BIOLOGICAL AMPLIFIERS 



filter which cuts off at a frequency below the chopping frequency but above 

 the upper limit of the required pass-band, an amplified copy of the original 

 input is obtained (Figures 39.3 and 39.4). Chopper amplifiers are charac- 

 terized by low drift. 



Input signal 



n 



Signal after 



chopping 



x 



A 



n 



Signal after 

 a.c. amplification 



Signal after 

 phase -sensitive 

 rectification ( half- 

 wave in this case) 



Signal after filtering 



Figure 39.3 



(3) Galvanometer amplifiers. This technique is due to A. V. Hill. The 

 input signal is applied to a moving-coil reflecting galvanometer and deflects 

 the beam of light across a differential photoelectric cell. The cell output is 



Sig.in 



Chopper 



ac. Amp 



Phase- 

 sensitive 

 rect. 



Low- 

 pass 

 filter 



Sig. out 



Reference 



wave 

 generator 



Figure 39.4 



amplified by valves and fed back negatively to series oppose the original 

 electrical input {Figure 39.5). This has the usual effect of increasing the input 

 resistance of the device — from a hundred ohms or so, the resistance of the 

 moving-coil, to the order of a megohm — and of raising the upper limit of 



620 



