BIOLOGICAL AMPLIHERS 



impossible, because if the input rises further, the output stage screen current 

 becomes so high that the stabihzer tube goes out. When this happens there 

 is heavy negative screen feedback and the anode current ceases to rise any 

 further. 



Loudspeaker amplifier 



Many workers like to be able to hear the result of an electrophysiological 

 experiment as well as see it on an oscilloscope ; when the eyes are engrossed 



Gain 

 control 



Figure 39.24 



in peering down a dissecting microscope at the site of the recording electrode 

 tip, auditory information about what is being recorded may be extremely 

 welcome. A straightforward audio-frequency amplifier and loudspeaker 

 enable one to hear action potentials, and in addition there is a sudden 

 change in noise level at the instant when the microelectrode tip, which is 

 usually invisible, first makes contact with the tissue. This reduction signals 

 the point at which to zero the micromanipulator dial gauges so that hence- 

 forward they are direct reading in penetration. 



A simple loudspeaker amplifier and power pack are shown in Figure 39.24. 

 This is another unit recommended for beginners, and in conjunction with 

 the single-sided pre-amplifier. Figure 39.1, enables a start to be made in 

 electrophysiological work. 



The bleater 



Useful as the above amplifier is, it cannot signal the magnitude or polarity 

 of a steady potential at a microelectrode tip ; on piercing a muscle or nerve 

 cell membrane the loudspeaker would merely emit a 'plop'. A device which 

 does, which is not really an amplifier at all but which it seemed appropriate 

 to mention here, is the 'bleater', first reported by Draper and Weidmann^^. 

 A possible circuit is shown in Figure 39.25. A thyratron relaxation oscillator 

 has its frequency controlled by a bias derived in part from a battery and 



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