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The Signal Sped Up

Stability Issues and Resolutions for High Speed Fully Differential Amplifiers (FDAs), Insight #8

New phase margin issues using RRout high speed VFA FDAs.

Michael Steffes
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5/3/2019 9:13:33 AM
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Open loop to closed loop mapping
Not too sure what you are saying, maybe talking about the internal open loop gain which nominally has a 1 pole frequency response. One of the points is the modern FDAs have multiple open loop poles that (when you go closed loop) depart from the simple gain bandwidth product idea in the close loop response shape. Actually, mapping from the LG to the expected close loop response has been too simplified with the gain bandwidth product idea where the curves I put into Insight #5 get a lot closer. The LG sims here capture all of that in what is actually a > 5th order system if you include the open loop output impedance effects. When I was doing the THS4551 TINA model, the open loop gain is a dominant pole and then 2nd order poles at higher F (and some other things). That plus the open loop output impedance into the RC load and feedback C's pushes you over 5th order, so no equations will help there, but the LG sims map to closed loop responses pretty well. 

5/3/2019 4:34:23 AM
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The FDA gain calculated at DC does not apply at higher frequencies. To a first approximation, the gain of a typical FDA is inversely proportional to frequency. This means that an FDA is characterized by its gain-bandwidth product. See to buy essays. For example, an FDA with a gain bandwidth product of 1 MHz would have a gain of 5 at 200 kHz, and a gain of 1 at 1 MHz. This low-pass characteristic is introduced deliberately, because it tends to stabilize the circuit by introducing a dominant pole. This is known as frequency compensation.

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Probably no single issue faces more high-speed signal path designers (and supplier support teams) than the risk of instability in higher speed op amps and fully differential amplifiers (FDA).
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