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it is asked to make a non ideal inverter opamp that has an input of 2mV or and get an output of -100mV, so I think the next circuit should suffice.

schematic

simulate this circuit – Schematic created using CircuitLab

But later there is extra questions: the error in the output voltage must be less than 0.1%, ¿What is the gain in open loop of this op-amp?¿What is the error if the gain is 10000?

I understand there should be a Beta parameter

\$A_{v}^{ideal}=\frac{1}{\beta}=\frac{-5000}{100}=-50\$

\$\beta=-0.02\$

and the closed-loop gain is

\$A_{v}=-\frac{R_{2}}{R_{1}}(\frac{A\beta}{1+AB})\$
but then how can be calculated he error or make it is between the limits required?

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\$A_v^{ideal}\$ is a special case of \$A_V\$ when \$A\$ is infinitely large.

In that case, \$\frac{A\beta}{1+A\beta}\$ equal one (because 1 is negligible compared to infinite \$A\cdot \beta\$, so they just cancel out).

If you plug a finite number for \$A\$, the fraction will be less than 1. If you want an error smaller than 0.1%, you need that fraction to equal at least 99.9%

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  • \$\begingroup\$ But then whats the meaning of the gain of 1000 that it has been told. I guess is refering to T=10000/50=200, then what? \$\endgroup\$ – avelardo Apr 15 at 20:19
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    \$\begingroup\$ As I understand the question, you should: 1) calculate open-loop gain needed for error less than 0.1% 2) calculate how big would the error be if open-loop gain was 10000 \$\endgroup\$ – Jiří Maier Apr 15 at 20:49
  • \$\begingroup\$ But if it is used the definition of open loop, it does have a meaning, i,e,:Aol=Vout/V+-V-=-100/0--2=-50, so there is no rom for the FGE to be calculated \$\endgroup\$ – avelardo Apr 15 at 21:50
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    \$\begingroup\$ I don't really understand what you meant by this comment. Open-loop gain is Vout/((V+)-(V-)) but (V+)-(V-) is not 2mV, 2mV is on the left side of R1. If it was an ideal op-amp, V+ would always equal V-. In case of real op-amp, difference between V+ and V- is Vout/Aol, which does not equal the 2mV input. \$\endgroup\$ – Jiří Maier Apr 15 at 23:25
  • \$\begingroup\$ Then how it is suggested to calculate(or proposed) the diference (V+)-(V-)? \$\endgroup\$ – riccs_0x Apr 16 at 4:49
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Solve for A with error on Av reduction, then visa versa.

I’d like to point out each R tolerance must be 0.05% with infinite gain or assume tolerance is 0% or some other combination.

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