DIRECT-COUPLED PRE-AMPLIFIERS 



the galvanometer frequency response about 10 times. Galvanometer ampli- 

 fiers are characterized by extremely low noise. 



The bulk of electrophysiological work is on exposed nervous tissue, and 

 for this reason the electrical signals which may be drawn off are quite large, 

 roughly in the range 50 /^V-50 mV. In these applications the 'straightfor- 

 ward' direct-coupled amplifier is quite satisfactory. Drift presents tiresome 

 but not insuperable difficulties, and in-so-far as the signals are relatively 

 large, the amplifier noise — provided reasonable precautions are taken — is 

 not excessive. In microelectrode work the drift and noise originating in the 

 electrode are in any case far more serious than that arising in the amplifier. 



Input 



Output 



■o 



Figure 39.5 



There are, however, fields in which much smaller steady potentials have 

 to be measured, usually from low-resistance surface electrodes. A case in 

 point is the study of eye movements by measuring the vertical and horizontal 

 components of electro-oculogram. These are of the order of a few microvolts 

 only. In work on brain or spinal cord activity there is a sense in which an 

 experiment is over in a single traverse of the cathode ray tube trace, perhaps 

 100 msecs; in this time the preparation has been stimulated and the response 

 elicited and recorded. In an eye movement study the time scale is likely to be 

 of the order of minutes. Clearly, then, only a very small drift rate indeed is 

 acceptable. This is the kind of work for which the chopper amplifier is suited. 



Galvanometer amplifiers are at present rarities and it seems likely that 

 they will remain so. Their freedom from noise gives them a position of 

 supremacy which is probably unassailable in studies of very feeble electrical 

 effects, but their bulk, necessary for elaborate resilient mounting and sound- 

 proofing, coupled with their use of special photocells and galvanometers of 

 extreme sensitivity, neither of which are readily available, may account for 

 their infrequency of appearance. 



Straightforward d.c. amplifiers 



An enormous amount of work has been directed towards the design of 

 better d.c. amplifiers, and descriptions of new circuits are published steadily. 

 Unfortunately most of these are developed for purposes other than — and 

 are not suitable for — bioelectric studies. The reader who is interested is 

 referred to a very useful study on the subject by Yarwood and Le Croisette^ 

 which includes a very long bibliography of work in this field. 



621 



