Radio mirror (Jan-Oct 1923)

March ij. 1923 RADIO DIGEST A. B. C. Lessons for Radio Beginners Chapter XI — The Vacuum Tube as an Amplifier By Arthur G. Mohaupt » MPL.IFICATION* in Radio refers to /\ stiviiKihening or increasing the inJ~\ tensity of Uie signals received and detected. Prior to the perfection of the ihree-«lectrode vacuum tube, numerous attempts haT been made to produce or develop son amplifier device, but all attempts w e without real success. With the advent of the vacuum tube, however, the problem was solved quickly, for not only can the vacuum tube be used as a detector of Radio signals, but under the proper conditions can also be used as an amplifier of the rectified oscillations existing in the output or plate circuit of the detector tube. Exactly how this can be accomplished will now be explained. The Vacuum Tube as an Amplifier Although the average detector tube can also be used as an amplifier with some degree of success, better results can be obtained if a specially prepared tube, known as an amplifier tube, is employed. Although an amplifier tube greatly resembles a detector tube in its mechanical construction, it differs, however, in that a much higher vacuum exists within the amplifier tube. In fact, the degree of vacuum is_practically as high as can be obtained with modern exhausting apparatus. On account of this higher vacuum the amplifier tube is often referred to as a "hard tube" to distinguish it from the soft detector tube in which there are still small quantities of gas left. The higher vacuum makes it possible to employ a much higher plate pressure, and hence the oscillations impressed on the g"rid can be greatly amplified without in any way distorting or altering the nature of the signals. General Operation of Tube In order to understand fully how a three-electrode vacuum tube adds as an amplifier, let us review briefly the general operation. "U"e will remember that as the filament of a tube is heated, negatively charged electrons are emitted which fill the interior of the tube. If then a positive potential is applied to the plate from some source of electrical energy, such as a group of dry cells, the electrons are attracted toward the plate and render the intervening space a good conductor of electricity. A current can then flow in the plate circuit, the energy being supplied by the dry cells. Between the filament and the the decreases, the signals would be distorted and the sounds heard would not be very pleasant. Increase and Decrease in Plate Current If then, an alternating electromotive force Is impressed upon the input circuit of the amplifier, that is, across its filament and arid, the positive and negative the filament and grid where they undergo rectification and are reduced to oscillations at an audio frequency. These audio frequency oscillations are then sent into the primary of the transformer and induce current oscillations of a much greater potential in the secondary circuit. These oscillations are then impressed Figure 41 voltage alternations will cause corresponding increases and decreases in the plate circuit current — with the difference, however, that the intensity of the electrical oscillations in the plate circuit is much greater than that of the oscillations initially impressed upon the plate and grid. The necessary additional energy is supplied by the battery supplying the high plate pressure. The alternating electromotive for operating the input circuit of the amplifier tube is generally obtained by means of a transformer connected into the output circuit of the detector tube. This transformer is of the step-up type, so that the change of voltage impressed on the grid will be as high as is practically possible. Very slight, practically no current is required to affect the grid, and hence large voltage variations can be created in the plate circuit by supplying only minute quantities of energy to the grid circuit. It is for this reason that the three-electrode vacuum tube can be used so effectively as an amplifier of electrical oscillations. Connections for Amplifying Tube In Figure 40 is shown a wiring diagram illustrating the scheme of connections used when an amplifying tube is used in connection with a transformer for amplifying the electrical oscillations in the output circuit of a detector tube. D is the detector tube with the connections M and N leading to the turner. In the plate Figure 48 plate is the grid in the form of a screw or woven network of fine wires. The electrical condition of this intervening grid has the ability to control the electronic emission and hence also the flow of current in the plate circuit. If the grid is negatively charged, it repels some of the electrons back upon the filament and thus weakens the plate current. On the other hand, if the grid is charged positively, it attracts the electrons on their way to the filament, inr creases the number that are emitted from the filament, and in this manner strengthens the current flowing in the plate circuit. Detector and Amplifier Action That the tube may function as a detector or rectifier of electrical oscillations, the relative electrical, conditions of the three elements or electrodes has to be such that a positive charge on the grid produces a greater increase in plate current flow than the decrease in plate current flow caused by an equal negative charge. In this manner a unidirectional p-ulsating current is set up in the plate circuit, and the envelope of these direct current oscillations correspond in all details to the electrical oscillations that are initially sent out at the transmitting station. That the tube may act as an amplifier of electrical oscillations, the relative electrical conditions of the three electrodes must be slightly different. The conditions must be such that equal positive and negative variations of the grid potential will cause correspondingly equal increases and decreases in the plate circuit current. This state o^f affairs is effected by applying a higher positive pressure to the plate of the tube. If the increases in plate current were greater or less than or output circuit, instead of having the telephone receiver, we have the primary winding of an iron-core step-up transformer. The secondary of this transformer, in turn, is connected across the filament and grid of the amplifier tube* T. Into the plate circuit of this tube the telephone receivers are then connected in series with the high-voltage battery B-2. The circuit arrangement operates in the following manner: When the receiving apparatus is tuned to the frequency of the incoming electrical oscillations, these are impressed upon Foote's Hand-made Triple -Test Crystal "Puts its Best Foot Foremose' Its supersensitive side is up and has loudly received broadcasting. Guaranteed QSA (your signals are strong). Look for the "T" on the back. Distributors and Travelers Wanted. Liberal Commissions. Large Sales. Quantity Production. Wire Territory Desired. F00TE MINERAL CO., Inc. S'ffiSs 107 North 19th Street, Philadelphia, Pa. Established 1876 upon the filament and grid of the amplifier tube T. Here they cause corresponding fluctuations in the plate circuit current, but since the battery B-2 is capable of supplying additional energy, the electrical oscillations are of much greater intensity and hence are capable of affecting the telephone receivers so that louder sounds are produced in them. In case one amplifier tube does not produce sufficient amplification, a second one can be used, but it is seldom advisable to employ more than two, for otherwise undesirable distortion of the signals is likely to result. Kinds of Amplification Amplification in Radio circuits can be effected in several ways. The first method was explained in the previous paragraphs, and consists of sending the electrical oscillations in the output circuit of the detector tube into one or more amplifier tubes until the desired strength of signals is obtained. This method of amplification is known as "cascade amplification," for the amplifier tubes are said to be connect, d in cascade — the word cascade meaning one on top or above the other. It is seldom advisable, however, to employ more than two stages of such cascade amplification, for as was stated, undersirable distortion of the signals is otherwise likely to result. This method of amplifying the audio frequency oscillations of the output circuit of the detector tube Is also commonly referred to as audio frequency amplification since it affects or amplifies the audio frequency oscillations. Connecting' Amplifying: Tube in Cascade In Figure 41 is illustrated the method of connecting two amplifier tubes in cascade. Such an arrangement is known as a two-step or two-stage amplifier. As is (ihown, the output circuit of the detector tube is connected through an audiofrequency transformer to the input circuit of the first amplifier tube. Into the plate circuit of this amplifier tul.3 is connected the primary of a second audio frequency transformer, and the secondary of this transformer is in turn connected into the input circuit of the second am(Continued on page 12) 'Here'syour opportunity. Radio needs you. Win success in this fascinating field. Trained men in demand at highest salaries* Learn at home, in your spare time. Be a Radio Expert I will train you, quickly and easily, to design. construct, install, operate, repair, maintain, and sell all forms of Radio apparatus. My newmethoda are the most successful in existence. Learn to earn $1,800 to $10,000 a Yea; prJCCT Wonderful, home-construction. : ■.-.:a ■ ■»■»*• receiving set, of latest de^^n. Write for"Radio Facts" free. 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