PUSH-PULL POWER STAGE 



anode current is constant the bias developed does not fluctuate and no 

 cathode bypass capacitance is required. The resistance required is half that 

 for a single-sided amplifier because it passes twice the standing current 

 (Figure 12.3). 



(4) Screen potential easily applied — If the valves are pentodes, the screen 

 potential may be derived from the HT by a simple resistance, no decoupling 

 capacitance (a.c. coupled amplifiers) or stabilizing tube (direct coupled 



HT+ HT+ 



11-r 



Bias 

 resistance 



— p/vV-" 



—\\ 



X 



vWV — ^' 



IK 



Hh 



HT+ 



HT* 



Figure 12.3 



Figure 12.4 



amplifiers) being required. This is because the screen currents of the two 

 valves also alter equally in opposite directions. The total screen current is 

 unaffected and the common screen potential may be supplied as in Figure 

 12.4. 



The appropriate stage to follow a push-pull amplifier is another push-pull 

 stage, either a further voltage amplifier or a push-pull power stage. Coupling 

 is achieved by any appropriate method from the selection available for 

 single-sided ampUfiers. 



PUSH-PULL POWER STAGE 



This is shown in the a.c. coupled 'class A' (see below) version (Figure 12.5) 

 and a direct (neon) coupled version (Figure 12.6). The high tension supply is 

 fed to a centre-tap on the transformer primary winding or load respectively. 

 Push-pull power stages have all the advantages already fisted for push-pull 

 voltage amplifiers, and three more in addition. 



(1) Absence of load polarization — ^We have seen that if the load is a magnetic 

 device, or is coupled to the valves via a magnetic device (transformer), then an 

 undesirable reduction of the handling capacity of the load or transformer 

 resulted from the steady magnetization produced by the mean anode current. 

 In the push-pull output stage the mean anode currents flow in opposite 

 directions and the net standing magnetization is zero. 



(2) Cancellation of certain forms of distortion — ^Valve anode characteristic 

 curves — even in the working region — are neither perfectly straight nor evenly 



177 



