LM111: Need help with positive feedback working

Nikunj Gupta

Part Number: LM111

I have the given circuit I made in Proteus software. It is just a basic question regarding the op-amp working. How does the 100k resistor in positive feedback affect the circuit. Why is this necessary?

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Nikunj,

what is your circuit expected to do? Can you elaborate?

Kai

0 Kirby Kruckmeyer96848 over 5 years ago

TI__Genius 17035 points

Hello Nikunj,
I was checking to see if you still had a question and if you could provide more information about the purpose of this circuit to help us answer your question.

0 Nikunj Gupta over 5 years ago in reply to Kirby Kruckmeyer96848

Prodigy 90 points

I have an RC circuit wherein capacitor charges and discharges. I'm comparing capacitor voltage against a fixed 2.5V I have obtained using voltage divider of 365 ohm resistors. I'm not able to understand the need for using feedback resistor from output to the input pin. I have attached the updated diagram. It does not show the RC network as I was trying to understand the op-amp part only.

Regards

0 Nikunj Gupta over 5 years ago in reply to kai klaas69

Prodigy 90 points

I have added a reply to Kirby's post. Kindly check the updated diagram. There is an additional resistor divider giving +2.5V reference at non-inverting pin of the op-amp.

0 kai klaas69 over 5 years ago in reply to Nikunj Gupta

Guru 140200 points

Hi Nikunj,

R15 and R16 are too small. This will result in a big supply current.

R8 provides a hysteresis and is very useful. But C5 makes no sense and must be removed. It hampers the operation of R8. If you want to filter the fixed 2.5V reference voltage, then you must do this between VDD and R15 by the help of an additional RC low pass filter.

Kai

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LM111: Need help with positive feedback working