D.C. RESTORING 



or c, and — V. The output of the filter is of necessity of the form of Figure 

 6.43a because this comprises the fundamental saw-tooth frequency and its 



IF 



Out put 



Figure 6.42 



+1/2V 



V2V 



+ V 



KhAKh^N 



-;hi^sh^^^^ 



Figure 6.43 



(a) 



(b) 

 (c) 



harmonics (all of which can be passed without distortion by the filter if RC 

 is large enough) but, unlike b and c, no direct voltage component. This is to 

 say, the direct component present in inputs b and c is lost because the" filter 

 cannot pass zero frequency. 



Electronic amplifiers often contain filters of this kind, not because they 

 are wanted as such but because the amplifiers are much easier to design if 

 they are there. The question then arises as to whether the lost direct com- 

 ponent can ever be recovered, and the answer is that in general it cannot; 

 but it can be in the two special cases where the positive or negative peaks of 

 the generator output — be they saw-teeth or any other waveform — are at 

 zero potential. That is to say, direct component restoration is possible for 

 the waveforms of Figure 6.43b or c but not for the waveform in Figure 6.44. 



/VVVV\ 



Figure 6.44 



The technique of restoration is extraordinarily simple : merely connect a 

 diode across the output as in Figure 6.45a to recover inputs of the Figure 

 6.43b type, or as in Figure 6.45b to recover the Figure 6.43c type. The 

 generators here are supposed to have lost the d.c. component, i.e. these 

 outputs swing between + ^/2 and — K/2. The method of working is merely 

 this : when the generator output in Figure 6.45b, say, goes positive, the out- 



^h 



t— o 



^h 



(a) 



(b) 



Figure 6.45 



put does not also go positive with respect to earth because the diode switches 

 to low resistance and substantially short-circuits the output: diode current 

 flows and quickly charges the capacitance instead. When the generator 

 output goes negative the diode cuts off" and the output is carried negative 



113 



