I have so far used opamps for near-DC work (connecting transducers to AD and the like) and have not studied electronics in school. Now i made voltage follower based on classic LF412.
When i passed 125KHz square wave to opamp noninverting input through 10k resistor then nice suqare wave was seen on output (like expected).
When i increased resistance on input only about four times - to 36k then the square wave got clearly rounded corners like is seen when higher frequency components of square wave are attenuated. When i used 681k reisistor there was nothing like square wave any more.
What is going on? Is this effect of input capacitance or anything else? What to do if one needs high input impedance on relatively high frequency?