SEMICONDUCTOR DIODE GATES 



1151 



and the output measured as a function of Ei. The results are shown on 

 Fig. 11. Since the diodes are not ideal, there is a transition region, but, 

 as predicted the output is very small at small negative control voltage 

 and the output is the full 10 volts when Ei is 10 volts. 



The curve also shows what happens if one of the enabling biases are 

 too small. A case is shown in which E2 was only 5 volts. There is no 

 significant difference until the output gets up to 5 volts. Above that 

 voltage the diode, D2, becomes conducting and the output is clamped 

 at that voltage. 



GATE CHARACTERISTICS 



The main virtues of this type of gate is that there is no pedestal and 

 a constant amplitude pulse is produced. It is also simple and has good 

 discrimination. There are limitations: 



1. Unless a very low control path resistance is used, there is a large 

 loss — that is the output pulse is much smaller than the control pulse. 

 For example, if the control resistance is equal to the output resistance 

 there is a two to one loss. 



2. A rather large load is put on the control generator, partly because 

 it must produce the enabling voltage across a small resistance and also 

 because, in some cases the total bias current flows in the control generator 

 output. 



3. A phenomenon called "hole storage", which is present to some 

 extent in all semiconductor diodes can make trouble. When a diode has 

 been resting in the conducting state with a current flowing in it and the 

 voltage is reversed, the diode does not immediately change to high im- 

 pedance. A reverse current flow for a short time — up to a few micro- 

 seconds. This can result in very inconvenient, spurious, output pulses 

 being produced by a gate which is supposed to be disabled. 



D3 



E, E2 



Fig. 10 — Switching type diode gate with two controls. 



