Tool/software:
I'm using the spice model in LTSpice, when I power the TL072H with single supply the output will not begin to move until 1V:
but if I use dual power (+/-12V) the output will be 200mV below the input, even if that is below the rail!
Is something wrong with this model?
Also, if I use it as a comparator, it works fine when VEE is negative (it doesn't need to be symmetric with VCC):
but it doesn't work when I connect VEE to GND:
With just -0.7V at VEE seems to be enough to make it to work:
Why doesn't the model like to work with single supply?
Hi Alejandro,
Why doesn't the model like to work with single supply?
TL072H is not RRIO (rail to rail input/output) op amp. The input Vcm and output voltage swing specification are shown below. The output voltage swing is a load dependent, but it may be used for certain regulator application as an error amplifier. You need to watch for input range and output swing as an error amplifier.
RRIO would be the better option for error amplifier (less design constraint from feedback and output consideration), for instance, TLVx197-Q1, OPA191, OPA197as examples.
With just -0.7V at VEE seems to be enough to make it to work:
Your simulation examples and different supply rails are required to inject the input in a linear operating range of the op amp. Linear output swing has some limitation. For a series regulator as an error amplifier, you would need a minimum RRO op amp type. TL072H may be used, but it depends on your output regulated voltage vs. supply rail of the TL072H.
If you have other questions, please let me know.
Best,
Raymodn
Hi Raymond, Thanks for your quick answer.
Yes, the difference between the values in the DS and what I was seeing in my simulation is what motivated the post.
Ok, I understand that even when the output swing seems to be out of spec that could be an effect of the input being too close to the negative rail (GND in this case). Also, in the first picture I sent (the voltage follower with VEE = GND), it seems like a glitch happened with LTSpice or something, after rebooting the computer (for other reasons) the simulation with VEE = GND looks the same as the one with VEE = -12V for the follower.
What still bothers me is the difference between output and input which happens either VEE is GND or -12V:
Vo (in gray) is ~400mV lower even when the input is 0V and that means it is below the rail, which should not be possible!
This makes me wonder if the difference in trigger point in the comparator simulation is a result of the simulation or if it will actually be like that in real life.
I chose the TL072H because in my application VCC is normally 24V, but abs max is 38V, and that op-amp has also become really cheap these days!
Of the ones you recommend OPA2197IDR is the closest to something viable for budget, but still 3x the price :/
Hi Alejandro,
Vo (in gray) is ~400mV lower even when the input is 0V and that means it is below the rail, which should not be possible!
I am unable to use LTSpice for the simulation due to the license agreement. Anyway, this is the simulation in Tina, I do not see issues.
I chose the TL072H because in my application VCC is normally 24V, but abs max is 38V, and that op-amp has also become really cheap these days!
Yes, TL072H will work for the error amplifier as integrator, however, it may have certain constraint and you need to work around depending on your feedback control point in your regulator.
It is close to be RRO amplifier, and you may need to have slightly higher Vcc rail than your regulator's output voltage in TL072H design as error amplifier. The TL072H error amplifier's output needs to integrate the regulator's output voltage range in order to work properly.
TLVx197-Q1 is the same family as OPA197/OPA196/OPA191/OPA192 etc., it will work well, but the cost will be a lightly higher. TLV9301/TLV9351 may work as well. If you are able to provide the performance and cost requirements, I may be able to recommend others for you.
If you have other questions, please let us know.
Best,
Raymond
