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.

 

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

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Uncle $crooge

 

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?

Attached Images

op-amp.jpg (29.0 KB, 206 views)

 

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.

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Uncle $crooge

 

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.

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Ron