# Op amp voltage follower saturates (high-rail) at 0V

I have 3 op amps, and they perform in two different ways, but I am not sure why. I want to buy another op amp that behaves a specific way, but I'm not sure what spec causes it to behave like this.

Here are the 3 op amps I'm using. I will refer to them as A, B, and C respectively from now on.

A) NJM072CAG-TE2 - Acts Strange

B) RC4558P - Acts strange

C) LM324DR - Acts Ideal

After a lot of hassle, I decided to use them in a very basic way to test their properties using a voltage follower circuit with supplies (V-)=0V and (V+)=5V (now we won't have to deal with gain and such).

I hooked up Vin to a variable power supply (0-5V), and I measured Vout with an arduino and made graphs. As I increased the voltage from 0->5V and back down, here's what the output looked like for each of the three op amps. Red=Vin, Blue=Vout

A) NJM072CAG-TE2 B) RC4558P C) LM324DR

Notice how A and B output a high voltage when the input is 0 (it stays this way until about 1.0V).

So my overall question is: How can I know if an op amp will "Saturate" or not at 0V (Like A & B) instead of following the voltage properly (Like C)?

Thanks in advance!

• 78M12 is a 12-volt linear regulator, not an opamp. Disregard...OP edited device A) Feb 26, 2018 at 14:38
• The first two are not "Rail to Rail", meaning that they cannot output voltages close to the supply rails, in this case 0V and 5V Feb 26, 2018 at 14:42
• What supply voltage are you applying to the devices? Feb 26, 2018 at 14:44
• Is there any way to know if it is "Rail to Rail" when researching them online? Sorry, op amps are very new to me. Feb 26, 2018 at 14:44
• Supply voltages are (V-) = 0V, and (V+) = 5V. I will update my post to reflect this Feb 26, 2018 at 14:45

## 2 Answers

The first device (7812) is a voltage regulator and not an op-amp so that's easy to answer. See EDIT below following OP changing their mind.

The 2nd device (RC4558) is not a rail-to-rail op-amp and it's output cannot get within 1 volts of either supply rail. See VOM (Maximum output voltage swing) in the data sheet. The VICR (Common-mode input voltage range) is also limited so you shouldn't use inputs too close to the power rails without unexpected results.

The LM324 is OK getting to the negative (0 volt) rail but has limits like the RC4558 when getting to the positive rail.

You also need to ensure that there is enough suipply voltage on the op-amp or it won't adequately perform. For instance, the RC4558 needs a minimum supply rail of 10 volts and not 5 volts. It's all there in the data sheet but you have to get used to the terminology used.

EDIT following change by OP.

The NJM072 has both input and output voltage restrictions specified in the data sheet. Input common mode range is not guaranteed to be better than +/-11 volts on a +/- 15 volt supply. The output swing is similarly problematic (+/-12 volts on a +/- 15 volt supply). Minimum supply for this op-amp is 8 volts.

Called phase reversal. It happens when you use the opamp outside its specified common mode input range.