Radio Broadcast (May 1927-Apr 1928)

JANUARY, 1928 THE SCREENED GRID TUBE 209 pendicularly and finally flattens out to become practically horizontal. All of this is contrary to what happens in standard tube practice and, to the student of physical phenomena, is extremely interesting. The slope of this plate voltage -plate current curve represents the plate impedance and, if plotted, an extraordinarily large scale graph would be required owing to the extent to which it changes. For example, in Fig. 1 it is 74,000 ohms near the origin, then it suddenly goes negative to the extent of 100,000 ohms, then positive about 10,000 ohms, and finally becomes about three-quarters of a megohm in value! The tube has a negative resistance, or a dynatron effect, at low plate voltages. These rapid and extensive changes in internal resistance are due to the varying proportions of current taken by the shield and the plate, both of which attract negative electrons, and to a certain amount of secondary emission which takes place within the tube. At the present moment, however, the detailed explanation of these effects must give way to the more practical information regarding the tube. We are more interested in this article in how the tube works than in "why." It is sufficient to state that the sum of the currents taken by the shield and the plate is constant, the plate current increasing when the shield takes fewer electrons, and vice versa. Under usual operating conditions, i.e., high plate voltages, the shield takes very little current indeed. Grid voltage -plate current curves appeared on page 1 1 1 of Radio Broadcast for December, 1927, and will not be repeated here. They conform to what one secures from other tubes of the general-purpose type. They indicate a mutual conductance of about 300 to 400 under average conditions, and an amplification factor of about 250 to 300, values which should be compared to those of standard tubes in Table 1. It is difficult to measure these factors on the ordinary bridge because of the extraordinarily high values of mu and plate impedance involved, and the better plan is to pick them from characteristic curves as we have done here. MATHEMATICS OF THE TUBE THE screened grid tube is designed primarily for radio-frequency amplification and, to understand its possibilities in amplifier circuits, we must examine somewhat more critically than usual the processes involved in the ordinary amplifier. Naturally we must have an input and output circuit, and for general analytical purposes we shall consider Fig. 3. The purpose of the transformer in such circuits is not, as many would have us believe, to increase the voltage step-up from tube to tube, but to obtain a proper impedance for the amplifier plate circuit to look into. Mathematics will show that the maximum voltage amplification will be obtained when the effective primary impedance of the transformer is equal to the internal resistance of the tube, and that under these conditions this amplification is: where Rp = inductance, 2V Rp Rs tube impedance, L = secondary Rs = secondary resistance, and to = 6.28 x frequency. If the effective resistance of the tuned circuit at resonance is higher than that of the preceding tube impedance, a step-down transformer must be used. This effective resistance may be found mathematically by substituting the proper values in the following expression: L2 0)2 FIG. 3 A diagrammatic representation of the ordinary interstage radio-frequency amplifier, consisting of a transformer, tuned to the frequency desired, connecting two tubes. The voltage gain at resonance is given in the form of two equations where L = inductance, Rs = high-frequency resistance, W = 6.28 x frequency. If we use an inductance of 250 microhenries having a resistance of 15 ohms at 1000 kc, this effective resistance will be: „ (250 x io-6)J x (6.28 x 106)2 Ko = ■ — = 177,000 Ohms 15 and if the previous tube is a 201 -a with an impedance of 12,000 ohms we shall be compelled to use a step-down transformer to secure maximum amplification and to prevent short circuiting the secondary, to the impairment of the selectivity. Using a tube and coil of these FIG. 4 For analytical purposes the two-winding transformer of Fig. 3 may be replaced by this auto transformer. The same conditions for maximum voltage amplification obtain characteristics, and with the proper primary. the maximum amplication will be: 8 250 x 10-6 x 6.28 x 106 K"max = x = ,5.0 (Approx.) 2 y 12.000 x 15 When any receiver designer states that he gets a uniform amplification per stage of much over this, he has neither used his mathematics nor Ro = Rs FIG. 5 In the ordinary tube, some electrostatic lines of force connect the plate and grid because they are at different potential. This means, simply, that some capacity exists between them and it is this capacity that causes trouble in the usual high-frequency amplifier his vacuum-tube voltmeter to substantiate his statement. This transformer, with its two windings, can be replaced by an auto transformer for all practical purposes, as shown in Fig. 4. An auto transformer, it will be remembered, is used in the "Universal" receiver previously described in this magazine and in the R. B. "Lab." Circuit, when the plate or primary coil is reversed. The impedance of the circuit, looked at from the preceding tube, must equal the impedance of that tube, and the position of the tap regulates the effective transformation ratio, so that this condition is realized, since the ratio of impedances across secondary and primary is equal to the square of the turns ratio. If we use a special radio-frequency tube with a higheramplication factorand higher impedance, such as the Ceco Type K, we must move the tap higher toward the grid end, or use more primary turns if we use a transformer, while if a 1 12 type tube is used with its lower impedance, the tap can be brought further down. Table 1 gives essential data on existing tubes. The approximate turns ratio, in the auto transformer case, can be found by substituting the value of plate impedance in the following equation: 60000 Turns Ratio V Rp TABLE 1 RP Gm TURNS TUBE \L RATIO 199 6.25 16,600 380 I .90 201— A 8.00 12,000 675 2.25 I 12 8.00 5,000 1 ,600 3-5 2 10 7.70 5,000 1.540 3-5 >7' 3 .00 2,000 1,500 5-5 222 250.00 700,000 400 1 .0 " K" 13 .00 16,800 780 1.9 Now all of this sounds simple to carry out but, practically, there are difficulties ahead — most of them due to the fact that the tube does not act like a one-way street. Some traffic always goes in the opposite direction, because of the grid-plate capacity. As soon as we get the tap on our auto transformer moved high enough toward the grid end to secure maximum amplification, we include sufficient inductance in the plate circuit of the amplifier to make it oscillate, and trouble begins. Therefore we must do one of two things: we must either move the tap down, and lose amplification because our equal impedance condition is no longer satisfied, or we must play neutralization tricks on the amplifier to keep it from oscillating, with perhaps slight loss in amplification as the price of stability. Here is where the screened grid comes in. Suppose, as in Fig. 5, we have the plate receiving electrons from the filament after passing through the grid in stiaight lines. Because of the fact that the grid and plate are at different potential there will be electrostatic lines between them, represented by the curved lines. In other words, there is some connection between the plate and the grid, other than that produced by the passage of negative electrons. Now if we surround the plate by a fine grid which is grounded, as shown in Fig. 6, these electrostatic lines do not reach the grid, and the latter is free to function only as a control on the flow of electrons. If, in addition, we make this shield positive with respect to the filament and grid, we neutralize some of the space charge which, in turn, boosts the amplification factor to a very high degree. If the plate is completely screened, the tube will be a one-way repeater, there will be no tendency to oscillate in the familiar tuned gridtuned plate circuit, and a little mathematics will