The grid nearest the cathode is the “manage grid” the voltage applied to it causes the anode present to differ. In standard operation, with a resistive load, this varying present will outcome in varying (AC) voltage measured at the anode. With appropriate biasing, this voltage will be an amplified (but inverted) version of the AC voltage applied to the control grid, hence the tetrode can give voltage obtain.
The second grid, referred to as “screen grid” or occasionally “shield grid”, gives a screening effect, isolating the control grid from the anode. This assists to suppress undesirable oscillation, and to lessen an undesirable effect in triodes known as the “Miller effect”, where the gain of the tube causes a feedback impact which increases the apparent capacitance of the tube’s grid, limiting the tube’s higher-frequency acquire. In normal operation the screen grid is connected to a good voltage, and bypassed to the cathode with a capacitor. This shields the grid from the anode, reducing Miller capacitance amongst these two electrodes to a really low level and enhancing the tube’s obtain at higher frequencies. When the tetrode was introduced, a standard triode had an input capacitance of about 5 pF, but the screen grid lowered this capacitance to about .01 pF.
As the screen grid is positively charged, it collects electrons, which causes present to flow in the screen grid circuit. This makes use of power and heats the screen grid if the screen heats up sufficient it can melt and destroy the tube. There are two sources of electrons collected by the screen gridn addition to the electrons emitted by the cathode, the screen grid can also collect secondary electrons ejected from the anode by the effect of the energetic main electrons. Secondary emission can increase enough to decrease the anode current, since a single major electron can eject more than 1 secondary electron. The reduction in anode current is because the external anode current (by way of the connection pin) is due to the cathode-to-anode present minus the secondary emission existing. This can give the tetrode valve a distinctive negative resistance characteristic, often named “tetrode kink”. This is typically undesirable, despite the fact that it can be exploited as in the dynatron oscillator. The secondary emission can be suppressed by adding a suppressor grid, producing a pentode, or beam plates to make a beam tetrode/kinkless tetrode.
The optimistic influence of the screen grid in the vicinity of the manage grid permits a designer to shift the handle grid operating voltage variety completely into the negative area (a triode of related geometry would likely demand good grid drive to attain the identical 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 manage grid to turn out to be positive (exactly where existing flow starts), nonlinearity is to be anticipated. (The control grid draws no current although negativeigh impedanceut draws existing although positiveow impedance.) With the handle grid operating completely in the negative region, and with the RF shielding afforded by the screen grid, tetrode input impedance is fairly high even at high frequencies. Gain can be almost flat from DC to full frequency. Linearity is excellent. Energy acquire in excess of ten,000 is feasible.
The triode vacuum tube also develops a “space charge” between the cathode and handle grid, which reduces its acquire, specially at low anode voltages. The screen grid of a tetrode neutralizes the space charge and increases the tube’s acquire.
Power tetrodes are generally used in radio transmitting gear, simply because the need for neutralization is much less than with triodes (see Radio transmitter style and Valve amplifier for far more particulars). Screen existing does represent loss. Some tube designers attempt to lessen screen present by placing every single wire in the screen mesh straight behind a corresponding wire in the handle grid mesh. Propagating electrons emerge from the handle 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 typical operation. Screen existing is negligible in a lot of styles. Shadow grids are employed in a variety of forms for a quantity of applications.
Far more than 1 screen grid can be used. For example the pentagrid converter has two. A tetrode can be converted to act as a triode by connecting the screen grid to the anode.
Circuit design and style considerations
Below specific operating situations, 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 area led to the term “tetrode kink”. In general, if the anode voltage exceeds the screen voltage, this region is avoided, and excellent efficiency 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 to have for each supply and sink present capability in the ideal screen energy supply. A bleeder resistor can generally be chosen to prevent the screen voltage from getting out of manage. Arcs from the anode generally hit the screen. As such, specific care is necessary in style of the socket wiring, to give a direct discharge path for arc existing. The undesirable nature of the tetrode kink led tube designers to add a third grid, referred to as the suppressor grid the resulting vacuum tube is referred to as a pentode. A lot more modern day tubes have anodes treated to minimise secondary emission.
At certain values of plate voltage and existing, the tetrode characteristic curves are kinked due to secondary emission
The adverse resistance operating area of the tetrode is exploited in the dynatron oscillator, though this was practical only with earlier tubes with higher secondary emission.
The tetrode tube was developed by Dr. Walter H. Schottky of Siemens & Halske GMBH in Germany in the course of Planet War I. Thousands of variations of the tetrode style, as nicely as its later improvement the pentode, have been manufactured since then, though vacuum tubes in low-power gear have been virtually totally superseded by strong-state semiconductor devices.
^ * L.W. Turner, (ed), Electronics Engineer’s Reference Book, 4th ed. Newnes-Butterworth, London 1976 ISBN 408 00168 page 7-19
^ Turner 1976, page 7-19
^ a b Turner 1976, page 7-20
^ http://www.ece.umd.edu/~taylor/Electrons3.htm A thumbnail history of electronics, retrieved 2009 Feb five
Categories: Vacuum tubes