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I'm trying to design simple voltage regulator with differential feedback. Vin = 12V Vout = 2-5V Iout = 0-7A

In simulator I can see large oscillations. The larger the output current, the higher oscillations. When I remove the first op amp, the regulator works fine.

How to fix oscillations in cascaded op amp schematic?

UPD sorry for bad schematic, I've redrawed it.

vreg sch

I was promped to add a capacitor C3 between negative input and output of U1B:

vreg sch no osc

And now it works fine, there is no oscillations. Is it right solution? And how exactly this capacitor stabilizes the schematic?

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  • \$\begingroup\$ What improvement were you trying to get by introducing a second op-amp in this design? \$\endgroup\$ – The Photon Jul 17 '17 at 18:46
  • \$\begingroup\$ The second op amp works as differential amplifier, I've updated the schematic so that it could be seen. \$\endgroup\$ – Ivan Peresedov Jul 17 '17 at 22:42
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The circuitry has a problem because you are getting your feedback from the driver output, which introduces a phase shift; when added to the phase shift of the two stages, your negative feedback becomes positive. The driver is only capable of driving current into your output capacitors; when U1-B output goes high, Q1 quickly charges C1 and C2 (and overshoots a little in the oscillating case). However, when U1-B drives in the other direction to compensate, Q1 does not drive the capacitors but can only turn off, and R7 must discharge the caps (hence the phase shift). U1-B goes into negative saturation before this occurs, and then must recover into its active region. When it does it overshoots again. The first stage does not add anything to this circuit but does slow the response and adds to the phase shift.

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