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Unit 1: Review of Basics of Digital Electronics

The figure 1.4 depicts how AND gates are placed within an IC. In the figure, pin number 1, 2, 4, 5, Notes
9, 10, 12, and 13 are the inputs to the AND gate, while 3, 6, 8, and 11 are the AND outputs. Pin
number 7 is connected to ground and pin number 14 is connected to the power supply.
OR Gate
OR gate is a type of digital logic gate which has an output that is normally at logic level ’0’, but
goes ’HIGH’ to a logic level ’1’ when any of its inputs are at logic level ’1’. The output of a logic
OR gate returns ’LOW’ again when all of its inputs are at a logic level ’0’.
The Boolean expression for OR gate is denoted as Q = A+B.
A simple 2-input logic OR gate can be constructed using RTL structure connected together as shown
in the figure 1.5 Either transistor T or transistor T must be saturated “ON” to produce an output
1 2
at Q. In the figure 1.5, we can observe that Vcc is the input to both T and T . The output of T and
1 2 1
T constitutes the final output - Q=A+B.
2

Figure 1.5: Design of OR Gate

Example: Commonly available OR gate ICs are as follows:
TTL Logic Types CMOS Logic Types
74L S32 Quad 2-input CD4071 Quad 2-input
CD4075 Triple 3-input
CD4072 Dual 4-input

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