differential op-amp

[faomari]

Im using a PWM, with a feedback op-amp supplied, im using DC input. My quistion regards v+ and v- (opamp-inputs), i want to control v- by adjusting input voltage to v+, the thing is this happens over a small voltage range. I was wodnering what factors can be done to increase that votlage range, and why in the first place is it being contolled for only a small voltage range.

thanks.

p.s i can add a pspise image of the real circuit, if u need more clarfications.

 

[audioguru]

Please attach the schematic as an ordinary graphic file (GIF, PNG or JPG) so we can see what you are talking about.

 

[fzn10]

Thats the differential op-amp circuit im talking about, the op-amp it self is integrated in a TL1451A PWM chip, so dont have to worry about the supply rails. Bassicly what im expecting is that when i vary V+ through the R25 pot, i want V- to equal that, what is acutally happening, is that V- is equal to V+ over an approximatly 0.1 votlage range, i.e when i have V+ set to 0.8 for example, and lets say i start varying v-, when v- is equal to 0.85V, v+ is the same, however if go bellow 0.75, v+ will stay at 0.75, even when i keep decreasing voltage applied to v-. The same thing applies when i increase v- to be above 0.85, v+ will get stuck at 0.85.
Is this normal?

 

[audioguru]

The negative feedback from the opamp's output to its inverting input will keep the inputs at the same voltage until the opamp's output reaches its max.

 

[Roff]

I think you have some misconceptions about op amps. Tell us what you are trying to do, instead of how you are trying to do it.