DIODE CIRCUITS 



with respect to earth by an amount — V. When the circuit is in equihbrium 

 there is a steady potential difference F/2 across the capacitance and diode 

 current flows only at the very positive peaks of the signal, just enough to 

 replace the charge lost by current flowing back round the circuit consisting 

 of resistance and generator. 



SHUNT DIODE SIGNAL RECTIFIER 



The shunt diode signal rectifier circuit (Figure 6.46) has a rather peculiar 

 performance which might lead one to wonder why anyone should want to use 

 it. Its output is a rectified version of the input modulated carrier wave plus the 



Figure 6.46 



modulated carrier wave itself, and usually has to undergo further filtration 

 to remove this carrier component. The reason for its appearance is as 

 follows: many pentode and triode valves include — for a particular purpose 

 in radio receivers — one or two thermionic diodes. These diodes share the 

 same cathode as the main valve and are at a fixed potential. They therefore 

 cannot be used for signal rectification of the series diode type — where both 

 electrodes are 'live' — but are suitable for shunt rectification where one side 

 of the diode is earthed. 



PHASE-SENSITIVE DETECTORS 



The function of these valuable devices is perhaps best illustrated by an 

 example : 



In recent work F. W. Campbell and W. A. H. Rushton (/. Physiol. 130 

 (1955) 131) have studied the absorption of light by the retina of the eye. 

 To do this, a red and a green beam of light were directed into the eye 

 alternately. The emergent reflected hght, which has twice passed through 

 the visual purple, was coflected by a photocell. If the amount of visual purple 

 changed, the brightness of the green beam altered with respect to the red. By 

 adjusting a purple wedge (in the path of the green beam) these intensities could 

 be matched {Figure 6.47a). 



The photocell output is a square wave, and matching is achieved when 

 the amplitude is zero. This is a good method since the characteristics of 

 neither light nor photocell, photocell amplifier or indicating galvo need be 

 weU known. Only the wedge must be accurately cahbrated. 



In order to deflect the galvanometer the signal from the ampHfier requires 

 rectification. If it were applied to an ordinary signal rectifier the galvano- 

 meter deflection would certainly be proportional to the square-wave ampli- 

 tude, but would always be in the same direction ; there would be no indication 

 as to which way to move the wedge in order to achieve balance. That is, 

 information about the phase of the 'error' square wave has been lost. What 



114 



