A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid amplifying vacuum tube, also called a screen grid tube. It consists of a sealed evacuated glass envelope containing four electrodes in this order: a cathode heated by a filament, a control grid, a screen grid, and a plate (anode).
The tube evolved from the triode tube, by the addition of the screen grid, which improved its electrical characteristics by reducing interelectrode capacitance. Walter Schottky invented the first double-grid tube in 1916, but his additional grid was placed between the control grid and cathode, to reduce space charge. The first true tetrode, with a screen grid designed to reduce grid-plate capacitance, was patented by Hiroshi Ando in 1919, and the first practical versions were built by N. H. Williams and Albert Hull at General Electric and Bernard Tellegen at Phillips in 1926. Tetrodes have higher gain and higher frequency capability than triodes, and led to a great improvement in high frequency radio equipment. However the screen grid also gave the tube a negative resistance characteristic which could cause parasitic oscillations, which led to the invention of the pentode. Tetrodes were widely used as amplifying tubes in consumer and industrial electronic equipment such as radios and televisions until the 1960s when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. Low power consumer tubes are still used in a few legacy and specialty vacuum tube audio devices such as tube guitar amplifiers.
How it works
The tetrode has four concentric electrodes: the cathode, a narrow tube down the center, heated by a heating element; two grids, the control grid and screen grid, consisting of wire screens surrounding the cathode; and the plate (anode), a metal cylinder surrounding the grids. It functions similarly to the triode, from which it evolved. A separate current through the heater heats the cathode, which causes it to emit electrons into the tube. A positive voltage is applied between the plate and cathode, causing a flow of electrons from cathode to plate through the two grids. A varying voltage applied to the grid nearest the cathode, the control grid, can control this current, causing variations in the plate current. With a resistive load in the plate circuit, the varying current will result in a varying voltage at the plate. With proper 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 tetrode was developed to correct deficiencies in the triode tube. In the triode, the control grid was next to the plate. Capacitance between these two electrodes caused instability and oscillation, and several undesirable effects. To reduce this capacitance, a second grid was added between the control grid and the plate to make the tetrode.
The second grid, called "screen grid" or sometimes "shield grid", provides a screening effect, isolating the control grid from the anode, reducing the parasitic capacitance between the two. In normal operation the screen grid is connected to a positive DC voltage slightly less than the plate voltage, and bypassed to the cathode with a capacitor, so it was at AC ground. The screen grid had several beneficial effects:
- It prevented unwanted parasitic oscillation due to feedback through the interelectrode capacitance. In triodes, the plate-to-grid capacitance served as a feedback path for energy from the plate circuit to get back into the grid circuit, causing the amplifier to become an oscillator. Triode amplifiers had to have complicated "neutralization" circuits to counteract this, and even with them the gain was limited.
- It increased the gain of the tube at high frequencies by reducing the Miller effect. This undesirable effect due to feedback through the grid-plate capacitance increased the capacitance at the grid of the tube, limiting the triode's high frequency gain. When the tetrode was introduced, a typical triode had an input capacitance of about 5 pF, but the screen grid reduced this capacitance to about 0.01 pF.
- It also served to increase the tube's gain by neutralizing the space charge. Triode vacuum tubes develop a "space charge" between the cathode and control grid, which reduces its gain, especially at low anode voltages. The positive voltage on the screen grid of a tetrode helps to neutralize the space charge and increases the tube's gain.
As the screen grid is positively charged, it collects electrons, which causes current to flow in the screen grid circuit. This uses power and heats the screen grid; if the screen heats up enough it can melt and destroy the tube. There are two sources of electrons collected by the screen grid—in addition to the electrons emitted by the cathode, the screen grid can also collect secondary electrons ejected from the anode by the impact of the energetic primary electrons. Secondary emission can increase enough to decrease the anode current, since a single primary electron can eject more than one secondary electron. The reduction in anode current is because the external anode current (through the connection pin) is due to the cathode-to-anode current minus the secondary emission current. This can give the tetrode valve a distinctive negative resistance characteristic, sometimes called "tetrode kink". This is usually undesirable, although it can be exploited as in the dynatron oscillator. The secondary emission can be suppressed by adding a suppressor grid, making a pentode, or beam plates to make a beam tetrode/kinkless tetrode.
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 relative to the cathode it can intercept electrons from the cathode, loading the drive circuitry. If the input signal causes the control grid to become positive (where current flow begins), nonlinearity is to be expected. (The control grid draws no current while negative—high impedance—but draws current while positive—low impedance.) 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 even at high frequencies. Gain can be nearly flat from DC to full frequency. Linearity is good. Power gain in excess of 10,000 is possible.
Power tetrodes are commonly used in radio transmitting equipment, because the need for neutralization is less than with triodes (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. Arcs from the anode generally hit the screen. As such, special care is required in design of the socket wiring, to provide a direct discharge path for arc current. 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. More modern tubes have anodes treated to minimise secondary emission.
The tetrode tube was developed by Dr. Walter H. Schottky of Siemens & Halske GmbH in Germany in 1919. Thousands of variations of the tetrode design, as well as its later development the pentode, have been manufactured since then, although vacuum tubes in low-power equipment have been almost totally superseded by solid-state semiconductor devices.
- L.W. Turner, (ed), Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976 ISBN 0408001682 page 7-19
- Brown, L. (1999). Technical and Military Imperatives: A Radar History of World War 2. CRC Press. pp. 35–36. ISBN 1420050664.
- Turner 1976, page 7-19
- Turner 1976, page 7-20
- http://www.ece.umd.edu/~taylor/Electrons3.htm A thumbnail history of electronics, retrieved 2009 Feb 5