DIODE CIRCUITS 



and B are at the same potential. Since B is earthed, A must be at earth 

 potential too. The action of the reference wave on the diodes is alternately 

 to make high and low the resistance of the path from A to earth. Now let 



Signal in 



"LTLT 



Earth' 



1 



Rectified 



^3\^ 1 n r 



out 



Earth 



nnnnnnnnnn 



"LTL 

 Reference wave in 



Figure 6.49 



the signal square wave be applied to the left-hand plate of the coupling 

 capacitor. If the capacitor is large, the same voltage waveform must be 

 present on the right-hand plate too, but as either the negative or positive 

 sectors are earthed by the diodes — depending on the phase of the input — 



Reference -f 

 wave —^ 



fq— pe^rth -f 



F 



Signal 



_r 



-Earth 



Output ~r 



-Earth 



n. 



-Earth 



Figure 6.50 



the output wave, as it were, 'stands' or 'hangs' from earth potential {Figure 

 6.50). By passing the output through a low-pass filter the average value may 

 be extracted, and this is the output required. 



AV 



Sig. in 



1 



VJ 



mMMM) 



nroo"o'o"oo"(ro 



ORcf. wave in 



] 



Output 



■A/VNAAAAaT I 



(b) 



(a) 



Figure 6.51 



This simple circuit has two snags: (1) if the diodes are not perfectly 

 similar the output is complicated by a component of the reference wave; 

 (2) the path to earth is not of very low resistance, since it involves the current 

 limiting resistors. The former objection can be overcome by replacing the 



116 



