12 



DOUBLE-SIDED AMPLIFIERS 



PUSH-PULL VOLTAGE AMPLIFIER 



In Figure 12.1 two similar voltage amplifying stages are shown back-to-back. 

 If instead of two separate signal sources it can be arranged that the same 

 generator supplies equal and opposite inputs to the two valves, from a phase 

 splitter, or in some such manner as shown in Figure 12.2, then the valves are 



HT + 



HT + 



•• l^oul, =>4Vjn, 



V'outz =AV'in2 



K.uf-^^n 



HT+ 

 Figure 12.1 



HT + 



Figure 12.2 



said to operate in push-pull. Kout is taken between the anodes. The gain is 

 the same as for one valve only, which might at first seem rather wasteful of 

 valves. In fact some very useful results follow. 



(1) Constancy of total anode current— When the anode current of one side 

 increases that of the other side decreases by a similar amount; the total anode 

 current is therefore constant. If the stage is a high level one there is therefore 

 no tendency for fluctuations to be impressed on the HT voltage, with con- 

 sequent motor boating. 



(2) Comparative immunity to supply voltage variations — Since the output is 

 taken from between the two anodes, any change of heater voltage of HT which 

 affects both sides in the same manner does not, to a first order, alter the 

 difference between the anode potentials. 



(3) Grid bias easily applied — Automatic grid bias is easily applied for direct 

 coupled amplifiers (where applicable) as well as a.c. coupled. Since the total 



176 



