Comparator Circuit
First of all, trivially, the comparator circuit is the circuit that compare 2 signals. It will tell you which signal is larger in terms of amplitude. The example of comparator circuit is found with the automatic system or the detection system. To understand this clearly, the math model is written easily in the figure. Before we have an operation amplifier, the comparator circuit is build using the BJT and it called the differential amplifier. This differential amplifier have the problem that the common mode gain is not zero. So, the op-amp is introduce. The basic comparator circuit uses the benefit from the ideal op-amp that the open loop gain is infinity which can tell which signal is larger.
In practical, we will set one of two terminal as a reference voltage as ground or using voltage divider to determine is value. Then, the left terminal will be the input of the signal that we want to compare with the reference value. For example, in the figure below, we use voltage divider to determine the voltage in the negative terminal which can written as V =Vcc* R1/(R1+R2). So, the positive terminal will be the input voltage. The result will give you the high and low value depend on the input voltage.
Now, we are going to demonstrate the application of the comparator circuit which is the zero crossing detection. The circuit will be high when the signal go cross the zero voltage. Therefore, in negative terminal, it will ground as the reference voltage. To implement, we use LM311 which is the differential comparators device.
The experiment and the circuit implementation is shown below. The yellow signal is the input signal which is sine wave. The blue one is the output signal. It can clearly see that if the yellow is go through zero volt, the blue will change. The resistor pull up is like the load resister because the current are going to increase while we reduce the resistance.
Thanks for reading.