# Solving for adding a voltage to eliminate the non-ideality of the op-amp

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.

• Sounds like homework, how far did you come and at what point are you stuck? Aug 28, 2018 at 12:08
• Cause there is no initial voltage given, so I have no idea how to solve this problem. Aug 28, 2018 at 12:09
• 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.
– LvW
Aug 28, 2018 at 12:24

• 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. Aug 28, 2018 at 20:53
• 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 $((I-0.7uA)R_2+R_FI)/V_{in}=1+R_F/R_2$ Aug 28, 2018 at 21:42