# OPA191: not getting expected output in Inverting OPA191

Part Number: OPA191

Hi Team,

Customer want to know the reason why in his simulation, they cannot get the correct output when using OPA191 as inverting. Below is the details.

"

I am using an OPA191 IN my design and trying to connect a voltage divider bias output to inverting input of the op-amp. I am not getting proper output.

If I put a buffer in-between I could get the output as expected or If I go for a non inverting configuration also the output will be as per my expectation.

I know it could be a a design mistake however if you help me to understand what is the cause behind that

Kindly refer to attached TINA: trial (1).TSC

"

Regards,

Maynard

• Hi Maynard,

the circuit works as expected:

Eventually, the customer has overseen that R9 is loading the voltage divider? It doesn't look like a voltage divider of 20k and 10k, but 20k and 5k, effectively.

Kai

• Hi Maynard-san,

Here is what the engineer has drawn in equivalent circuit. The Thevenin resistance is R1||R2 or 10k||20k=6.67k, and Thevenin voltage is the voltage divider or 3.33Vdc.

The circuit's gain is -39k/(10k+6.67k) = -2.34 V/V and VF2 = -2.34*3.33V = -7.79 V.

When a buffer is used in place of the voltage divider, the buffer's output is low impedance, therefore, VF2= -3.33V* 39k/10k = -12.987V, which it will saturate the output. I changed the +/-12Vdc to +/-15Vdc and getting -13V.

Best,

Raymond

• Hi Maynard-san,

I Have not heard from you for over a week. I am going to close this inquiry. If you have additional questions, you are still able to open this thread.

Best,

Raymond

• Hi Raymond,

Thank you for you support, this resolved the issue.

Regards,

Maynard