Why is there a negative voltage?
In this case the &quot;negative&quot; voltage is just one of the supply rails.  Ground is defined as 0V by convention, but signals can go below ground and become negative.  In most circuits, they will have to be supported by voltages that are negative, meaning having a negative potential when compared to ground.
The voltage sources aren't necessarily DC, even though they have polarity markings.  The plus and minus just indicate that, if the voltage source has a negative value, the &quot;+&quot; side has a voltage lower than the &quot;-&quot; side.  There's nothing in the symbol that indicates the source doesn't vary with time.