Re: output buffer?
Yihsiu,
It's not clear from your post whether you are looking for a single buffer that sums two inputs, or an analog switch that provides the input to a buffer. In any case, the Elantec EL2005 provides the bandwidth and voltage range that you need. I don't know of an analog mux that provides the high voltage and wide bandwith that you need. You could implement the switches using descrete FETs. The biggest problem will be with feed through (signal leaking thru the off capacitance of the switch). This can be mitigated by using a series-shunt arrangement where the unused input is shorted to ground. Also, board layout is very critical at these frequencies.
Regards,
Jon