# Is there current flowing out of an Operational Amplifier output?

I came across the following self-study problem, taken from The Analysis and Design of Linear Circuits (8th edition).

I was not intending to solve the problem, since I am just looking over all problems in the book. Something caught my attention, however. I was looking at node c, the one labeled with $$\v_c\$$ in the Figure. If I were to apply Kirchhoff's Current Law to then perform node-voltage analysis, I would not know what currents to write down. I know the current flowing into the positive input of the second OpAmp is zero, so that means there is either current going into/coming out of the output of the first OpAmp, and current through the 150K resistor; or else, no current goes through the 150K or out of/into the first OpAmp output. Then, what is the point of cascading two OpAmps?

• As far as $v_b\ne v_c$, there will be a current flowing through 150k resistor and hence to/from op-amp 1 output. Think. – nidhin Oct 18 '18 at 15:45
• Thank you. I actually wrote the equations down, solved them, and got results that I later verified with LT Spice. My intuition just failed me, I guess. – Bee Oct 18 '18 at 15:59

Yes, current can flow into and/or out off the output of an op-amp.

However, an op-amp provides a voltage output. It is the circuit that surrounds the op-amp that dictates what current will flow into/out of the op-amp's output.

To apply hand analysis to your circuit you would assume:

• $$\V_b = 0\$$ (due to op-amp action)
• $$\V_e = V_c\$$ (due to op-amp action)

Apply KCL at the $$\V_b\$$ and $$\V_o\$$ nodes.

You now have 2 equations with 2 unknowns ($$\V_o\$$ and $$\V_c\$$).

• Thank you. I did that and got to a plausible solution. I would suggest that you don't need to apply KCL at $V_0$, since you get one equation from applying KCL at $V_b$, and the other from using the voltage divider equation to get a relation between $V_0$ and $V_e$. – Bee Oct 18 '18 at 16:12
• @Bee Absolutely use whatever is most comfortable to you. You could also solve $V_c$ as a function of $V_o$ (non-inverting amplifier) and apply KCL once at $V_b$. – sstobbe Oct 18 '18 at 16:22