Fig. 1.
A simplified schematic of a traditional patch-clamp amplifier in voltage-clamp mode. A field effect transistor (FET)-input operational amplifier (op amp) uses feedback to hold the pipette voltage (VP) at the user-specified potential (Vclamp). The whole cell current (Icell) passes through feedback resistor RFn, selected by switch S1. The resulting output voltage (Vout) is linearly proportional to the measured current. Several different feedback resistors are needed to accurately measure different current ranges from picoamps to nanoamps. These resistors must be in the high mega- and gigaseal range, making them relatively large and impractical to integrate on a chip. See text for definition of variables.