البوابات المنطقية .. الالكترونيات

Logic Gates .. Electronics

Logic Gates

Logical operations are performed by "gates" whose output depends on the logical operation or "correctness" required.

Computers use a large number of these gates. In Figure (283) we see the gate "and" which contains two inputs A and B to which the input pulses reach. In normal conditions, transistors 1 and 2 are not connected, and no current passes through the resistor, and no voltage drop occurs. As for the base of transistor 3, it is negative, and the current passing through the resistor M2 causes a voltage drop through it. The result is that the output level remains close to zero (positive). When a certain pulse reaches A "and" B, one of the transistors becomes connected. However, the resistance does not remain without current unless both are in a state of conduction, and this happens when the pulses reach A and B together. In this case, the collector C becomes positive and transistor 3 stops conducting and a negative pulse appears at its output. As for Figure (248), it is a two-input "OR" gate. This gate does not contain a specific pulse at its output unless there is a specific pulse on each of its two inputs A "or" B. Such units are capable of giving 500,000 pulses per second or more.

283 - Two-input "OR" circuit and an inverter circuit.

284 - Two-input "OR" circuit and an inverter circuit