The invention of electronic lamp is directly connected with the development of lighting technology. In the early 80-ies of the XIX century, the famous American inventor Edison was engaged in improving the incandescent lamp. One of its drawbacks was a gradual decrease in light output due to the tarnishing of the cylinder due to the appearance of dark spots on the inside of the glass. Exploring in 1883. The reasons for this effect, Edison noticed that often tarnished glass bottle in thread loop plane remained light, almost unobscured band, and the band is always found on the side of the lamp, where the positive input of filament circuit. The thing looked like a part of the coal filament adjacent to the negative input, emitted from itself the smallest particles of matter. Flying past the positive side of the thread, they covered the inside of the glass bulb everywhere, except for that line on the surface of the glass, which, as it obscures the positive side of the thread. The picture of this phenomenon has become more evident when Edison introduced into the glass cylinder small metal plate, placing it between the inputs of the filament. Combining this record through the galvanometer with a positive electrode filaments can be seen flowing through the space inside the cylinder electric current.
Edison suggested that the flow of coal particles emitted from the negative side of the thread, makes part of the way between the filament and plate conductive introduced them, and found that the flow is proportional to the extent of the filament, or in other words, the light power of the lamp. This, in fact, and ends with a study of Edison. American inventor could not then imagine, at the threshold of the greatest scientific discovery he stood. It took almost 20 years before Edison observed phenomenon has its proper comprehensive explanation.
It turned out that when strong heating lamp filament placed in a vacuum, it begins to emit electrons into the surrounding space. This process is called thermionic emission and it can be regarded as the evaporation of electrons from the filament material. The thought of the possibility of the practical use of "Edison effect" for the first time came up with the English scientist Fleming, who in 1904 created based on this principle, the detector, known as the "two-electrode tube" or "diode" Fleming. Fleming lamp is a conventional glass bulb filled with rarefied gas. Inside the cylinder was placed filament with covering her metal cylinder. The heated electrode lamp continuously emits electrons, which formed around the "electron cloud". The higher the temperature of the electrode, the higher the density of the electron cloud provided. When connecting the lamp electrodes to a power supply, an electric field between them. If the positive pole connected to a cold source electrode (anode) and negative - of the heated (cathode), then an electric field, the electrons leave the electron cloud partially and rushed to the cold electrode. Thus between the cathode and the anode electric current is installed. When you turn on the source of the opposite anode negatively charged electrons are pushed away, and the positively charged cathode - attracted. In this case, the electric current does not arise. That is, the Fleming diode has a pronounced unilateral conductivity. Being included in the receiver circuit, the lamp is operated like a rectifier, a current flowing in one direction and not the reverse flowing and could thus serve volnoukazatelem - detector. To increase the sensitivity of a lamp fed appropriately chosen positive potential. In principle, the receiving circuit with a lamp Fleming almost no different from the others of the radio at the time. She gave way to sensitivity of the magnetic circuit with the type of detector, but had a much greater reliability.
A further outstanding achievement in the field of technical improvement and application of electronic lamp was the invention in 1907 by American engineer De Forest lamps containing an additional third electrode. This third electrode was named the inventor of "net" and the lamp itself - "audinom", but in practice it was fixed for another name - "transistor". The third electrode, as can be seen from its name, was not continuous and can pass electrons flying away from the cathode to the anode. When between the grid and the cathode comprises a source of voltage between the electrodes, an electric field is strongly influenced by the number of electrons reaching the anode, that is, the strength of the current flowing through the lamp (power of the anode current). When reducing the voltage supplied to the grid, the power of the anode current decreases with increasing - increased. If the grid was supplied negative voltage, anode current general stopped - lamp are "confined". A remarkable property of the transistor was the fact that the drive current can be many times smaller than the main - insignificant change of voltage between the grid and the cathode causing quite a significant change of the anode current. The latter allows to use the lamp to enhance the small AC voltage and provides it with extraordinary opportunities for practical application. The appearance of a three-electrode lamp resulted in a rapid evolution of radio circuits, since there was the possibility of tens and hundreds of times amplify the received signal. Repeatedly increased sensitivity receivers. One of the early schemes tube receiver was proposed in 1907 by the same De Forest.
Between the antenna and the ground circuit is enabled LC, at the terminals of which there is a high frequency alternating voltage, formed by the action of the energy received from the antenna. This voltage was applied to the lamp and controlled mesh anode current fluctuations. Thus, in the anode circuit receives amplified received signal fluctuations which may cause the membrane to move phone included in the same circuit.
The first three-electrode lamp audin De Forest had many drawbacks. Location electrodes so it was that most of the electron beam do not enter the anode and on the glass bottle. Control the effect of the grid is not enough. The lamp was evacuated and poorly contained a significant number of gas molecules. They are ionised and continuously bombarded filament, giving her a devastating impact.
In 1910, a German engineer created Lieben-enhanced triode vacuum tube in which the grid has been formed as a perforated aluminum sheet placed in the center of the cylinder, dividing it into two parts. The bottom of the lamp filament was at the top - the anode. This arrangement allowed the grid to strengthen its control action, as it passed through the entire electron beam. The anode in this lamp was shaped like a twig or a spiral of aluminum wire, and served as the cathode platinum filament. Particular attention is paid to increase Lieben emission properties of the lamp. For this purpose it was first proposed to cover the filament with a thin layer of calcium oxide or barium. Additionally, we introduced into the balloon mercury vapor, which created additional ionization and thus increased cathodic current.
Thus, the vacuum tube, first came into use as a detector, then - amplifier. But a leading role in the radio it has won only once been found possible to use it for generating undamped electrical oscillations. The first tube generator created in 1913, a remarkable German radio technician Meissner. On the basis of the transistor Lieben he also built the world's first radiotelephone transmitter and implemented a radiotelephone link between Nauen and Berlin at a distance of 36 km in June 1913.
Tube generator comprises an oscillating circuit consisting of an inductor L and the capacitor C. It has been said that if a capacitor is charged, the damped oscillations occur in the circuit. To fluctuations do not fade, you need to compensate for the energy loss in each period.
Consequently, the energy from the DC voltage source must periodically enter the circuit. For this purpose, an electric circuit oscillation circuit included triode vacuum tube, so that the oscillation circuit fed with a grid on it. The anode lamp circuit comprises a coil Lc, inductively coupled with the coil L of the oscillatory circuit. At the time of the circuit current of the battery is gradually growing, moving through the transistor and the coil Lc. Thus according to the law of electromagnetic induction in the coil L will be an electric current which charges the capacitor C. The voltage from the capacitor plates, as is evident from the diagram, is supplied to the cathode and grid. When the positively charged plate of the capacitor is connected to the grid, that is, its positive charge, which contributes to increase the current flowing through the coil Lc. This will continue as long as the anode current reaches the maximum (because the current of the lamp determined by the number of electrons, evaporated from the cathode, and their number can not be indefinitely - increasing up to a certain maximum, the current no longer increases with the grid voltage). When this happens, it flows through the coil Lc DC. Since the inductive coupling is carried out only for AC current in coil L will not. In this regard, the capacitor begins to discharge. The positive charge of the grid, will therefore be reduced, which immediately affect the value of the anode current - it will also decrease. Consequently, the current through the coil Lc is decreasing, creating a current in the coil L in the opposite direction. Therefore, when the capacitor C will be discharged, decreasing the current through Lc will still induce a current in the coil L, whereby the capacitor plates will be charged, but in the opposite direction so that the plate associated with the mesh, will accumulate a negative charge. This will cause eventually a complete cessation of anode current - the current flow through the coil L again stopped and the capacitor starts to be discharged. As a result, the negative charge on the grid will be less and less, will reappear anode current, which will increase. So the whole process is repeated again. From this description it is seen that through the grid of the tube will flow alternating current, whose frequency is equal to the natural frequency of the oscillation circuit LC. However, these fluctuations are not damped, and constant, as they are supported by continuously adding battery power through the coil of Lc, inductively coupled to the coil L.
The invention of the generator tube allowed to make an important step in the technique of radio communications - apart from the transmission of telegraph signals, consisting of short and longer pulses became possible reliable and high-quality telephone connection - that is, transmission using electromagnetic waves of human speech and music. It may seem that the radio communication is not a big deal. In most cases, the sound vibrations via the microphone easily converted to electric. Why, strengthening them and submitting to the antenna, transmit over a distance of speech and music the same way as passed before Morse code? However, in reality this method of transmission is not feasible, as well radiated through the antenna only high frequency energy. A slow oscillation frequency sound excited in the space is so weak electromagnetic waves that take them there is no way. Therefore, to create a vacuum-tube oscillators that produce oscillations of high frequency radio communication is extremely difficult. To transmit the vibrations or sound change is said to be modulated by low (sonic) frequency. modulation point is that the high and low frequency oscillations from the oscillator microphone overlap and thus fed to the antenna.
Modulation may occur in different ways. For example, the microphone included in the antenna circuit. Since the microphone resistance changes under the influence of sound waves, the current in the antenna will in turn change; In other words, instead of oscillation with a constant amplitude, we have oscillations with varying amplitude - modulated high frequency current.
The received modulated RF signal receiver even when the gain is not capable to cause fluctuations in the membrane or phone horn speaker with a sound frequency. It can only cause high-frequency vibrations, is not perceived by our ear. Therefore, the receiver is necessary to make the reverse process - select audio signal from the high-frequency modulated waves - or, in other words, produce a detection signal.
Detection was performed by a vacuum diode. Diode, as already mentioned, was passed a current in only one direction, transforming the alternating current into a pulsating. This pulsed current is smoothed by the filter. The simplest would be a filter capacitor connected in parallel with the handset. The filter operation occurred since. At that time, when the diode current is missed, some of it was diverted to the capacitor and charge it. In the intervals between pulses when the diode is trapped, the capacitor is discharged to the tube. Therefore, in the interval between the pulses of current to flow through the tube in the same direction as that of the pulse itself. Each next pulse recharges the capacitor. This current flows through a tube sound frequency, the form of which is almost completely reproduces low-frequency waveform to the transmitting station. After amplification, the electrical oscillations of low frequency converted into sound; The simplest crystal set consists of oscillating circuit connected to the antenna, and connected to the loop circuit consisting of a detector and a phone.
The first vacuum tubes were still very imperfect. But in 1915, Langmuir and Gede offered an efficient way of pumping light to very low pressures, thereby replacing an ion vacuum lamps come. This electronic equipment raised to a much higher level.