There are to ways to earn Ethereum. First is to join bounty campaign which pays u in ethereum. The 2nd is to join Airdrop & bounty campaign. From these, u got some valuable tokens, in exchange you can get ethereum. But for that u have little amount of ethereum in your wallet.
I got 150 $ from all Airdrop, but I cannot exchange them because I don't have ethereum.Then I join a bounty which paid me in ethereum.
As far as I know,no bounties are paying directly in the ethereum,they will pay us in tokens which can be converted it into ethereum or bitcoin based on where they were listed but most of the time it won't get listed that is the real problem here.
but today, i see some bounties use eth as their payment for bounty reward, although only 50 percents from total token value.and i see your signature campaign paid with ethereum too.