enter image description here

The circuit as shown above.

Conditions known:

  1. The op-amp is nonideal.
  2. Both inputs have current flow, lets say 0.7 \$\mu\$A.
  3. Only \$R_F\$ is known, which is 450 \$k\Omega\$.
  4. Open-loop gain is high.
  5. Close-loop gain needs to be around 13.
  6. Initial \$V_{in}\$ and \$V_{out}\$ is unknown.

Question: How much voltage should be added to \$V_{in}\$ to eliminate the impact from the non-ideal op-amp to the circuit? (So that \$V_{in}\$ will be equal to \$V_{in}+V_x\$ after the modification, and solve for \$V_x\$).

I used the formula, \${V_{out}/V_{in}=1+R_F/R_2}\$ to find \$R_2\$, then I stuck.... I have no idea how to apply the current to solve this problem.

  • \$\begingroup\$ Sounds like homework, how far did you come and at what point are you stuck? \$\endgroup\$
    – PlasmaHH
    Aug 28, 2018 at 12:08
  • \$\begingroup\$ Cause there is no initial voltage given, so I have no idea how to solve this problem. \$\endgroup\$
    – markable
    Aug 28, 2018 at 12:09
  • \$\begingroup\$ V_in equal to (V_in + V_x) ?? Result: V_x=0. More than that, if the opamp has to be considered as non-ideal, the open-loop gain must be given. \$\endgroup\$
    – LvW
    Aug 28, 2018 at 12:24

1 Answer 1


As this is homework, I'm just going to give a couple of hints.

The only non-ideal parameter given quantitatively is the input current. That's a hint that you need to consider that.

You'll need both resistor values. You have one and can calculate the other. The result should follow.

  • \$\begingroup\$ I used the formula, \${V_{out}/V_{in}=1+R_F/R_2}\$ to find \$R_2\$, then I stuck.... I have no idea how to apply the current to solve this problem. \$\endgroup\$
    – markable
    Aug 28, 2018 at 20:53
  • \$\begingroup\$ @markable that's an equation to find gain in an ideal opamp. An ideal opamp has no current into its inputs. Using this, you should be able to re-derive that ideal gain equation by looking at the current flowing from Vout through Rf and R2. Now consider the non-ideal case, with input current. Draw the current going into the - input at V1, and see how that changes the current flowing through Rf and R2. \$\endgroup\$ Aug 28, 2018 at 21:09
  • \$\begingroup\$ The current goes through R2 and Rf in ideal case is I then \$(R_2+R_F)I/V_{in}=1+R_F/R_2\$, For the non-ideal case, this is \$((I-0.7uA)R_2+R_FI)/V_{in}=1+R_F/R_2\$ \$\endgroup\$
    – markable
    Aug 28, 2018 at 21:42
  • \$\begingroup\$ So you look at the current flowing in or out of the inputs. 700nA each (sign is not clear from your re-statement of the question but it should be clear in the question). We only care about the current related to the inverting input. You can use superposition to determine the change in voltage at the inverting input due to the input current, and for balance you know the voltage at the non-inverting input must be equal, so that number is the change. \$\endgroup\$ Aug 28, 2018 at 22:35
  • \$\begingroup\$ Please Please, My mind is gonna below up.... but still have no idea. \$\endgroup\$
    – markable
    Aug 29, 2018 at 8:46

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