Electronic – Inverting Op-Amp Circuit Problem

operational-amplifier

I am trying solve the question shown in the picture below

enter image description here

(I am assuming the switch is down in the picture, I can't tell)

The equation that I am using is $$\frac{V_{\text{out}}}{V_{\text{in}}} = – \frac{R_{\text{f}}}{R_{\text{in}}}$$ because there is negative feedback.

For 2a), I ignored \$R_4\$ because there is no current input to the op amp, so I am assuming \$R_4\$ can be neglected. I combined \$R_2\$ and \$R_3\$ so that the total resistance for \$R_{\text{f}}\$ is \$1000\Omega\$. I then substituted \$V_{\text{in}} = 20\text{ mV}\$, \$R_{\text{in}} =1\text{ }\Omega\$ and rearranged to get \$V_{\text{out}} = -20\text{ V}\$. I am unsure if I applied the right logic to this question and I am don't know how to solve 2b).

This circuit is different from other inverting op amp circuits I have seen which is why I am struggling to answer it. How do I approach a question like this?

Best Answer

Redraw the circuit like this

schematic

simulate this circuit – Schematic created using CircuitLab

The questions you have are essentially the same as finding the voltage at point a and b.

You worked question a correctly.

Now you can easily find out the current through R3 and use Ohm's law to find the drop across it. From the known voltage at a and the drop across R3, you can find the voltage at b.

Alternately, you know the voltage at the op-amp inverting input, and you can use the voltage drop across R2 to find V(b).

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