Hello,
Excuse my english, I'm French ...
The LM 358 is a mono-voltage amplifier capable of approaching its negative supply (there is a NPN transistor in the output stage). I had to build a mo no-voltage subtractor (a voltage substractor built with a LM358 powered wit h 0v-12v supply) and with the consent of the electronic simulator ISIS I b uilt (wired) such a subtractor. I was surprised that it could not entirely do the job. For some subtractions, like 2v - 1.8v for example, the substra ctor said 0.62v instead of 0.2v. But for some other, it answered the right thing, wich was 0.3v (a voltage lower than 0.62v) sometimes. So it was able to go under 0.62v !
Why does a 2v - 1.8v substraction gave 0.62v instead of 0.2v !
Resistors, all identical, were 2.5k one, so I do not suspect bias currents problem. I tried many other chip, it didn't change anything. I changed the
0v suply into a -12v one and the substractor worked well for every input vo ltage.Does someone have an explanation?
cdlt.
Michel.