instrumentational opamp

[rudie]

hello guys,

I want to make a circuit that gets rid of 0.7V on a dc signal. In the attachment you will find a picture of the circuit that i made. In the simulation it works perfect. But if i built it up, it doesn't, It works until the source V1 is 1.3V if it goes below 1.3V (0.6V difference, so 0.6V is the output voltage). The output remains at 0.6V it doesn't go lower. In the schematic you see that i used a Ua741 i replaced that on my board with a LM358 which has a lower voltage swing. But still it doesn't go lower that 0.6V.

The second problem is that the voltage that is produced by the resistor divider of R9 and R10 (the 0.7V that i need), changes if if change V1. How is this possible V1 isn't even in connection with R9 and R10.

I hope that someone can help me

greets rudie

 

[Audioguru]

Look at the datasheet of an LM358. Its output voltage cannot go lower than about 0.6V unless it does not sink any current or it has a pull-down resistor.

The inputs of a 741 opamp don't work if they get within about 3V from a supply voltage. Its output cannot go closer than about 1.2V from a supply voltage.

 

[rudie]

oh ok. Thank you. if i looked better i could have figured that out myself. But what about the second problem. About that the voltage caused by the resistor divider is influenced by the voltage source V1. How is that possible?

greets Rudie

 

[Audioguru]

R4 plus R6 are the load on the voltage formed by the divider of R9 and R10. The voltage of the divider changes when the load current changes which happens when V1 is changed.