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BELL SYSTEM TECHNICAL JOURNAL 



18. Resistance Coupled Amplifier. This simplest of all amplifier 

 circuits (Fig. 23) is particularly useful where a wide range of frequencies 

 is to be amplified without selective amplification of any particular 

 frequencies. For this reason, it is often used as an amplifier in con- 



Output 



Fig. 23 



nection with an oscillograph. It is also one of the few types of circuit 

 which can be used for direct current amplification. However, as will 

 be pointed out later, a special type of push-pull circuit makes a more 

 satisfactory d.c. amplifier for many purposes. 



One or more stages of the resistance coupled circuit may be sub- 

 stituted for transformers in voltage amplification. Voltage amplifier 

 tubes having an amplification constant a< = 30 are common and it fol- 

 lows from Equation G that such a tube can readily produce a voltage 

 amplification of from 20 to 25. It can, therefore, take the place of an 

 input transformer in one of the other types of amplifier circuit. Unless 

 special considerations require another adjustment, it is customary to 

 arrange all but the last stage of an amplifier for voltage amplification, 

 the last stage being designed for power amplification. (See Sees. 14 

 and 15.) 



Since in resistance coupled amplifiers there is a d.c. path between 

 the plate of one tube and the grid of the following tube, a negative 

 grid battery large enough to counterbalance the plate battery must 

 be used in every stage in order to supply the necessary negative grid 

 potential. As shown in Fig. 23, a common plate battery can be used 

 for two or more stages. A more complete discussion of battery re- 

 quirements is given under Power Supply. 



19. Resistance-Condenser Coupled Amplifier. This type of circuit 

 (Fig. 24) is similar to the preceding in all respects except that con- 

 densers are inserted between the plates and grids of adjacent stages. 

 This makes the employment of large negative grid batteries unneces- 

 sary although it is still important that steady negative potentials be 

 applied to the grid of each tube sufficiently large to prevent their 

 being carried positive by the variations. For example, in Fig. 24, 

 r' g may be two meghoms and the grid battery emf 2 to 3 volts. This 



