I have a first order filter. It is supposed to have unity gain at frequencies below the 3dB frequency of 50Hz. I have simulated two of these first order RC filters. One where the signal is fed into a universal op-amp acting as a voltage follower and another as a simple RC network. I have attached images of both the schematic and the response.
When conducting an AC sweep of the circuit the signal given into the op-amp voltage follower there is a constant -2dB attenuation. Universal opamp 2 has been chosen from the LTSpice component list. For the RC network not fed into a voltage follower there is 0dB gain below the cut off frequency which is what is desired.