AIEEE 2008 and AIPMT 2008 Questions on Digital Circuits

Questions from digital circuits at your level are simple and you should never omit them. The following question appeared in All India Engineering/Architecture Entrance Examination 2008 question paper:

In the circuit below, A and B represent two inputs and C represents the output.

The circuit represents

(1) AND gate

(2) NAND gate

(3) OR gate

(4) NOR gate

If the inputs A and B are low, the diodes nwill not conduct and the potential at the output point C will be low (ground potential). If either A or B is high (at high voltage level), the corresponding diode will conduct and will place the output point C at the high voltage level. If both A and B are high, again the output will be high. The circuit therefore implements OR operation [Option (3)].

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In the above question we have taken, as usual, the high voltage level as the logic 1 level and the low voltage level as the logic 0 level. This system is the positive logic system.

If the low voltage is taken as logic 1 level and the high voltage is taken as logic 0 level, the systerm is called negative logic system. Unless specified otherwise, all systems are positive logic systems.

In the negative logic system, the above circuit will become AND gate.

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The following question appeared in All India Pre-Medical/Pre-Dental Entrance Examination (AIPMT) 2008 question paper:

The circuit

is equivalent to

(1) NAND gate

(2) NOR gate

(3) OR gate

(4) AND gate

The second gate is a NAND gate. Since its inputs are shorted, it functions as a NOT gate. So the circuit is a NOR gate followed by Two NOT gates. NOR followed by NOT is OR and OR followed by NOT is once again NOR. So the correct option is (2).