Tetrode
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A tetrode is an electronic device having four electrodes. The term most commonly applies to a two-grid vacuum tube. It has four electrodes instead of three, as in the case of a triode. Tetrode transistors also exist, although they are rarely used in modern electronics.
The first grid of a tetrode is the "control" grid, because the voltage applied to it causes the plate current to vary. A varying (AC) voltage measured at the plate will be an amplified version of the AC voltage applied to the control grid, thus the tetrode provides voltage gain.
The second grid, usually called a "screen grid" or "shield grid", provides a screening effect, isolating the control grid from the plate. This helps to reduce an undesirable effect in triodes called the "Miller effect", where the gain of the tube causes a feedback effect which increases the apparent capacitance of the tube's grid, thus limiting the tube's high-frequency performance. The screen grid is connected to a positive voltage, and bypassed to the cathode with a capacitor. This shields the grid from the plate, thus reducing Miller capacitance to a very low level and improving the tube's performance at high frequencies.
The triode vacuum tube also develops a "space charge" between the cathode and control grid, which reduces its gain, especially at low plate voltages. The screen grid neutralizes the space charge and increases the tube's gain.
The tetrode tube was developed by Dr. Walter H. Schottky of Siemens & Halske GMBH in Germany during World War I. Thousands of variations of the tetrode design, as well as its later development the pentode, have been manufactured since then.
A tetrode is a group of wire bundles used in electrophysiological studies in the neurosciences to record extracellular field potentials from nervous tissue, e.g. the brain. They consist of bundles of 4 thin (e.g. 30 µm diameter) wires glued together. The idea is that the wires are spaced close enough to each other to 'see' overlapping populations of neurons, but wide enough so that the exact waveform of the signal recorded would be different on each of the wires. These differences would then be used to distinguish the contributions of different neurons based on the shape of their spikes (the extracellular correlates of their action potential) by spike sorting.