The difference between "knowledge" and "advantage" can be explained using pronunciation keys, but it might help to think of it like where the emphasis is placed on the word.
Saying "knowledge", the emphasis is placed on the first of the word's two syllables. That means you say it a bit louder and the rest sort of trails off, giving the impression of a -ədʒ sound.
Saying, "advantage", there are three syllables. The first and third are less pronounced, while the "van" is more emphasized. For me, this results in a more of a -ɪdʒ sound.
I think @TRomano hit it on the money when he said to think of it like -tdʒ. In real conversation, the differences between these pronunciations shouldn't matter.
EDIT: Just wanted to add in that this is coming from the US northeastern shore.
EDIT 2: One does not technically say -tdʒ, but thinking of it as running together might give you a better idea into how it's pronounced in fluent speech.