Monday, April 23, 2007

Two Multiple Choice Questions on Digital Circuits

Most of you will like the section on digital circuits, especially because at the class 12 level you have to study very simple circuits. Consider the following MCQ:

For the circuit shown, logic level 1 is +5 volts and logic level 0 is 0 volt. This circuit is

(a) an EXOR gate (b) an AND gate (c) an INVERTER

(d) an OR gate (e) a NOR gate

If both inputs A and B are zero, the diodes will not conduct and the output point will be at ground potential so that the output Y = 0.

If at least one input is at logic 1 level (+5 volts), the diode connected to that input will conduct. The diode connected to the output also will conduct making the output high (+5V). Thus the output Y=1. The same thing happens if both inputs are high.

So, the circuit is an OR gate.

Now, consider the following question:

The digital circuit shown in the figure implements

(a) OR operation (b) EXOR operation

(c) NAND operation (d) NOR operation (e) AND operation

The first gate is a NAND gate. The second gate also is a NAND gate whose inputs are shorted. But when the inputs of a NAND gate are shorted, it becomes an inverter (NOT gate). So, the circuit is a NAND followed by an inverter which is altogether an AND gate [Option (e)].

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