The type of op amp I am using is Voltage-to-Current Amp. Under DC voltage input, it works perfectly well, boosting the current of a signal generator (DC mode) up to 250mA with the help of single supply working at 25V. However, when I turn the signal generator in AC mode, hoping to achieve AC current oscillating between the value of +125mA and -125mA by adjusting the offset, I found that the negative current won't decrease. By decreasing the offset, the positive value keep decrease (from 250 to 200mA), but the negative one get stuck at -4mA. When the offset is lowered to a certain value, the positive value suddenly drop to negative. I have no idea what's going on. It is my first time to play with a op amp, I didn't expect it to be so complicated.