Actually constitutional amendments need to also be ratified by 3/4 of the states. There are amendments passed by Congress that were not ratified, including the original 13th Amendment which protected the right to slave ownership and was designed to prevent the Civil War. Also, the Equal Rights Amendment against sex discrimination.
The US Constitution was written before there was the idea of popular referendum, and was generally designed so that the system would not be too democratic.
The eighteenth century notion of 'democracy' was a tad different from today, perhaps closer to Athenian democracy: only male landowners could vote. Not sure how much land was needed to vote. The hoi poloi, women and slaves could not vote. The American Revolution was fomented by middle class intelligentsia who thought themselves unjustly treated by the crown (they were, by the standards of the day) The 'representation' they sought bore little similarity to the modern meaning in a democracy.
One of the most impressive things, to me, about the US constitution ,is a feature which seems to escape many Americans: The Constitution was designed to be somewhat fluid, capable of being changed with relative ease, as needed.Hence claims about 'what the founding fathers intended (IE the status quo) kinda miss the point, imo.
At the time of the US Constitution, they didn't actually talk about democracy. There were property requirements, which were not as severe as in Britain, as it was easier to own land in the US. Also, you declared your vote in front of the candidates. So the wealthy planters and merchants in the Continental Congress often were not really democratically elected. The situation with elections being controlled by the aristocracy. was worse in Britain. Also, in Britain, there was an inheritted king and upper house, which had parallels in colonial governments' governors and governors' councils.
The US Constitution established an indirectly elected President and Senate. There was opposition to it that the President, who essentially had the powers of the British king at that time, would become a king or dictator. There were also concerns that the Senate would be aristocratic, and that federal judges would have too much power and be appointed for life.
Several of the states ratified with the provision that they wanted a Bill of Rights to limit the new federal government. The Bill of Rights is the same name as the document that the English parliament got the usurpers William and Mary to agree to in 1689. The English Bill of Rights contained some of the same provisions as the US wons. However, the English Bill of Rights was more oriented towards limiting the power of the monarch, whereas the US Bill of Rights mentions Congress shall make no law.
Most states have their own Bill of Rights, but the US Bill of Rights did not originally apply to the states. The US Supreme Court has changed its interpretation claiming the 14th Amendment made much of the US Bill of Rights apply to the states.