BIOLOGICAL AMPLIFIERS 



(3) Mechanical convenience. Most of the controls in frequent use are on 

 the pre-ampHfier. The latter can therefore be arranged close to the experi- 

 menter, the main amplifier being relegated to some convenient and unob- 

 trusive position. In the Cambridge School of Physiology under Matthews, 

 the experimenter, preparation and pre-amplifier with batteries are all to- 

 gether in a wire-mesh screening cage. The remainder of the electronic 

 apparatus — apart from the RF stimulator probe — is kept outside. 



If the display device is to be a cathode ray tube it may be worth while 

 considering buying a commercial oscilloscope, such as the Cossor 1049 or the 

 Nagard DTI 03. Both these have internal direct-coupled deflection ampli- 

 fiers, making the provision of a separate main amplifier unnecessary. Simi- 

 larly, manufacturers of penwriters commonly supply a main amplifier 

 especially designed to go with their products. Pre-amplifiers, on the other 

 hand, have in general to be home made. 



The distribution of labour between pre- and main-amplifier is usually 

 arranged so that the signal level is raised to the order of 1 V in the former, 

 further amplification as necessary being carried out by the latter. 



A.C. COUPLED PRE-AMPLIFIERS 



A very simple single-sided capacitor-coupled pre-amplifier is shown in 

 Figure 39.1. This is suitable for recording action potentials from nerves via 



+ 120V 



0-1 fiF 



Input iii:i I 



-| 1M 



3_ 



lOOk^ipOOpF 



I 



1,000 pF 



41 



-3-7 



EF86 +60V 

 2-7k 



3^ Output I 



6uF 

 12 V 



To -^ 

 heaters-^ 



-v^ 



-o + 



Figure 39.1 



To 6V 



car 



battery 



low resistance electrodes, and is very suitable for beginners' use. Because it 

 is single-sided, it is unable to discriminate against interfering a.c. fields, and 

 consequently the preparation and electrodes must be totally screened. 

 Unless a screened room is available this rather restricts its application to 

 small animals. As an added measure of protection against 50 cycles pick-up 

 the coupling time constants are chosen so that this frequency lies well below 

 the bottom end of the amplifier pass-band. The first stage is triode connected 

 to eliminate partition noise, and the signal is fed to it via a CR network; 

 this keeps the valve's grid current out of the preparation. The gain lies 

 between 1,500 and 2,000 times; it will depend upon the particular valve 

 specimens in use. The upper limit of frequency response is largely deter- 

 mined by the output cable; it is profitless to have the pass-band excessively 



618 



