DOUBLE-SIDED AMPLIFIERS 



Depending on the nature of the input circuits, it may be possible as an 

 alternative to carry out gain balancing at the grids {Figure 12.34). R^ and i?2 

 form a fixed potentiometer which reduces the input to the lower valve by a 

 factor 0-9. If R^ is a quarter of R^, then when the slider is in the middle the 

 input to the upper valve is also reduced to 0-9. When the slider is at a the 

 feed to the upper valve is 10 per cent high, and when at ^ it is 10 per cent low. 



Static balance 



This is a problem which occurs only in direct-coupled amplifiers, and refers 

 to the requirement that when the difference of potential between the input 

 terminals is zero the difference of potential between the output terminals is 

 also zero. An a.c. coupled amplifier is self-adjusting in this respect, since in 

 the absence of any input the potentials of the output terminals automatically 

 return to that of the point P, usually earth, and so become equal to each 

 other {Figure 12.35). 



HT+ 



X[P Output 



Output 



Static balance 

 adjustment 



HT + 



Figure 12.35 



Figure 12.36 



Lack of static balance occurs as a result of asymmetries in the circuits of 

 three kinds: 



(1) lack of balance in the d.c. resistance of valve pairs in each stage when 

 the grid potentials are equal; 



(2) lack of balance in the circuit resistors deUberately introduced to achieve 

 gain balance; 



(3) numerous other effects, many of which fluctuate, to produce 'drift'. 



It is possible but unwise to use direct-coupled amplifiers in a state of 

 static unbalance. Quite a small amount of unbalance originating in an early 

 stage is ampUfied by the later stages and the final stage may be operating 

 under conditions of serious distortion — one valve passing grid current and the 

 other cut off. Since the final stage is affected first, static-unbalance is best 

 detected by a sensitive and high resistance voltmeter connected between the 

 final stage anodes. If the final stage is operating in a statically balanced 

 condition there is seldom cause to worry about the earlier ones. 



190 



