DIODE CIRCUITS 



The effectiveness of this circuit depends on the attenuation being large 

 when the diode conducts, i.e. R large and (r + Rf) small. It thus appears 

 that any diode, however indifferent its forward resistance, may give as good 

 a performance as is wanted merely by making R large enough. In fact, two 

 considerations impose a restriction on R: 



(1) Stray capacitances across the load. These will form a low-pass filter 

 with R, having a cut-off frequency within the required spectrum if R is too 

 large, and frequency distortion will ensue. 



(2) If the diode is of the semi-conductor variety the back resistance R^, 

 will form a potential divider with R and there will be a loss of voltage delivered 

 to the load even when the potential at A is within the prescribed hmit. Since 

 Rf, is a non-linear function of diode voltage there will in addition be intro- 

 duced amplitude distortion, of an amount proportional to R. 



Design procedure is therefore as follows. When the diode is of the semi- 

 conductor kind, choose R at about the geometric mean of R^ and R,,, bearing 

 in mind that in any particular case either efficient catching or absence of 

 amplitude distortion may be more important, then check that loss of high 

 frequencies will not be excessive. If it seems that this will be the case, R 

 will have to be made smaller. If the diode is thermionic, use the highest value 

 of R that will not cause intolerable high frequency loss. If the catching level 

 is to be made adjustable, a potentiometer is added to the circuit (Figure 6.35). 

 With this arrangement there is one important point to watch: Efficient 

 catching depended on r, the internal resistance of the battery being low. 

 With the new arrangement we have to substitute (/?ii?2)/(^i + ^2) the 

 effective internal resistance of the source of variable potential. If this is 

 always to be low, whatever the potential setting, R^ + i?2 i^^st also be low, 

 and much power from the battery is wasted in flowing down the potentiometer. 



If the output of the generator is a complex of frequencies extending down 

 to zero then this fact has to be accepted and tolerated. However, if there is a 

 lowest frequency coj^, as is often the case, then we can employ a capacitor 

 (Figure 6.36) and the potentiometer can be of as high a resistance as we wish, 

 for the 'catching attenuation' cannot now be less than 



,— ^ + Rf + R J. 



1 ~ ^ 1 



In practical circuits the j term can often be made negligible by using quite 

 moderate values of C 



The circuit in Figure 6.36 might be described as a 'positive catcher', in 

 which case the circuit in Figure 6.37 is a 'negative catcher', i.e. the potential 

 at F is prevented from going more than a certain amount negative. 



CLIPPING 



When a positive and negative catcher are combined the result is called a 

 clipper (Figures 6.38 and 6.39). The design of these is a perfectly straight- 

 forward extension of the procedure for catchers, and requires no further 

 comment. 



110 



