A low side clamp circuit has a control portion, a sense portion, and a clamp portion. When the sense portion detects that the input voltage of an output stage of a buffer has gone below a threshold voltage, it triggers the control portion to quickly turn on a clamp transistor (in the clamp portion) to clamp the output voltage to the clamp voltage. The control portion and sense portion have cross-coupled transistors that create increased speed and a sharp response with little or no voltage offset with a wide range of load currents. A clamp current source draws current through a resistor coupled in series between the base of the output transistor in the control portion and the collector of the output transistor in the sense portion. The clamp current is set to ClLo/R, where ClLo is the clamp voltage. A high side clamp is also described.