Abstract: An extension of the op-amp concept featuring two differential inputs is presented. In a closed-loop environment this circuit forces two floating voltages to the same value, and thus has many interesting applications in the analog circuit domain. A description of such a circuit, its nonidealities, and restrictions are given. A monolithic integration of this differential difference amplifier is implemented in a double-poly CMOS technology, its measured characteristics are described. Many applications of this circuit, including a voltage comparator with floating inputs, a voltage inverter without resistors, and an instrumentation amplifier with only two external gain determining resistors, are discussed.