A comparator is an integrated circuit used to compare voltage (or current) and whose digital result indicates which of the values is higher. It can be realized by an operational amplifier which is a linear integrated circuit, consisting of a multitude of connected transistors, diodes, resistors, and capacitors. Linear integrated circuits work with analog values, where the output signal is a linear function of the input signal value. This is exactly where they differ from digital circuits, which work with discrete values, 0 and 1, as we have already explained. But, when you think about it, when used as a comparator, the operation amplifier works with digital values, which makes it perfect for usage in digital circuits. In this experiment, we will try to realize a primitive voltage comparator with transistors.
Components:
- breadboard
- 5V power supply
- 3mm red LED
- 7 x 10KΩ resistors
- 2 x BC547 NPN transistors
- 10KΩ potentiometer
The base of transistor Q2 is exposed to a voltage of ~2.5V due to the voltage divider and this voltage value is our reference. The base of transistor Q1 is exposed to variable voltage because the voltage divider is defined by a variable resistor. The emitters of the transistor are connected and grounded through a 10kΩ resistor. In case the voltage based on transistor Q1 is less than the reference voltage ~2.5V, transistor Q1 is cut-off, transistor Q2 conducts and output is 0 which is demonstrated by the LED that does not light up. The variable resistor allows the voltage at the base of transistor Q1 to exceed the value of the reference voltage ~2.5V, and then the Q1 transistor conducts and the transistor Q2 cuts off. In that case, the output is 1 and the LED is lit.
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