DOUBLE-SIDED AMPLIFffiRS 



output stage is used in a commercial radiogram^. A direct-coupled version is 

 used in a Velodyne circuit^. 



PUSH-PULL POWER STAGES FOR LOADS 

 WITHOUT A CENTRE-TAP 



Here we are concerned mainly with direct-coupled loads, such as meters and 

 some kinds of penwriters. Two possible schemes are shown in Figure 12.43. 



HT+ 



HT+ 



R, 



^N ^V 



'OtW^ 



Load ^/^^^ 



^^7 



-rr 



(a) 



(b) 



Figure 12.43 



Figure 12.43a requires less input to the grids to secure a given load current, 

 but b may be preferable in cases where the load has to be fed from a source of 

 low impedance (as is sometimes necessary to get the correct electromagnetic 

 damping with penwriters): neither of these circuits has very good power 

 efficiency. Suppose as a result of a very large input signal one valve is cut off 

 and the other becomes for all intents and purposes a short circuit, then the 

 maximum possible load current which can flow in either case is F/(7?^ + Rj) 

 (Figure 12.44a and b), so that we would like to have Rj^ small compared with 



(a) 



Figure 12.44 



(b) 



Ri. Also, the maximum voltage swing at the output is clearly obtainable 

 when the no-signal potential of each load terminal is -\-{yl2), for then the 

 stage can theoretically operate until one terminal rises to + F and the other 

 falls to earth potential. It follows that the optimum resting current through 

 each valve in either configuration is given by ^VjRj^ and is small when Rj^ is 

 large. Thus the requirement for a low standing HT current conflicts with the 

 requirement for good maximum load current. 



194 



