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Circuit digest published interesting information concerning this circuit. The circuit shown above was a very simple implementation of the OR gate, but this circuit is seldom used in the manufacture of ICs, for a few reasons. If we connect one input to VCC, a switch to the other input and probe the input and the switch and trigger on the rising edge of the switch, we notice a small problem.
The output goes high only after some time, i.e. does not respond instantly to the input. The time taken for the input to be seen at the output is called propagation delay. The same thing happens when the input is removed. The output takes some time to go back to ground.
This happens because of the transistor’s base capacitance. One solution would be to decrease the values of all the resistors, so that more current flows and the capacitances are charged quickly. But this would result in too much power dissipation. However, the placement of two small (<10nF) ‘speedup’ capacitors across the base resistor to reduce the ‘storage’ time can circumvent this problem.
Another problem is that this circuit cannot sink as much current as it sources. Sourcing is not a problem because there’s at least one transistor turned on (when at least one input is high) that is connected directly to the output, so that the output can provide a decent amount of current.
However, when the transistor is turned off, only the 1K resistor pulls the output down, and sink current is limited. To make the drive symmetrical, an output push-pull stage is added. Both these modifications greatly reduce rising and falling propagation delays.
Applications of OR Gates
Along with the AND gate, the OR gate forms an integral part of all logic circuits. For example, if there are ten inputs that a microcontroller needs to monitor, a 10 input OR gate will tell the controller if any one of the inputs is high without needing ten input pins.
Another interesting applications of logical OR is in your car. The seatbelt warning light turns off only when all the doors are closed, in other words, if any one (or at least one) of the doors are open, the alarm lights up.
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