Op-Amp Voltage Follower Outputs ~+Vcc when IN+ is 0V

Thread Starter

rhaggart1

Joined Apr 25, 2020
6
Hi all,

I'm fairly new to working with operational amplifiers and so I'm just messing around with a few basic circuits. Had a comparator working fine to switch an LED on/off based on some variable reference voltage, but now I'm trying to get a working voltage follower.
I have 2 different Op-Amps.
Texas Instruments TL072CP
Texas Instruments LM741


I'm connecting the Op-Amp as follows
+Vcc from 5V supply on an Arduino Uno
-Vcc connected to Arduino Uno GND
Non-inverting input connected to the middle pin of a 10K pot (with the other 2 pins connected to +Vcc & GND)
Output directly fed back to the inverting input (as per voltage follower)
I've then connected both inputs of the Op-Amp to pins A0 and A1 of the Arduino so I can read the voltage out of the pot and the output of the Op-Amp.

Obviously, in this configuration, we'd expect these 2 voltages to be pretty much the same through the Op-Amp supply range as I twist the pot. As I understand, the LM741 has some limitations (due to BJT?) which means it loses this behaviour towards the ends of the supply range, and this is what I've observed. All good.

When trying the TL072CP, if I start turning the pot from maximum output to decrease the IN+ voltage, the output decreases accordingly... until I hit ~1V. After this point, the output climbs rapidly up to +Vcc.

I've attached a portion of the serial log below.

I've given it a good think, and I've double-checked my connections but I can't quite work out what's going on here! If anyone has any idea, please let me know :)
Thanks!

opAmpLog.png
 

OBW0549

Joined Mar 2, 2015
3,457
Verbal descriptions of circuits are absolutely worthless. Post a schematic diagram, complete with all components and connections (including power). With that, someone might be able to help you.
 

Thread Starter

rhaggart1

Joined Apr 25, 2020
6
Apologies! Power for op-amp omitted but is connected to the 5V DC supply. Gain of op-amp isn't as given here. Voltmeters represent the analog readings on the Arduino.
opAmpCircuit.png
 

Thread Starter

rhaggart1

Joined Apr 25, 2020
6
Phase reversal because you exceed the input common mode range.
https://www.edn.com/are-you-violating-your-op-amps-input-common-mode-range/
Ah hah! This looks like it's the culprit. Thanks so much for sharing. I can't say I fully understand it immediately but I'll give it a proper read through and a good think ASAP.

Luckily the application I'm looking at using this op-amp for should be clear of the violating range (will be DC biasing, V_p-p is approx. a few hundred mV, and the supply will be 9V, not 5V)

Glad to see this problem shouldn't raise its head when it comes to the actual application (so long as I design with this in mind)

Thanks for introducing this to me - I'm sure it'll be very useful info to have going forward :)
 

Audioguru again

Joined Oct 21, 2019
1,602
Here is a photo showing the output suddenly going as high as it can when the input voltage gets too close (within a few volts) to the negative supply voltage. The common mode voltage range of a TL072 is as much as 4V above its negative supply voltage.
This is one reason that its recommended minimum supply is 10V but I have used hundreds powered from a 9V battery that drops to 6V.
 

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ronsimpson

Joined Oct 7, 2019
726
There are types of opamps designed to live on 5 volts.
There are types of opamps designed to have a input range of "rail to rail". R-R supply to supply
You probably also need R-R on the output.
MCP6001 is an example. (watch out there are many different pin-out versions)
 
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