SMOOTHED RECTIFICATION 



performance of the circuit in its ability to fulfill its task, namely, to produce a 

 direct voltage which follows faithfully the ampHtude of an alternating voUage. 

 Figure 6.19b, a carrier wave, has been modulated by the pulse in Figure 6.19a 

 and this modulated carrier is supposed to be the output of the generator. 



When the carrier peak voltage rises from V-^ to V^, provided the generator 

 and diode forward resistances are low, Kout reaches V^ at the peak of the 

 next positive excursion of point A, that is to say the response time cannot 



(a) 



Figure 6.19 



exceed the time of one cycle of the carrier {Figure 6.19c). At the end of the pulse 

 matters are rather different, for now the potential at A never reaches that of 

 B and the diode is continuously cut off until Font falls once more to V^. 

 This it does by approaching zero with a time constant CR until such time 

 as its decay is arrested by renewed conduction in the diode. 



Two points emerge: (1) distortion is more serious at the end of the pulse; 

 (2) distortion at the end of the pulse is lessened when CR is small. R cannot 

 be made small if the generator and diode forward resistances are to remain 

 relatively negligible, therefore the reduction must be made in C. 



The magnitude of the ripple in the output depends on the carrier frequency, 

 the load and the value of C. For small ripple we need a high carrier frequency 

 (there is then less time for the charge on C to leak away between cycles) or a 

 large R (which we have already got) and a large C. 



Requirements for low distortion at the pulse end, small c, and low distortion 

 during the pulse, large C, are therefore conflicting. Signal rectifier circuits 

 are quite easy to design when the carrier frequency is much higher than 

 the modulation frequency. The procedure is to arrange that CR is much 

 greater than the time of the cycle of carrier but much less than the time 

 of the cycle of modulation: the geometric mean would be suitable. When 

 the two frequencies are closer, the carrier being perhaps only ten times the 

 highest modulation frequency, it is best to have a low C, which preserves 

 the back edge of the pulse, and remove the ripple by subsequent low-pass 

 filtering. 



When the generator and diode forward resistance are not negligible com- 

 pared with the load, the effect is twofold: (1) the rising phase of Kout will 

 also be slowed up — several cycles of carrier at the new value of Kg may be 

 required before Kout reaches its final value; (2) Fout will not reach V^ but 



103 



