DRIFT 



to ensure the amplifier is stable. Do not skimp on decoupling capacitance 

 with battery operated gear. Remember that as the battery gets older the 

 internal resistance will increase and so will the positive feedback. Parsimony 

 here will lead to an ampHfier which is stable with new batteries but begins 

 to motor boat while there is still useful Ufe in them. 



Input 



Figure 10.4 



Figure 10.5 



With direct coupled amplifiers decoupling does not work because there is 

 no frequency so low that there is no amplification. Furthermore, with long 

 time constant a.c. coupled amplifiers of the kind used in electroencephalo- 

 graphy and cardiography, decoupling may be impracticable because capaci- 

 tances of prohibitive size are required. In these cases it may be possible to 

 do something about motor boating by the widespread use of stabilizer tubes, 

 but it is probably cheaper to begin again with an amplifier of the double- 

 sided type. 



DRIFT 



Even if the amplifier of Figure 10.2 could be prevented from motor boating 

 it would be extremely prone to 'drift'. This is the blight with all direct 

 coupled amplifiers and refers to the tendency for the output to alter all the 

 time even when there is no change at the input. It is caused by minor fluctua- 

 tions in the supposedly constant parameters, such as HT voltage, tempera- 

 ture of valve cathode, and component values generally. Drift is impossible 



161 



