CATCHING 



CATCHING 



'Catching' is a diode technique for ensuring that the potential of a point 

 in a circuit shall not exceed a certain prescribed limit, and might be regarded 

 as analogous to buffer stops at the end of a railway siding. Catching circuits 

 are most practicable when the load resistance is high, e.g. a valve, and we 

 shall assume this is the case. 



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Figure 6.32 



In Figure 6.32 we have some kind of generator feeding some kind of signal 

 to a load Rj^ whose resistance is great. If it is required that the potential at F 

 shall not exceed -\-E volts, then we arrange a series resistance R, a diode of 

 forward resistance R^ and an E volt battery — or some other direct voltage 

 source — (of internal resistance r) as shown. 



When the potential at A is less than -\-E the diode is cut off and the circuit 

 behaves as if it were not there. Since R is negligible compared with 7?^ the 

 generator is effectively feeding straight into the load. When the potential 

 at A is greater than E the diode conducts and a potential divider is formed 

 having in the top limb R and in the lower r in series with Rf (Figure 6.33) and 

 further positive excursions of A will be attenuated by the factor 



R + r+R, 



r+Rf 



Potential at A 







Rf 



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+E 



Earth- 



Figure 6.33 



Figure 6.34 



Thus in Figure 6.34 the potential at A and the potential at F are seen to part 

 company as soon as the former exceeds -\-E volts, the latter being prevented 

 from rising far above the 'catching level' +£". 



109 



