We got a jetpack a few months ago. There are more than one, the "free" one does NOT have any ports for an external antenna. The $50 one does, so if you are in an area with limited signal or coverage you can easily connect it to an external antenna or an amp with external antenna input.
the free one can be used with an external amp with a passive antenna coupling, but you won't get quite as much amplification.
And older amps will not amplify the 4G signal band, but it will amp a 3G signal, which for many uses is just fine.
My experience is that the jetpack must have a better antenna or more sensitive reciever than many phones. I get a 10db or so better signal level with the jetpack than either of our phones. In some cases that means we can get 3G data when the phones will only give us voice coverage.