GAIN AND BALANCE CONTROLS 



As to how to correct for static unbalance, it is surprisingly difficult to 

 devise methods for doing this which do not also upset the gain balance. One 

 often sees arrangements such as the differential anode load being advanced as 

 static balancing devices, and there is no question that they can be used as 

 such ; but if one then seeks some other method for doing one's gain balancing, 

 it is sure to turn out that it throws the static balance off again. The only 

 reliable method known to the author is to have a special source of variable 

 e.m.f. with which to inject into the amphfiers at some appropriate point; a 

 signal to correct the off-balance, for example Figure 12.36. This may be 

 conveniently a small dry cell and potentiometer. These components have an 

 appreciable capacitance to earth which affects the high frequency rejection 

 properties of the amplifier unless corrected for by a similar capacitance 

 between the other input terminal and earth. In order to prevent undue loss of 

 high frequency response as a result of these capacitances, the input terminals 

 of the amplifier should be fed from a pair of cathode followers. Since electro- 

 physiological amplifiers are usually preceded by cathode followers anyway, 

 this is not a disadvantage. 



There remains one difficulty. If a single correcting voltage at the input is 

 used to estabhsh static balance at full gain, then when the gain setting is reduced 

 the amplifier will in general go off balance. For example, consider Figure 12.37: 



there is a lack of static balance, represented here by an equivalent unbalancing 

 signal V in the stages following the gain control stage. At full gain this is 

 offset by a static balancing e.m.f. A at the input, which emerges from the first 

 stage as A-^A and from the second stage as A^A^A. If the amplifier is statically 

 balanced, the output of stage 3 is zero, so A^A^A must equal C/, and A is 

 adjusted until this is true. If now A.^ is altered, a new value of A will be 

 required. This is unsatisfactory. 



Fortunately there is another adjustment scheme which has the property 

 that static balance correction may be carried out without upsetting the 

 differential gain balance, and which only works at low gain settings. This is 

 the differential cathode resistor. Consider Figure 12.38. At full gain i?„ = 

 and the effect of manipulating the differential cathode potentiometer is to 

 alter in opposite directions two resistances in parallel. The resistance of the 

 parallel connection is at a very flat maximum when the potentiometer is at 

 mid-setting, and the gains of the two halves are g^^Ri ^"^^ gvii^-i independent 

 of this setting. That is, operation of the control does not affect the gain 

 balance. At low gain R^ is large and the effect produced by altering the 

 cathode potentiometer is quite different, for it alters the bias on the valves 



191 



