Solution 2:

(i) To determine the critical voltages for the standard TTL with totem pole

output and plot the VTC:

VIL = VBE,O(FA) - VCE,I(SAT) = 0.7 - 0.2 = 0.5V

VIB = VBE,O(FA) + VBE,S(FA) - VCE,I(SAT) = 0.7 + 0.7 - 0.2 = 1.2V

VIH = VBE,O(SAT) + VBE,S(SAT) - VCE,I(SAT) = 0.8 + 0.8 - 0.2 = 1.4V

VOL = VCE,O(SAT) = 0.2V

VOH = VCC - VBE,P(FA) - VD(ON) = 5 - 0.7 - 0.7 = 3.6V

VOB = VCC - IRD x RC - VBE(FA) - VD(ON)

= 5 - (0.7/1)2.1 - 1.4 = 2.13V

 

Voltage Transfer Characteristics

 

 

(ii) A Knee region exists on the plot.

This is because of the passive discharge resistor RD. When the transistor

switches state the minority carriers present in its base region has to be drained.

This is done using a discharge resistor RD. A discharge current flowing through

RD produces a Knee.