DIFFERENTIAL POWER AMPLIFIER 



differentially and hence the anode currents, producing differential alteration 

 of the anode potentials and therefore of the amplifier static balance condition. 

 By combining the schemes of Figures 12.36 and 12.38, satisfactory static 

 balancing becomes possible. If the ampHfier is balanced when i?^ = (high 

 gain) by the floating supply A, and when Ry is at maximum (low gain) by the 

 differential cathode resistor, then it will remain balanced at intermediate 

 settings of R^. A possible outhne design for an a.c. coupled differential 

 voltage amplifier is given in Figure 12.39, and a d.c. version with cathode 

 follower input in Figure 12.40. 



DIFFERENTIAL POWER AMPLIFIER 



This is an application to a power output stage of the property of the 

 differential voltage amplifier of providing a push-pull output from a single- 

 sided input. An a.c. coupled version is shown in Figure 12.41, and a d.c. 



To load 



LfijuuJ 



HT + 



Figure 12.41 



version in Figure 12.42. The advantage of the method is its simplicity in that 

 no phase-splitter is required. The disadvantage is low power efficiency; the 

 valves have to be operated in class A for the scheme to work, and for the 



HT+ 



HT + 



Balance 



HT- 

 Figure 12.42 



HT- 



signal to be nearly equally divided between the valves, R^ should be large. 

 Thus a standing current flows both in the valves and in R^, whether there is 

 any signal passing through the stage or not. An a.c. coupled cathode-coupled 



193 



