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Problem with Differential Amplifier

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momokochan

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I am trying to amplify a differential voltage (several tens of mV) and I'm focusing on a differential amplifier with opamps. The circuit is attached. Here, R1 = 2k and RF = 100k. In this circuit, instead of the IN+ reference resistor going to GND, I have it tied to VR = 0.5V. The equation turns out as: Vout = VR + RF/R1*(V2-V1). The reason this is done is so that I can shift the output by 0.5V so that when the differential voltage is zero, the output is 0.5V.

However, here is my problem. From tolerances in the preceding circuitry, V2 may become lower than V1 (this may at most be 50mV). I want to keep the output at 0.5V even if V2 < V1. What kind of circuitry is recommended to do such function?
 

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I want to keep the output at 0.5V even if V2 < V1

What does this mean? The output must remain at 0.5 volts independent on V1 and V2 ?
The question/problem is not clear to me.
 

Normally V2 (+ input) would be higher that V1(- input) and the output will amplify the difference.
He says that there will be times when V1(- input) will have a higher voltage than V2 (+ input) and he wants to have an output that doesn't go below 0.5v in that case.

Alex
 

alexan_e said:
Normally V2 (+ input) would be higher that V1(- input) and the output will amplify the difference.
He says that there will be times when V1(- input) will have a higher voltage than V2 (+ input) and he wants to have an output that doesn't go below 0.5v in that case.
Click to expand...

Correct. In other words, mathematically the output voltage can be described as
Vout = VR + (RF/Ri)*(V2-V1) if V2 >= V1
Vout = VR if V2 < V1​
This can be achieved if V2 is at minimum limited to V1.

I'm not very experienced in circuit design so I only know the basic circuits, and the only way I could think of is somehow limiting V2 compared to V1. Something like a comparator and then an opamp feedback? Although the comparator design will be pretty complicated as the difference is only ~50mV. I just feel like something like this would have been dealt with before by someone else.
 

Feed a bit of -ve bias into the -ve input of the amp. The feed resistor will have to be taken into account when doing gain calculations. Likewise its value must be calculated with Vo= + .5 V and Vin = 0V.
Frank
 
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