THERMIONIC VACUUM TUBES 



51 



Referring to Fig. 14, for an input potential of 10 volts (peak value) 

 and v p = 6,000 ohms, the point B (but neither A nor C) is evidently a 

 satisfactory mean position about which to operate. 



Since both voltage and power amplification increase as E p is in- 

 creased (because r p is decreased) it is frequently desirable to have 

 the value of E p considerably larger than the lower limit just indicated. 



17. Amplifier Circuits. The fundamentals of thermionic amplifier 

 circuits may be gathered from Fig. 22. The variable input voltage 



i— Jk— i- 



Hh 



Fig. 22 



e between grid and filament of the first tube A modulates the plate 

 current of this tube. The circuit is evidently such that the variable 

 I p of tube A varies the grid potential of B and, due to the properties 

 of the three electrode tube, not only is the power applied to the grid of 

 B greater than that applied to A, but the potential variation may be 

 many times as large as e . Hence the variations in I p of B will be 

 larger than those of I p in A, thus yielding an amplifying action. 

 Tubes A and B and their associated circuits are known as the first and 

 second stage respectively. 



Amplifier circuits may for convenience be divided into six general 

 classes: 



1. Resistance coupled circuits (Fig. 23). 



2. Resistance-condenser coupled circuits (Fig. 24). 



3. Retard-condenser coupled circuits (Fig. 25). 



4. Transformer coupled circuits (Fig. 28). 



5. Feed-back circuits (Fig. 31). 



6. Push-pull circuits (Fig. 32). 



The Sections immediately following will point out the advantages 

 of each type of circuit and general design considerations. Equations 

 6 and 7 show that the amplification of which a single tube (or "stage") 

 is capable is definitely limited. For greater amplification than a 

 single stage can produce, it is necessary to arrange two or more stages 

 in cascade. Multistage amplifiers frequently consist of combinations 

 of certain of the above types of circuits as will be pointed out in the 

 following paragraphs. 



