Is the rise time of the output of a logic IC (e.g. flip-flop or inverter) or optocoupler independent or dependent on the rise time of the input? If the answer is dependent, are there any other devices where the rise time of the output is independent of the rise time of the input?
For example, I have a 555 timer output running in astable mode with a 50% duty cycle producing a 1.5kHz square wave where my scope says the rise time is approximately 400ns (and this changes if I change the frequency of the 555 output). I'd like to hook something up to the output of the 555, like a flip-flop or inverter or optocoupler (or anything that does the trick) to get a square wave with a rise time that's: 1) less than 400ns and 2) preferably, independent of the 555 frequency (i.e., same whether I configure the 555 to output at 1.5kHz or 10kHz).