DOUBLE-SIDED AMPLIFIERS 



in earlier stages may be driven outside their working regions — with attendant 

 distortion — by large but not excessive input signals. 



If the amount of amplification present is not great, and noise presents no 

 problem, the gain control may confidently be placed at an early stage, 

 possibly even directly after the input terminals {Figure 12.26). For high- 



Input 

 terminals 



Input 

 terminals 



or 



Figure 12.26 



gain electrophysiological and similar amplifiers the author favours controUing 

 the second amplifying stage, either the second stage input potentiometrically 

 or the second stage gain by feedback. 

 A.c. amplifiers — F/gw/-e 72.27 shows in outline a potentiometric continuously 



HT+ 



HT+ 



>--VW-A/V— 1 



^^- 



HT + 



HT + 



Figure 12.27 



variable gain control for an a.c. coupled amphfier which ought theoretically 

 to be satisfactory. In practice it is not, because the ordinary double potentio- 

 meter is not ganged sufficiently accurately, that is, the gains of the two sides 

 are not maintained sufficiently similar as the control is turned; we have seen 

 that under these circumstances the rejection ratio is adversely aflFected. 

 Nevertheless the potentiometric method may be adapted successfully to 

 achieve control in discrete steps by using a pair of potential dividers composed 

 of precision resistors, and the range of control may be made as wide as 

 desired in this manner {Figure 12.28). It is then usually necessary to 'fill in' 

 between the steps with a second continuously variable control, of limited 

 range only, which may be of the feedback type. To achieve gain control by 



186 



