Radio Broadcast (May 1928-Apr 1929)

The 222 Tube as an R. R Amplifier Part JN THIS, the second article from ■*■ Mr. Browning s notebook on the 222 tube, the author discusses two common methods of coupling a screen-grid tube to a detector or to a following amplifier. The equations by which Mr. Browning arrived at his conclusions should be interesting to the mathematically inclined; the results of those equations and the laboratory data will be interesting to anyone who likes to keep up to date in radio. — The Editor. 32 30 28 ^26 o S24 o i22 I 20 <: a is < 5 16 o "14 12 10 > 5 4 _ 2 i -=: 3 300 350 400 WAVELENGTH IN METERS 450 IN THE article on page 252 of September Radio Broadcast the characteristics of d.c. and a.c. types of screen-grid tubes were discussed and their performance in untuned amplifiers was also considered. It is the object of this article to treat of two types of tuned radiofrequency amplifiers, one the common radiofrequency transformer where a primary and secondary winding is used, and the other the auto-transformer usually termed tuned impedance. The function of a tuned radio-frequency amplifier is not only to amplify incoming signals but also to give the desired amount of selectivity. There is also the question of the tendency of the preceding circuits to oscillate, which is very important with tubes which have a great deal of FIG. 3 Theoretical and actual voltage amplification curves over the cast band for the types of r.f. coupling discussed in this cerned Fig. 1 reduces to Fig. 2, where a voltage of ■j.Eg is applied in series with the plate resistance, Rp, and the primary of the transformer. Analyzing this circuit and making certain simplifications the voltage amplification is (0 Where T L, V2 lg + ill V2 amplification factor of the tube coefficient of coupling between primary and secondary Secondary inductance in henrys Primary inductances in henrvs Rp/U" R2/L, w 2x frequency It will be readily seen by equation 1 that there is a relation between fii, ^2 and t that will make the amplification a maximum. This relation is = tlu t)2 (2) for maximum voltage amplification. The amplification obtained by the transformer and tube when this relation is satisfied is (3) 2.25 VAC. + B67 +B135 THE ABOVE FIG. I. TO LEFT — FIG. 2. The circuit diagram and its schematic equivalent circuit for a stage of r. f. using a t. r.f. transformer capacity between grid and plate. This effect, however, is minimized with the screen-grid tube, and consequently will not be dealt with at length here. To determine the design of an r.f. transformer for the screen-grid tube the mathematics for a one-stage amplifier such as shown in Fig. 1 should be examined and the voltage amplification, i. e., output voltage, E, divided by input, Eg, calculated. As far as alternating current is con Ee 2 V r;i Tj2 From this analysis it may be seen that L2 should be as large as possible consistent with tuning down to the lowest wavelength desired. Li should be as small as possible consistent with satisfying the relation t2 = 1)1 Tfe. It should be noted that when Li is small that r)i is large and consequently the coupling must be increased. Thus it is advantageous to make the coupling large consistent with keeping the capacity between the primary and the secondary windings small, as this capacity between the two circuits has the effect of introducing a voltage in the secondary circuit somewhat out of phase with the voltage induced by the magnetic coupling. With the ordinary 199, 201 a, 226, and 227 type tubes, the plate resistance is sufficiently low so that all the above relations may be satisfied, and maximum gain may be obtained. (See Proceedings of Institute of Radio Engineers, December, 1926.) However, with the screen-grid tube the plate resistance is between 400,000 and 700,000 ohms so that rn is very large and the relation t2 = yjx tj2 can never be satisfied. Of course the primary inductance of the r.f.t. may be increased up to the point where the distributed capacity of the winding itself tunes the primary to some frequency in the wavelength band. 112 is made as small as possible but can never be reduced below a value of about 0.003 except with regeneration. Therefore, it is essential in the design of a transformer for the screengrid tube to make the coupling very large. This problem was attacked by the writer some months ago and by careful design the coefficient of coupling was increased from its usual value of about 0.5 to 0.91. This factor depends upon the geometrical relation between primary and secondary in such a way that the shorter the secondary winding with the primary in a given position the larger x becomes. The coils which showed a t of .91 were wound on a 2" form and had a winding length of tV The primary was slot wound and placed J" from the low potential end. With these coils in the one-stage amplifier the circuit of which is shown in Fig. 1, and using a CeCo a.c. 22 tube, an amplification of about 20 550 broadarticle 32 30 28 26 24 22 0 20 1 18 _l Q_ 1 16 S 14 o 12 10 8 6 4 2 41 4i ■ 1 — 1 — ■ r■ ■ 1 H 1 — i • -i 1 J l t -i • 4 r / — / 50 40 30 20 10 0 10 20 30 40 50 BELOW RESONANCE "ABOVE RESONANCE MMFD. FIG. 4 The solid curve is the actual resonance curve of the transformer shown in Fig. 1 and 2 at 400 meters. The dotted peak is the portion of the calculated curve which does not coincide with the actual curve. The dot-dash curve is the resonance curve of the tuned impedance shown in Figs. 5 and 6 359