PUSH-PULL POWER STAGE 



{Figure 12.12). This is the reason for the dictum that second harmonic 

 distortion is associated with triodes and third harmonic with pentodes. 

 With the push-pull output stage second harmonic distortion (and higher 



Figure 12.11 



Figure 12.12 



even harmonics) cancels out, as indicated in Figure 12.13. Since the inputs are 

 in anti-phase, when one valve is working in the distortion region the other is 

 not, the contributions towards the output of the two valves add, as shown, to 

 produce an undistorted total output. Unfortunately third (and higher odd) 



Contribution 

 of one valve 



Contribution 

 of other 

 valve 



Total 



Contribution of 

 one valve 



Contribution of 

 other valve 



Total 



Figure 12.13 



Figure 12.14 



harmonic distortion does not, as shown in Figure 12.14. It is thus clear that the 

 best type of output stage for all-round performance ought to comprise 

 triodes in push-pull, and at one time this view was frequently held. In modern 



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