62 BELL SYSTEM TECHNICAL JOURNAL 



should be operated from a single battery. In the case of a retard 

 coupled amplifier whose stages are tuned with condensers, each stage 

 should preferably have a separate plate battery to reduce the tendency 

 to "sing." 



A generator as a source of plate voltage is frequently used for 

 power amplifiers. In case a direct current generator is used, a filter 

 is generally necessary in the plate circuit to remove commutator 

 ripples. 



26. Filament Voltage Supply. A source of constant filament voltage 

 is not necessary in order to insure constant space current within the 

 tubes at temperature saturation, but in general any variation in fila- 

 ment current will affect the relative potential difference between 

 filament and grid, and is, therefore, equivalent to a variation in the 

 input voltage. This possible source of trouble must be particularly 

 guarded against in such a circuit as that shown in Fig. 25, in which a 

 portion of the adjustable resistance of the filament circuit is included 

 between the filament and the "common point" A. If storage batteries 

 are available, they form the best source of filament current; generators 

 have been used satisfactorily however. 



27. Sources of Grid Potential. A flow of electrons to the grid of a 

 tube is liable to result either in distortion or a loss of amplification or 

 both (see Sees. 15 and 16). The steady negative grid potential re- 

 quired to prevent the input voltage carrying the grid to a positive 

 potential may be obtained from either one of two sources: by a grid 

 battery or by an IR drop in some resistance in the circuit. The re- 

 quirements for a grid battery are very light since it is called upon to 

 give no appreciable current. As was pointed out in Sec. 26, use of 

 an IR drop for grid voltage pre-supposes steady filament or plate 

 battery according to circumstances, and since the proper grid battery 

 is readily obtainable the use of an IR drop is likely to prove desirable 

 only in very unusual circumstances. 



VI. Troubles in Amplifier Circuits 



28. Noise. The noise in amplifier circuits is due to several causes 

 which may, in general, be grouped into two classes. Certain noises 

 originate within the tubes and other noises find their origin in the 

 circuit. The amount of noise in any amplifier limits the minimum 

 input voltages which it will handle satisfactorily, for obviously input 

 voltages which produce output currents of the same order of magni- 

 tude as the currents giving rise to noise will not be satisfactorily 

 amplified. 



