1 - Al Kossow's Bitsavers

TESTER LIMITATIONS WITH PROGRAMMED INPUT LOW LEVELS OF LESS THAN - 0.7 VDriver distortion occurs when input low levels are programmed for values below - 0.7 V. The input diodellateral NPN transistorshares a common PN junction which becomes forward biased at - 0.7 V and below. The transistor collector, which istied to VSS, carries most of the forward-bias current (see Figure 3).VssFIGURE 3 -FORWARD- TO REVERSE·BIAS CURRENT, The diode exhibits a classical forward- to reverse-bias recovery delay due to a momentarily large reverse current of amplitudelimited only by the programmed reverse voltage VIH and driver output impedance R, i.e., the input approximates a momentaryshort circuit. An initially large short-circuit current plateau (lR =VIH/R) subsequently relaxes to the normal dc reversebiascurrent IS (see Figure 4). The time from the positive transition edge, corresponding to t= 0 in Figure 4, to the pointwhere the reverse current surge relaxes to 10% of the plateau value is the diode recovery time (toft).VIHR..,.,------ toft -------iP4f10% IRISc:o'';:;CO..E.... o.5t/)c:o'';:;CO,~0.c.