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Current clamp is a way of recording biological voltages such as the action of an excitable cell with a microelectrode while keeping electrical current through the recording electrode very small (or at least constant). Its simplest form is a relatively low impedance microelectrode and an indifferent or "ground" electrode that are connected by low-resistance and low-noise connections to a differential voltage amplifier (a high-impedance electronic voltage follower circuit or operational amplifier). The output from the amplifier over time is then displayed on an oscilloscope or computer for analysis.
This type of recording can be used for both extracellular and intracellular recordings. Typically, changes in voltage of about 0.1 mV to 200 mV are recorded in this manner, and changes in voltage lasting only a fraction of a mS are often recorded. The term "current clamp" is usually used to describe recordings from a very confined region (such as a single cell) and often it is used to mean the voltage of the inside of a single cell in comparison to the outside (an intracellular recording).