Suppose I have a circuit like the one below. In simulation, at the point of maximum current draw, I measure the current through R1 to be ~-10mA, R18 to be ~28mA, R19 to be ~28mA, R3 to be ~-25mA, and R5 to be -25mA (where positive currents flow to the right).

If I were considering each of these op amps individually, I would design my power supply to be able to deliver at least 28mA to U1 (neglecting quiescent current draw), |28mA|+|-25mA| to U6 (neglecting the bottom branch), and 25mA to U4. However, using simple junction rules, I can see that at the junction at the output of U6, 25mA flows in from the right while 28mA flows out, so therefore the output current need only be 3mA (this is in the low frequency regime, the capacitors are not conducting). This 28mA flows to the left through R18, and at the junction to the left of R18, 10mA flows out, so the op amp needs to supply (or sink) only ~18mA.

Is this the proper approach? How should I properly analyze the output current requirements of my op amps so that I can choose a proper power supply for circuits like this?

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  • \$\begingroup\$ You talk about the "junction at the output of U3", but I don't see any U3 in your schematic. Can't you just measure the current through the sources producing VN10 and V10? \$\endgroup\$ – The Photon May 6 '18 at 4:17
  • \$\begingroup\$ Sorry, meant U6 - edited as such. Also sure I could, but I'd like to understand the principles behind it. \$\endgroup\$ – Billy Kalfus May 6 '18 at 4:20
  • 1
    \$\begingroup\$ You are using LTSpice - just see what it tells you for supply current. And double check the quiescent current for the opamps to see that it correctly models that current. \$\endgroup\$ – Andy aka May 6 '18 at 8:26

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