DIRECT-COUPLED PRE-AMPLIFIERS 



wide, as response above, say, 10,000 cycles per second, adds little to the 

 signals but contributes unwelcome noise. If we arrange the response to be 

 about 3 dB's down at 10,000 cycles it turns out that the cable capacitance 

 can be 160 pF. This allows one to use 10 ft. of the usual polythene 

 insulated variety, which is quite convenient. 



For work on large preparations — including human beings (e.g. cardio- 

 graphy and encephalography) — which are difficult to screen properly, the 

 use of differential amplification becomes essential. A very typical design is 

 given by Dickinson^ and is reproduced in Figure 39.2. The first stage valves 



+60, 



lAOk 



1 + 60 



50k ^3Q <iM 



X" 



Mm 



EF37A 



Output 



Figure 39.2 



are triode-connected and facilities are provided for gain balancing. The time 

 constant of the interstage coupling is 4 seconds, rendering the pre-amplifier 

 suitable for studying slow phenomena such as spinal cord root potentials, 

 electrocardiograms, etc. The heater supply is 6 V, but the first stage heaters 

 are connected in series so that each valve receives only 3. A low cathode 

 temperature and hence low noise are thus obtained. 



DIRECT-COUPLED PRE-AMPLIFIERS 



As soon as the frequency response of amplifiers has to extend down to zero 

 frequency there is an immediate increase in complication, which seems to be 

 inevitable and makes it worth while to consider carefully before embarking 

 upon it. There are three approaches, each dictated by the requirements. 



(1) ^Straightforward'' d.c. aniplifiers. These have been discussed in Part I 

 and attention has been drawn to the necessity for distinct static balancing 

 and gain balancing controls and to the existence of drift. 



(2) 'Carrier' amplifiers. These are not true direct-coupled amplifiers but 

 behave as if they were. The input signal is converted into an alternating 

 one ('chopped') at a fixed frequency which is much higher than the highest 

 frequency to which the apparatus is to respond. The alternating voltage 

 thus produced undergoes conventional capacitor-coupled amplification, fol- 

 lowed by phase-sensitive rectification. If it is then passed through a low-pass 



619 



