Michele, this is not really a differentiator circuit, it's more a summing amplifier, though the 100nF capacitor (which will have a reactance of almost 10 Kohms at 160 Hz, and about 100 ohms at 16 000 Hz, is having some effect as well.Bear in mind that at point A you will have 5V (peak to peak it would seem) of the 160 Hz sine wave and at point B you only have 0.1 V peak to peak of the high frequency waveform.Each signal has 2K of resistance to get through to get to the other point (ie A to B and B to A). So the low frequency waveform A is 5V and it will have less than 100 mV of high frequency signal getting to it. You can see some raggedness in waveform A which is the low level high frequency signal getting to it.Now pass to point B, You have only 100mV of the 16000 Hz. signal there, whereas some of the very high level 160Hz signal can get through those 2 x 2K resistors to it. You can see from the point B waveform that your 16000 Hz signal is about 100 mV pk-pk, whereas the 160Hz signal is about 200 mV Pk-Pk. I don't knwo if this was a simulation of oscilloscope waveforms from a real circuit, but try making both waveforms the same level (say 1v) and see what happens then. Your problem, in fact, is not a problem, it is just that your signals are at very different levels.
