is a noise gain of +2V/V. In this case, the gain or phase margin should be based on a noise gain of 6dB, and the gain shaping technique shown in the app note should be based on noise gain of 6dB.
Gain and phase margin are measured with the device open loop, which is difficult because the op amp saturated easily, or by using a servo loop to keep the op amp from saturating and then calculated based on the gain of the loop. I hope this helpful.
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