DOUBLE-SIDED AMPLIFIERS 



potential is fixed, that is, the tube is suited only for 'asymmetric' deflecting 

 drive. In this case we have no option but to take the output from one anode 

 only (Figure 12.23). The anti-phase gain is now only 11,250, because half 

 the output is wasted at p, but equally the in-phase gain is only \ and the 

 rejection ratio is 11, 250/ J = 22,500. 



In point of fact it will not be as high as this, and the rejection for the 

 symmetrical case will certainly not be infinity; we have assumed hitherto that 



HT+ 



HT+ 



HT+ 



—II 



HT+ HT+ 



Figure 12.23 



HT+ 

 Figure 12.24 



the two sides of the amplifier are perfectly similar in all respects. In a 

 practical circuit with real-life components this cannot be the case. What 

 happens is that an in-phase input produces at the first stage anodes potential 

 changes which are in the same direction but not quite equal — that is, an in- 

 phase output with a small anti-phase component. This anti-phase component 

 will suffer amphfication by the second stage, so that the practical rejection 

 ratio is always inferior to the theoretical. 



The situation can be mitigated by introducing special gain-balancing 

 arrangements. The principal offenders in causing inequaUty between the two 

 sides are the valves, whose g^ values are subject to rather wide variations both 

 in manufacture and in aging. The object of gain balancing networks is to 

 correct for differences in the performance of the two valves comprising each 

 stage; hence theoretically each stage requires a balancing adjustment. In 

 practice, amplifiers must not have too many knobs ; it is clear that since lack 

 of gain balance in the first stage — when handling an in-phase input — leads to 

 amplification of the concomitant anti-phase component by all the succeeding 

 stages, the first stage is the one which requires balancing most, and in practice 

 the provision of first-stage gain-balancing only is usual. 



In the a.c. coupled differential voltage amplifier no HT negative supply with 

 respect to earth is necessary, and the stage has the form of Figure 12.24. 



MAKING R^ A PENTODE 



A good ratio of anti-phase to in-phase gain with a differential stage depends 

 on having i?^ large. Unfortunately, if -^^ is an ordinary resistance and is to 

 be large, the negative supply must be very negative indeed if the valve currents 



184 



