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HOWARD J. CURTIS 



one tube is balanced by the drift of another. The second is to use 

 constant voltages, especially in the first stages. This can be done 

 moderately well by the ase of batteries, but even these drift some. A 

 much better way is by the use of an electronically i-egulated power 

 supply (see Sect. C6). The third precaution is the selection of tubes. 

 The best tubes for this purpose ai-e of course the electrometer tubes. 

 However, they are expensive and have characteristics that are not 

 always suitable for particular amplifiers. The author has found that 

 it is reasonably satisfactory to use tubes such as the type 12SJ7, run 

 as triodes in the first stages and as pentodes in subsecjuent stages. 

 It is best to purchase a number of these tubes all at the same time 

 and from the same dealer, rim them at a moderate plate current for 



Input 



Output 



Fig. 7. Diagram showing common mode degeneration. 



100-300 hours, and then carefully match them in pairs. Put the 

 best pair in the first stage, the next best pair in the second stage, etc. 



There are a number of other things that can be done to minimize 

 drift. The plate voltages and currents should be kept as low as pos- 

 sible; voltages as low as 20 v. are not unreasonable. Filament cur- 

 rent on individual tubes can be carefully adjusted. Precautions 

 can be taken to keep the temperature changes the same on both tubes 

 of a pair. Wire-wound resistors can be used throughout. Indeed, 

 there is almost no end to possible refinements, most of which are un- 

 necessary for biological work. 



Another troublesome feature of amplifiers is pick-up. Very 

 elaborate precautions have been taken in the past to prevent induced 

 currents from being amplified along with the potentials under study. 

 These precautions usually consisted of extensive shielding, and even 

 elaborately shielded rooms. However, the use of amplifiers with 

 common mode degeneration has made the use of such shielding com- 



