comparatoroperational-amplifiervoltage
I am using a UA741CP opamp IC as a simple opamp voltage comparator.
I initially supplied it with 5V and tested the circuit as shown but even though I give voltage less than inverting input to non-inverting input I am getting 1.8 V or near not 0 V as output.
I hope someone knows the answer.
Regards,
The datasheet is really quite clear about this:
This opamp isn't specified for only 5 V supply.
These parameters shown above are for ±15 V supplies. The common mode input range only goes to within 3 V of either supply. Nothing is said how that scales to lower supply voltages, so assume it is at least that much. That means there is no common input range left with only a 5 V supply.
Likewise, the output can't drive to within 3 V of the supply rails with a 10 kΩ load, and not to within 5 V with a 2 kΩ load.
You need to actually read the datasheet before designing a part into a circuit.
the maximum voltage output is around 3.3V and not at 5V. How would you explain that ?
The LM311 has an open collector output so the it does not "drive" the output high. For your circuit, the output high is given by:
$$V_{OH} = 5V(\dfrac{2.7k + 1k||2.7k}{2.97k + 2.7k + 1k||2.7k} + \dfrac{1k}{1k + 2.7k}\dfrac{2.97k}{2.97k + 2.7k + 1k||2.7k}) = 3.31V$$
how would you explain that the output is not exactly at 0V when the input is high?
Best Answer
This is expected behaviour from the ancient 741.
From the datasheet:
As you can see, the output voltage cannot reach the supply voltages
The thing of it is, that it isn't proportional to the supply voltage. The output can never get closer to the voltage rails than 1 or 2 volts.
Since you are working with a supply voltage of 5V, you have very little room for the voltage to change. The output will probably vary between something like 2V and 3V. Not really all that useful.
There are many reasons not to use the 741. I won't repeat them here, but pretty much every reason not to use the 741 applies in your case.
You should use a rail to rail opamp, or, since you need to buy a part anyway, just go ahead and get a comparator rated to work on a single 5V power supply.
The LM393 is a comparator rated to operate on 5V. It is commonly available -it is nearly as common as the 741.