Page 89 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Electrical equivalent circuit of an AND gate
The output is available only when both the switches are closed. IC7408 is a T.T.L quad AND gate IC. (Refer data
book for pin diagram). The electrical equivalent of AND gate and AND gate using diodes are shown in Fig 6a and
6b.
Two input AND gate using diode
I condition
A=0, B=0, Y=0 as in Fig 7.

Fig 6 Fig 7

During the above condition I/P A and B are connected to ground to make logic low inputs. During this condition,
both the diodes conduct, and pulls the O/P Y to logic-0.
II condition
A=0, B=1, Y=0 as in Fig 8.

Fig 8

In the II condition shown in the figure above, diode D1 is connected logic-0 input and diode D2 is connected to
+5V [Logic high]. Diode D1 is in forward bias and conducts. Diode D2 is having equal potential (+5V) at anode and
cathode. So potential difference between anode and cathode is 0. Hence diode D2 does not conduct. The output
Y is pulled down to logic zero, since D1 is conducting.
III condition

A=1, B=0, Y=0 as in Fig 9.
The III condition is similar to the II condition. D2 is forward biased. D1 is reverse biased. Hence output Y is pulled
to logic-0.

IV condition
A=1, B=1, Y=1 as in Fig 10.
In this condition both the diodes are reverse biased. So both the diodes act as open circuit. Therefore output y is
+5V i.e y is in logic-1 condition.
AND gate as an Enable/Inhibit device
A common application of the AND gate is to enable (i.e to allow) the passage of a signal (pulse waveform) from
one point to another at certain times and to inhibit (prevent) the passage at other times.

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CITS : Power - Electrician & Wireman - Lesson 60-69