Finer Details of Radio

Radio Record, Volume II, Issue 43, 10 May 1929, Page 30

Finer Details of Radio

The Three-Electrode Valve as an Amplifier

By

PENTODE

T can truthfully be said that 7; the valve plays the greatest part in most sections of radio, and modern methods of speech reproduction. Unless some alternate component had been invented and used. radio could not have pro gressed so far and so rapidly. As a rectifier the crystal cannot hold its own against the valve when properly used. One or two so-called valveles: amplifiers have been introduced, tried and then cast aside in preference t: ones using the valve. Since the valve plays such a grea part in the receiver, it is just as wel to get a fair idea of its performance not only when tested by the usua means, but also under actual operatin; conditions. This article will deal wit: the electrode valve when used as a1 amplifier, the next issue with the valv: as a detector, followed by a discussio: on the new screen grid and pento" valves. What the Graph Indicates. (THERE are quite a few really enthi. siastic amateurs who do not full: understand and realise the importanc of the graphs generally supplied with each valve, and to these a few line. will be devoted to a simple explana tion. As the name st&gests, the thre electrode valve has three elements-the filament, the grid and the plate. Two circuits are employed, the grid and the plate and. filament, the latter being common to both. The one is called the input or grid circuit, and the other the output or plate circuit. When the filament is heated by the low tension supply, electrons are given

off, which, being of a negative character, are attracted by the positive potential applied to the plate by the high tension or B supply. Each of these electrons carry a small charge of electricity, and.a current flows from the filament to plate, through the meshes of the grid. Now consider what would happen it various potentials were applied to the srid. If of a negative character, this

Would oppose tue negutive civcirons elmitted from the filament, and reduce the current flowing in the plate circuit. while if a positive potential was ap. plied to the grid, electrons would be attracted to the grid and a current would flow in the grid to filament circuit. This is undesirable, as wil] he explained later. A§ an example, let. us tuke a general purpose valve and see what relationship exists between the applied grid potential and the plate current. Starting with a constant "B" voltage of 90 volts, the normal resistance of the valve allows a current of 7.3 milli amperes to flow with the grid at 0 volts. Apply 4 volts negative bias to the grid. and the anode current drops to, say. 2.75 milliamperes. Still further increase the negative grid bias, and a point wil) be found where the applied grid potential completely cancels the effect of the 90 volis "B" battery, and no current at all flows in the plate circuit.. In the example taken this occurs when 7 volts negative bias are applied. Now, it can be seen that seven volts applied to the grid controls tr plate circuit to the same extent that the 90 vols "B" supply do, and from this the voltage amplification of the valve can be determined. In this case it amounts to 90/7 or 13 approx. Reference to the diagram shows how this can be depicted on paper in graph form. The horizontal lines indicate the anode or plate current in milliamperes, while the vertical lines show the applied grid voltage. The negative on the left and the positive upon the right. The two dotted lines indicate the curve when plotted with the various anode voltages shown. Neglect for a moment the third curve plotted aeross the graph and consider the performance. of

the valve when the grid is made positive. Space Charge. ON first thoughts it would appear that the grid, being positive, would attract the electrons to itself, robbing the anode and keeping the plate current more or less constant. When the filament is heated electrons fiy off with a comparatively small velocity, forming what is known’ as a space charge around itself. Without the attraction caused by the positive plate potential these electrons would merely fly off, circle round and return by the filament once more to the filament. As the plate voltage is in‘reased so the number of electrons vhich pass from filament to plate inrease. This continues until the space harge is no longer present, all the .vailable electrons being drawn away wy the positive plate voltage. In this ‘ondition the valve is said to be satuated. Now, instead of increasing the date voltage to proauce this effect, it an-be done by increasing the grid voltge in a positive direction. The reason hat the electrons do not'all fiy to the rid is due to the fact that their velo‘ity carries them past the grid into the lirect influence of the plate voltage, 1d only a small proportion actually ettle on the grid producing grid cursents. So much forthe characteristic curves given with various valves, and let us, see if these curves still hold good when ‘the valve is used in a receiver. Under Working Conditions. VEN a valve possessing characteristics as shown in the accompanyin, graph, let as see what happens when used in the position of an audio frequency amplifier, a transformer primary being in the plate circuit. With a plate voltage of 90 a grid bias of 4 volts can be given and the valve is operating at the position marked x. Alternating current voltages are applied to the grid, which for our argument can amount to 1 volt on either side of the operating point. These are reproduced in the plate circuit as voltage variations of this value multiplied by the voltage amplification of the valve. In this case it is 18, and the output would be 26 volts alternatin# current of the same frequency as that of the input. Neglecting the steady anode current of 2.75 milliamperes, the alternative anode voltxs would be associated with varying wunode amperage, as can be seen from the graph. On the positive 1 volt, half of the grid swing, the anode current would vary by approximately 1 milliamp. This current variation would be applied to the primary of the transformer in the anode lead of the valve. This primary has a fairly high A.C. resistance or impedance, causing a voltage drop across it. This voltage drop depends upon several factors, namely, the impedance of the transformer primary, the A.C. current flowing, and the frequency of the alternating current. Without knowing the value of all these the plate voltage cannot be defined, but it is lower than that originally used and will move the operating point of the valve

aol to some point to the right. For our ‘example let the voltage drop be equal to about 30 volts, thus for this halfcycle the valve would be working on the point marked X. On the other half-cycle the current flowing through the transformer would be decreasing and, due to the dynamic effect of this component, anode voltages would be built up on the plate exceed-ing-the normal B battery voltage. Thus the valve would operdte on a slightly different curve, represented by the point X2. The difference between character- .. istic curves obtained by the tésts made on the bench and the working curve actually made use of when. a load is included in the plate circuit is now quite apparent. f Across the Zero Line. Yr has been asked often, why, as .the characteristic curve is straight on both sides of the zero grid point, cannot the grid swing be allowed to _run over the other side to the positive "even slightly? Reference to the characteristic curves on the graph make it seem that even if the grid worked at zero potential then the variations

in the plate circuit would be in direct proportion as the line runs quite straight on either side for a considerable difference. This is so for charuacteristic curves only. If the grid becomes: positive, then .grid currents would flow. As a resistance or impedance is included in the grid circuit of any working valve, this would interfere with any current flowing in the grid circuit, It has been shown before that a voltage drop occurs across the | anode impedance or resistance and the Same would occur across the grid impedance if current was allowed to flow and alter altogether the straightness of the working curve. These currents certainly would be comparatively small, but being in the grid circuit would be amplified by the valve itself Anotner point that is made apparent is that the correct operating point of a valve is obtained by. biasing the grid at a point to the left of the middle of the straight portion of the characteristic curve.