Radio broadcast .. (1922-30)

How to Build a ResistanceCoupled Amplifier An Inexpensive Three-Stage Amplifier Unit Which Will Give Distortionless Amplification By G. Y. ALLEN Westinghouse Electric & Manufacturing Company HE three-electrode vacuum tube is in itself a very good amplifier of electric voltages. Tubes have been constructed whose plate-to-filament voltages range as much as thirty to forty times the grid-to-filament voltage variations. Furthermore, within certain limits, this voltage amplification is substantially a linear relation. Where power must be absorbed from the output side of a tube, however, high amplification factors, with their accompanying high plate impedances, require high B battery voltages in order to maintain the linear relationship between the input and output sides. These considerations limit the design of practical tubes to those having an amplification constant of from 6 to 8 and plate impedances ranging around 20,000 ohms. Such tubes work very well as amplifiers using from 45 to 150 volts B battery. THE PLACE OF THE TRANSFORMER IN AUDIO AMPLIFICATION NOW it is a well defined law in electrical engineering that the most efficient conditions are met in any device supplying electric power when the impedance drop across the power consuming device is the same as the impedance drop across the device supplying the power. When using two or more tubes in cascade amplification, the grid circuit of any tube is the load consuming device for the plate circuit of the preceding tube. But the grid impedance of a vacuum tube as it is used in an amplifier is very much greater than the plate impedance. Thus, if two or more tubes are connected directly in cascade without intervening apparatus, great amplification will not result. This condition has made very popular the practice of using step up transformers between the plate circuit of one tube and the grid circuit of the next tube, which transformer presents to both the grid and plate circuits impedances permitting the tubes to be worked at their most efficient point. Furthermore the inherent voltage amplification factor of the tube is multiplied by the step up ratio of the transformer resulting in greatly increased amplification per stage. Were amplification all that were to be considered, transformer-coupled amplification would be ideal. With the advent of good broadcast transmission, however, and with the production of good loud speakers, the broadcast listener's standard of quality has crept steadily upward. A transformer, being essentially an inductance cannot transform all voice frequencies with exactly the same amplification no matter how carefully designed, with the result that all amplifying transformers suppress the lower and favor the higher frequencies. True, great improvements have been made in transformer design, but the fact remains that fundamentally the transformer cannot ever give distortionless amplification. WHEN RESISTANCE-COUPLED AMPLIFIERS ARE DESIRABLE FOR those who demand quality at all cost, the resistance-coupled amplifier is unquestionably the most desirable. The amplification per stage is not as great as with transformers, but a nearly absolute linear relation holds between input and output. A curve showing amplification plotted against frequency for a transformer as compared with the linear relation existing in a resistor is shown in Fig. 2. It will be noted that the amplification of a transformer, and the curve is shown for one of the best transformers on the market, falls off very rapidly below 200 cycles. In other words, all notes below middle C on the piano are reproduced much below their normal amplitude with resulting loss in quality. On the other hand (Fig. 3), the amplifier using resistance-coupling shows nearly constant amplification throughout the range of frequencies plotted, although some