Radio broadcast .. (1922-30)

RADIO BROADCAST. (4) above this value. The primary resistance was 1300 ohms and the secondary 12,000. The effect of a change in either plate voltage or grid bias materially modified the shape of the characteristic, as shown by Fig. 14. The amplification increased with an increase of plate voltage and a decrease of negative grid bias. For a plate potential of 45 volts and a grid bias of 3 volts the curve rises rapidly to a maximum and then decreases rapidly to a very low value as the fre- quency is increased. Series V: The five trans- formers here compared have the constants given in Table I on this page. A comparison of the three transformers having 5000 turns in the primary and 15,000, 20,000, and 25,000 turns in the secondaries, re- spectively, is given in Fig. 15. There are two curves for each transformer but for the present consider only the higher one which is for normal conditionsof operation. Atlow frequencies the amplification is almost exactly proportional to the number of turns in the secondaries but as the fre- quency is increased the ampli- fication gradually increases to a maximum of approximately 175, 150, and 115 per cent, of the low frequency values and then decreases quite rapidly. These transformers, reach their maximum amplification at 6000, 4000, and 3000 cycles, respectively. It happens that the first and last trans- formers have almost exactly the same average amplification over the nine- kilocycle band while the intermediate one has a somewhat higher average and on the whole is probably the best. Al- though the average amplification is not necessarily the best measure of a trans- former, especially if taken over a wide band of frequencies, frequencies below five thousand may be far more important in some cases than those above. An added capacity of 50 mmfd. across the secondary has slight tendency to increase the ampli- fication at certain frequencies. This tend- ency, which is greatest in the case of the low-ratio transformer, is negligible for the intermediate, and vanishes for the high ratio. Above a certain frequency, depending on the transformer, there is a marked decrease in the amplification due to the added capacity indicating the de- sirability of keeping the distributed capa- city of the secondary and the capacity of the leads as small as possible. Different Turns Ratios Compared In Fig. 16 are compared transformers having a ratio of 3:1 with primaries of 5000 and 7500 turns and transformers having a ratio of 5:1 also with primaries of 5000 and 7500 turns. It will be observed that the cut-off is much more marked in the case of the high-ratio transformers. This is due largely to the increased dis- tributed capacity of the secondaries as indicated in Fig. 15. In the case of the 5:1 transformer with the 7500-turn primary, the Transformer Equations Fig. 16 ance and, therefore, tends to cause a larger portion of the total amplified voltage in the plate circuit to be consumed by the induced electromotive force of the primary, and, therefore, a greater induced electro- motive force in the secondary. The prim- ary current may be found from equa- tion (3) and the secondary current is: W'M* •R 2 Zl ''TF^FF-^f^-^] 2 Ei M/C 2 E2 °r = ~TT/T~ w'M 2 *2 I 2 T w.*M» (wL* - -±- /RVt- K« —H / R + —/— rr*1 + v *c 2 J " V wC 2 // R.+ R2+ ( wU __L-) «u — — — R2 ^( wL! ~^c7 Table I Ratio Primary turns Secondary turns Primary resistance Secondary resistance Size of wire Ti 3:1 5,000 15,000 1,750 7,300 40-40 Tz 4:1 5,000 20,000 1,750 13,500 40-41 5:1 5,000 25,000 1,750 27,000 40-44 T 4 3:1 7,500 22,500 6,200 25,000 44-44 50OO-I5000 5000-20000 5000 -£SOOO -FREQUENCY IN KILOCYCLES Fig. 15 cut-off is at 2000 cycles and is quite sharp. In Fig. 17, the same comparison is made but with 45 volts on the plate, and this shows that there is considerable difference between the characteristics for the two plate voltages. In analyzing these curves in greater detail than time will permit here it will be helpful to recall the equations for the equivalent transformer constants. Where the primary of a transformer has resistance and inductance but negligible distributed capacity and the secondary has resistance, inductance, and capacity which includes the distributed capacity of the secondary and that of the leads and grid-filament of the associated tube, for the sake of sim- plicity, taken as a fixed value across the secondary, and further that the mutual capacity between the two windings be neglected, one may replace the transformer with an equivalent resistance and react- ance in so far as the plate circuit of the amplifying tube associated with the prim- ary is concerned. The equivalent constants then have the values indicated by equations (1) and (2). For the low values of frequency and the capacity usually associated with the secondary, the total reactance of the secondary is capacitive and when trans- ferred to the primary has the effect of in- creasing the equivalent inductive react- • NOVEMBER 1929 • and what we are more concerned with is the voltage across the term- inals of the secondary which is given in equation (4). For low frequencies uL^ is less than l /u>Ci and as the frequency is. w. 2 —£—) increased these two factors approach equality until a con- dition of resonance is reached in the secondary and the first radical in the denominator decreases to a minimum value of Rs as a result of which a large output voltage is ob- tained. As the frequency is further increased ul/j is greater than l /<*>C.i and the denomina- tor increases, thus reducing the output voltage. An increase of secondary capacity due to the added condenser causes the condi- tion of resonance to be reached at a lower frequency after which the output voltage de- creases rapidly as may be seen from the equation. These re- sults will be somewhat modi- fied by the mutual capacity between the two windings of the trans- former. T 5 5:1 7,500 37,500 6,200 46,000 44-44 Bibliography JOHNSON: Transmission Circuits for Telephone Communication. D. Van Nostrand Co. CASPER: "Telephone Transformers." American Institute of Electrical Engineers, March. 1924. MARTIN AND FLETCHER: "High-Quality Trans- mission and Reproduction of Speech and Music." American Institute of Electrical Engin- eers, March, 1924. KELLOGG: "Design of Non-Distorting Power Am- plifiers." American Institute of Electrical Engineers, May, 1925. — The Editor. Fig. 17 45