Hello,

For a school project we're trying to build an analog calculator using operational amplifiers. However, we can't get it to work. We have 3 different changeable voltage sources (V1, V2 and V3 as you can see in the second picture) and Vout should be equal to V1 + V2. V3 is there to supply voltage to the op amps. However, we can't get the circuit to work; Vout wont display V1 + V2, instead it will display some weird values we couldn't even explain, usually 1-2V lower than the highest voltage source. Unfortunately couldn't get a picture of the breadboard but we're pretty sure the issue isn't a faulty connected wire, since we've been stuck on this for weeks and tried different ways to fix it, even with the help of people who knew what they were doing, but couldn't seem to find the issue. However, we suspect there might be something wrong with the way we linked the op-amps to a voltage source. In the first picture you can see the circuit we're trying to build:




And in this second version is an elaborate circuit scheme in which I tried to exactly show what we built on the breadboard (unfortunately couldn't get any clear pictures of the breadboard itself, but it's exactly like in this picture).



Thanks in advance to whoever can help us find the issue with this circuit or the way that we built it!

 

Your power supply is the problem. You need two batteries, with the centre point connected to 0V, to give a ± supply.

 

Furthermore, if it's exactly like the picture, then your second opamp has no feedback and is running open loop.

 

Furthermore, if it's exactly like the picture, then your second opamp has no feedback and is running open loop.

Sorry, mistake in the second picture. The second op amp is like in the first picture, forgot to draw the negative feedback.

 

Your power supply is the problem. You need two batteries, with the centre point connected to 0V, to give a ± supply.

Could you clarify this a bit more? Sorry, we're pretty new to this.

 

 

Could you clarify this a bit more?

The input and output connections must have a path to the power supply, otherwise there's no place for the currents into and out of the op amp to go.
In your circuit this is no such path.

 

Considering that the OP has little or no experience with opamps I feel it would be much more productive if he implements the last stage first using a fixed voltage as the input. Once it is working going then to the summer.

In fact my learning experience went like that.

Good luck.

 

Could you clarify this a bit more? Sorry, we're pretty new to this.

@wouter368 Welcome to AAC.

What was missing from my colleagues correct suggestion of a dual power supply is why its needed.

Your opamps are configured as inverting amplifiers, specifically unity gain because input and feedback resistors are the same value. The opamp is always trying to keep the voltage difference between its inputs as zero. Since the + input is fixed at zero volts, to make the - input zero with a positive input voltage the output must be negative with respect to the +input.

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Your first stage output will therefore be:

Vout1 = -1 x (V1 + V2)​

Without the -ve power supply the output will never go below 0v, so the opamp will not give you the correct answer.

Your 2nd stage opamp behaves the same way so:

Vout2 = -1 x Vout1 = -1 x -1 x (V1 + V2),​

ie Vout2 = (V1 +V2) as expected.​

​

You could do the same with a non-inverting configuration and a single opamp...