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Inverting Op Amp Circuit

  • Thread starter AdkinsJr
  • Start date
1. The problem statement, all variables and given/known data

Write output voltage in terms of input voltages for the circuit drawn.

I drew the circuit and then feed back loop.

circuit-9_4 (1).png

2. Relevant equations

Ohms Law V=IR
Basic Properties of Op Amps (ideal), voltages are assumed equal at both inputs...

3. The attempt at a solution

The difficulty is that there should be two currents that need defining, but I'm not sure how to eliminate both of them. I wrote two equations for the current through the feedback loop and then equated to write Vout in temrs of the resistances and Va.

[tex]I_a = \frac{V_{out}-V_a}{R_2} [/tex]
[tex]I_a = \frac{V_a-V_{out}}{R_1}[/tex]
[tex]V_{out}=\frac{V_a-V_{in}}{R_1}R_2 +V_a[/tex]

I can't figure out how to write Va in terms of Vin and the resistances at this point. The only other relationships between Va and other quantities come from the fact that [tex]V_b=I_bR_3[/tex] which means that [tex]V_a=I_bR_3[/tex] also, but I still need to get rid of the current Ib now, so that doesn't help much. I can't figure out what I'm missing. Any help would be great.
There's a mistake in the feed back loop in my drawing, the ground at the end of R2 should not be there, that should be Vout.


You've listed one useful property for the op-amp in a negative feedback situation. What other property might be useful?


Have a look on the second equation. Obviously, it is wrong (Vout mixed-up with Vin).
More than that, you can forget the influence of R3 (because an ideal amplifier with infinite input resistance is assumed). Hence, you can set Va=Vb=0.

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