BIOLOGICAL AMPLIFIERS 



may be purely electronic. Electronic modulators have not been dealt with 

 in Part I because they have little relevance to the subject of this book, but 

 the reader who is interested will find them discussed in any textbook on radio. 

 Vibrating relay or 'Chopper' amplifiers — Provided the gain required is not 

 too high a simple form of chopper amplifier may be made using an existing 

 a.c. coupled amplifier, a high-speed change-over relay and an oscillator of 

 sufficient power to operate it. A possible scheme is shown in Figure 39.17, 



HT + 



|— T-AAAWV^ 



Output 



Figure 39. J 7 



where the amplifier is a simple two-stage affair using a double triode. The 

 relay moving contact 'buzzes' back and forth between the fixed contacts, 

 alternately earthing the input and the output. The periodic short-circuiting 

 of the input converts it into a square wave of amplitude proportional to 



flftntin 



""mm 



(a) 



(b) 



Figure 39.18 



itself and of phase depending on its polarity. This emerges from the ampli- 

 fier about a thousand times greater and is phase-sensitive-rectified by the 

 other relay contact in a manner similar to the Cowan bridge described in 

 Part I. The output is now of the form shown in Figure 39.18a, and if this is 

 filtered it has the appearance of Figure 39.18b and is a magnified copy of the 

 input. The chopping frequency should be the highest at which the relay will 

 work properly, and the filter can then be designed to cut off at, say, one-third 

 of this, and the amplifier will then handle input frequencies up to, say again, 

 one-tenth. 

 A difficulty with chopper amplification of this rudimentary type is this : 



630 



