# Inverting Op-Amp Circuit Problem

I am trying solve the question shown in the picture below (I am assuming the switch is down in the picture, I can't tell)

The equation that I am using is Vout/Vin = - Rf/Rin because there is negative feedback.

For 2a), I ignored R4 because there is no current input to the Op-Amp so I am assuming R4 can be neglected. I combined R2 and R3 so that the total resistance for Rf is 1000 ohms. I then substituted Vin as 20mv, Rin as 1ohm and rearranged to get Vout = -20V. 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 to other inverting Op-Amp circuits I have seen which is why I am struggling to answer it. Does anyone know how to approach a question like this? Thanks very much for your time

• AFAIK your logic seems right in lower position, for switch in Upper position, that formula doesn't apply and you have to derive new one. Use ideal op-amp's equivalent circuit for that.. – Deep Oct 1 '17 at 15:41
• Simply treat the opamp circuit and the resistive divider separately. – Brian Drummond Oct 1 '17 at 15:47

Redraw the circuit like this 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).

• Thanks very much for your informative answer, I really appreciate the diagram too. I have done what you said and calculated the voltage to be 8V at b, is this correct? – s.twenty Oct 1 '17 at 16:15
• @s.twenty you are wrong. It is not 8V. It is -8V. – Chupacabras Oct 1 '17 at 16:21
• @Chupacabras Thank you for the correction, I forgot the sign must be inverted too – s.twenty Oct 1 '17 at 16:24

Or much simpler simulate this circuit – Schematic created using CircuitLab

Any output resistance inside the feedback loop will be hidden away by "infinite open loop gain" a.k.a. ideal operational

You can revert to studying the well known inverting amplifier on the right side of the diagram