BIOLOGICAL AMPLIFIERS 



Of the d.c. amplifiers which have been published for electrophysiological 

 work, a notable example is due to Bishop and Harris*. This might perhaps, 

 without impertinence, be described as the ultimate in biological amplifiers. 

 Both main- and pre-amplifiers are described. The latter comprises a pair 

 of cathode followers, followed by two stages of balanced amplification by 

 double triodes of a type (6J6 or ECC91) in which both valves share the same 

 cathode. The object here is presumably to mitigate the effects of emission 

 fluctuations. These two stages have pentode valves in the common cathode 

 lead to secure a very high rejection ratio. The fourth stage, a pair of pentodes, 



, N.F.B. 



Double 

 triode 



Double 

 triode 



Pentode 



Cathode 

 follower 



Pentode 



Cathode 

 follower 



Gain 



Variable 

 low - 

 pass 



filter 



Output 



N.F.B. 



Figure 39.6 



provides additional amplification and the fifth is a pair of output cathode 

 followers. From the CF outputs the signal is passed back to the first double 

 triodes to control the gain by variable negative feedback, and is passed for- 

 ward to the main amplifier via a single stage of low-pass filtering in which 

 the capacitors are switched to limit the upper frequency response as required 

 (Figure 39.6). The pre-amplifier includes a stabilized HT supply and uses 

 15 valves in all (including 1 stabilizer tube). 



Not all physiologists would agree that so much complexity is desirable, 

 though there is no question that this amplifier is capable of a very fine 

 performance. A much simpler amplifier has been described by Asher^ 

 which possesses the valuable feature that it may be switched to capacitor- 

 coupled operation as required. An unusual feature is the absence of any 

 division into pre- and main sections ; a pair of cathode followers is followed 

 by 4 stages of balanced pentode amplification, after which the signal is of 

 sufficient magnitude to deflect a cathode ray beam. The whole amplifier is 

 supplied from batteries. It is of the 'climbing' type and 240 V HT are 

 required. A mains-driven unit in which beam-tetrodes arranged as cathode 

 followers drive a penwriter is also described. 



A pre-ampHfier designed by the author for general purposes, including 

 microelectrode work, is shown in Figure 39.7. Before discussing it we will 

 consider for a moment the question of input cathode follower grid current 

 in direct-coupled apparatus. 



Considerwhathappens when a microelectrode pierces a hypothetical spheri- 

 cal cell of radius 10/^. The surface area A of the cell is A-nr^ = 12-6 X lO'^ cm^. 



Taking the resistance of cell membrane, p, as 1,000 Q. cm^, we see that the 

 resistance of the membrane of this particular cell is 



622 



