Abstract: A programmable gain instrumentation amplifier includes first and second differential subcircuits, each of which includes first and second input transistors, a first constant current source, first and second gain selection transistors, an output transistor, and a second constant current source. The bases of the first and second input transistors of the first and second subcircuits are connected, respectively, to first and second input terminals. The emitters of the first and second input transistors are connected to first and second gain resistors, respectively, and also are connected to collectors of the first and second gain selection transistors, respectively. The bases of the first and second gain selection transistors of the first and second subcircuits are coupled to first and second gain selection signals, respectively. Collectors of the first and second input transistors are connected to the first constant current source.