In my chip, there is a 200-MHz input signal with an amplitude of 400mV. It's connected to the gate of an NMOS transistor. Is it OK to directly connect the signal pad with the transistor gate? Is it necessary to put some buffer? What kind of buffer would be appropriate? Thanks.
You can directly connect the input signal to the gate of transistor as long you match the 50ohm line transmission which we consider as input matching. In case you want to isolate the signal from loading the input, then you can use the source follower (buffer) before the input stage.