DOUBLE-SIDED AMPLIFIERS 



designs, however, pentodes are used because of the advantages they possess — 

 enumerated earlier — and the output waveform distortion is controlled by the 

 application of suitable negative feedback. 



(3) Possibility of securing high efficiency — the class B stage — The power 

 amplifiers which we have discussed so far have been biased to the middle of 

 the working region. Operation in this mode is called class A. In single-sided 

 circuits class A working is essential but with the push-pull arrangement it is 

 not. With class A push-pull output stages there is a considerable standing 

 anode current which flows whether there is any signal input or not and 

 consequently represents a waste of power. 



Class B 

 working 

 point 



Figure 12.15 



In class B working the valves are biased to anode current cut-off, so that 

 the working point of each valve is near the bottom right-hand corner of the 

 working region {Figure 12.15). The standing anode current in the absence 

 of a signal at the input is therefore zero. On the arrival of a signal, each valve 



Contribution 

 of one valve 



_/V7V 



Contribution 

 of other 

 valve 



Total 



A^A^V 



Figure 12.16 



conducts alternately — passing a large anode current when the grid is swing 

 positive and remaining cut-oflF when the grid goes negative {Figure 12.16). 

 High tension power is consumed by the stage only when required, and it is 

 therefore extremely efficient. 



180 



