Tetrode
102451
219935532
2008-06-17T15:03:34Z
86.134.62.249
{{Unreferenced|date=August 2007}}
{{otheruses4|the 4 element vacuum tube|the 4 element electrode used in biology to sample neural signals|tetrode (biology)}}
[[Image:Tetrode.PNG|thumb|right|80px|Schematic diagram of a tetrode]]
A '''tetrode''' is an electronic device having four active [[electrode]]s. 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]].
== Grids ==
The first grid of a tetrode, i.e. the grid nearest the cathode, is the "[[control grid]]", because the [[voltage]] applied to it causes the [[anode]] [[current (electricity)|current]] to vary. In normal operation, with a resistive load, this varying current will result in varying (AC) voltage measured at the [[anode]]. With proper [[Biasing (electronics)|biasing]], this voltage will be an amplified (but inverted) version of the AC voltage applied to the control grid, thus the tetrode can provide voltage [[gain]].
The second grid, usually called a "[[screen grid]]" or "shield grid", provides a screening effect, isolating the control grid from the [[anode]]. This helps (1) to suppress oscillation and (2) 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 gain. The screen grid is connected to a positive voltage, and bypassed to the cathode with a capacitor. This shields the grid from the [[anode]], thus reducing Miller capacitance to a very low level and improving the tube's gain at high frequencies. The positive influence of the screen grid in the vicinity of the control grid allows a designer to shift the control grid operating voltage range entirely into the negative region (a triode of similar geometry would likely require positive grid drive to attain the same maximum [[anode]] current). When any grid is driven positive, it can intercept electrons from the cathode, thus loading the drive circuitry. If the input signal causes the control grid to transition into the positive region (where intercept current begins) nonlinearity should be expected. (The control grid is high impedance while negative and low impedance while positive.) With the control grid operating entirely in the negative region, and with the RF shielding afforded by the screen grid, tetrode input impedance is quite high. Gain can be nearly flat from DC to full frequency. Linearity is good. Power gain in excess of 10,000 is possible. As a result tetrodes are commonly used in radio transmitting equipment, because the need for neutralization is less when compared to the triode (see [[Radio transmitter design]] and [[Valve amplifier]] for more details). Screen current does represent loss. Some tube designers attempt to minimize screen current by placing each wire in the screen mesh directly behind a corresponding wire in the control grid mesh. Propagating electrons emerge from the control grid as a projected image of openings in the grid. By placing the screen in the shadow of the control grid, interception of electrons by the screen is minimized in normal operation. Screen current is negligible in many designs. Shadow grids are used in a variety of forms for a number of applications.
Circuit design considerations:
Under certain operating conditions, the tetrode exhibits [[negative resistance]] due to [[secondary emission]] of electrons from the [[anode]] (to the screen). The shape of the characteristic curve of a tetrode operated in this region led to the term "tetrode kink". In general, if the [[anode]] voltage exceeds the screen voltage, this region is avoided, and good performance can be expected. But this lower limit on total tube voltage drop prevents widespread adoption of tetrodes for consumer amplification applications. Secondary emissions from a screen have the effect of pulling the screen upward, toward the [[anode]] voltage. This implies the need for both source and sink current capability in the ideal screen power supply. A bleeder resistor can usually be selected to prevent the screen voltage from getting out of control. Caution: arcs from the [[anode]] generally hit the screen. As such, special care should be taken in the design of the socket wiring, to provide a direct discharge path for arc current, when tube arcing is a possibility. The negative resistance operating region of the tetrode is exploited in the [[dynatron oscillator]]. The undesirable nature of the tetrode kink led tube designers to add a third grid, called the [[suppressor grid]]; the resulting vacuum tube is called a [[pentode]].
== Operation ==
The triode vacuum tube also develops a "[[space charge]]" between the cathode and control grid, which reduces its gain, especially at low [[anode]] voltages. The screen grid neutralizes the space charge and increases the tube's gain.
== Invention ==
The tetrode tube was developed by Dr. [[Walter H. Schottky]] of [[Siemens AG|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.
==See also==
*[[Beam tetrode]]
[[Category:Vacuum tubes]]
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