So in my SKIL course, (SKIL is the name of the lab courses (1 through 6) physics majors take. This course is about non-linear circuits) we need to develop some type of project that incorporates what we learn in the labs. The course deals with.. -Operation Amplifiers (Op Amp's) -Diodes -BJT Transistors. (Or BJT's.. I guess saying BJT transistors is a little redundant.) So I was looking up project ideas that incorporate all of these things and I found something I am very much interested in. http://www.electronics-project-design.com/DCMotorDriver.html [Broken] Since my knowledge of transistors and op amps are limited, I was wondering if anyone could answer a few questions I have about the workings of this circuit. My understanding of OpAmps is that they look at the voltage difference between the + and - sides, and if the input (+ side) is negative with respect to the - side, the op amp will put out a positive current to balance the potential between the two points. Likewise, if the input is more positive with respect to the - side, it would put out a negative current. The amount of amplification depends on the ratio of the resistors the op amp is connected to. Is this an accurate description? I feel like it is not completely correct. So I can pretty much see how this circuit works I think. The main source of confusion is the way the potentiometer works. Why would changing the resistance of the input change the polarity of the signal input to the OpAmp? Thanks!