[Solved] Unity Gain Differential Amplifier not working as expected

[king.oslo]

Hello there,

I made an Unity Gain Differential Amplifier circuit. This tutorial told me that Vout = V2-V1: http://www.electronics-tutorials.ws/opamp/opamp_5.html

My exact schematic is attached. R1-R4 are 22R3

The problem is that Vout != V2-V1.

I built it up using a Microchip MCP6041 on a bread board. This is what I found:

Case 1:
V2: =1.475V
V1: =2.835V
Expected Vout = -1.36V
Vout = 1.936V

Expected Vout != Vout

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Case 2:
V2: = 0.982V
V1: = 2.829V
Expected Vout = -1.847
Vout = 1.930

Expected Vout != Vout

This is the datasheet for the opamp: http://ww1.microchip.com/downloads/en/DeviceDoc/21669c.pdf

What have I missed?

Thanks for your time.

Kind regards,
Marius

[IanB]

Are you supplying the op amp with a dual rail supply? You have not said how you are powering it.

You cannot expect to get a negative voltage out of an op amp unless you have a negative supply voltage fed into it.

[king.oslo]

Are you supplying the op amp with a dual rail supply? You have not said how you are powering it.

You cannot expect to get a negative voltage out of an op amp unless you have a negative supply voltage fed into it.

Opamp is powered by 5VDC to gnd. I think you just pointed out the problem.

[slateraptor]

Expected Vout = -1.36V

Here's your problem, viz. your lower rail is tied to ground, thus cannot produce negative voltage.

[king.oslo]

That's great.

It says my opamp can only do  1.4V to 6.0.

Does that mean that I power it like this: VDD = 6V, VSS = 0V. Then pull inverting input to 3VDC, and then measure Vout between Vout and 3VDC?

1. That should work, right?
2. Is this the traditional way to do it?

Thanks.M

[slateraptor]

Instead of tying R4 to ground, tie it to Vcc/2. Then measure your output with respect to Vcc/2. This is standard single-supply practice.

EDIT: If you analyze the circuit assuming the op amp is ideal, you can easily see why this will work. I encourage you to give it a shot.

[king.oslo]

Thanks.

Since I wasn't doing this, I was measuring Vout with respect to gnd, and tying R4 to gnd as well. What values was I actually measuring?

Thank you for you time. Both of you.

Kind regards,
Marius

[slateraptor]

What values was I actually measuring?

The op amp's output wasn't within its normal operating region; you were effectively measuring junk.

[king.oslo]

What values was I actually measuring?

The op amp's output wasn't within its normal operating region; you were effectively measuring junk.

Ok, thanks!   M