. Electronic apparatus for biological research . Figure 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 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. D
. Electronic apparatus for biological research . Figure 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 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 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
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