Voltage follower op-amp hits the lower output limit at 180mV, by spec it should be as low as 20mV

Thread Starter

kralg

Joined Aug 13, 2017
4
Hi,
I am getting confused with my first circuit with an op-amp. I am trying to voltage-amplify a signal from a PT100 sensor using an LM358AM dual op-amp, see attached schematic.

For some reason, the 117mV input on the voltage follower causes a 180mV output. As a result the inverting amplifier correctly sets 4V at its output.
Increasing the input voltage above 180mV, the voltage follower starts working as I expect.
Reaching 235mV, the inverting amplifier fails to keep the balance as it cannot decrease the output further. At 245mV input voltage I measure 12,7mV at the output, so the inverting amplifier is able to go under 20mV with its output.

So what is the reason that the voltage follower can not? At this point it is sinking less than 1mA current, which it should do without problem.

(Obviously I tried to replace the op-amp, but obviously it did not help, so the op-amp is OK.)

Hope somebody can point me to what I do wrong. Thanks.

Kralg
 

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ericgibbs

Joined Jan 29, 2010
9,532
hi kralg,
Welcome to AAC.
The problem is that you are using a single voltage supply for that type of OPA.
It will not go down to 0Vout.
Try it with dual supplies.

E
 

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OBW0549

Joined Mar 2, 2015
3,203
So what is the reason that the voltage follower can not? At this point it is sinking less than 1mA current, which it should do without problem.
Watch out with that "should"; the Devil is in the details, as they say.

The LM358's output will go down to within a few mV of V-, but ONLY if it's sinking a very low current; beyond about 80-90 μA it "bottoms out" at about 600 mV. In your circuit it would be sinking ≈180 μA which is too much.

LM358.png
 

ronsimpson

Joined Oct 7, 2019
439
The data sheet: with no load the output can get to 150mV. With 1mA load 1V worst case 0.7 typical.
If you look at the internal workings of the amp, the output transistor shuts down below about 1V. And the 120uA pull down also shuts down below some voltage. (200mv)
1578494327841.png
 
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DickCappels

Joined Aug 21, 2008
6,185
1578498503432.png
The LM324 is the quad version of the LM358. You can extend the output swing by sinking current either through a grounded resistor as shown above, or with a current source or resistor from a negative power supply voltage.
 

crutschow

Joined Mar 14, 2008
24,318
Below is the LTspice simulation of the circuit with a current-mirror load of about 1mA on the left op amp to pull its output to near zero with a 0V input (Q1 saturates if the sink load current becomes less than that near 0V).

I also increased the value of the right op amp resistors to reduce the load current.

The transistors are operated in the inverted mode, since BJT's have a slightly lower saturation voltage at low currents when operated inverted (I learned that from the old GE transistor manual).

1578499340737.png
 
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Audioguru again

Joined Oct 21, 2019
857
It is a sunny day and I can not see your negative schematic image. The dark blue lines on the black background cannot be seen even with the windows covered and the room lights off.
I tried inverting the colors and ended up with bright yellow lines on a white background that also could not be seen.

Have you ever seen a datasheet that has a negative schematic? I have not. I wonder why yours is a negative.
 

Thread Starter

kralg

Joined Aug 13, 2017
4
Thank You all for your quick and helpful responses.

As ronsimpson suggested I removed the 680 Ω resistor and the voltage follower gave the correct output. Not a big surprise I guess but at least it proves that the load is causing the higher voltage.

As DickCappels suggested I added a 1kΩ resistor as an extra load to the voltage follower and now the complete circuit works as I expected. I understand that this resistor is basically a relief to the voltage follower (so less current it has to sink), but on the other hand there will be a voltage drop on the resistor. So I believe it is more like an experimental work here to find the ideal resistor value, and not something that can be easily calculated.

Thanks OBW0549 for pointing out what I completely ignored in the datasheet. I believe the most difficult part in learning op-amps is accepting that they are not ideal at all.
 

Thread Starter

kralg

Joined Aug 13, 2017
4
Below is the LTspice simulation of the circuit with a current-mirror load of about 1mA on the left op amp to pull its output to near zero with a 0V input (Q1 saturates if the sink load current becomes less than that near 0V).
Thanks for fixing my design, I did not even expect for it. Understanding the behaviour of the transistors will take me a little time to digest though... :)
 

Thread Starter

kralg

Joined Aug 13, 2017
4
It is a sunny day and I can not see your negative schematic image. The dark blue lines on the black background cannot be seen even with the windows covered and the room lights off.
I tried inverting the colors and ended up with bright yellow lines on a white background that also could not be seen.

Have you ever seen a datasheet that has a negative schematic? I have not. I wonder why yours is a negative.
I completely agree and sorry about that. I made a quick screenshot (of gEDA schematic) and since I am used to it and on my screen it is visible, I did not bother turning it to white background. But next time I will do it.
 

Audioguru again

Joined Oct 21, 2019
857
The LM358 is old and was made to have a very low idling current which is why it has crossover distortion, a low bandwidth and a saturation voltage that rises even when the load current is fairly low.

I have never seen gEDA before. I wonder why they selected schematics to be negative images?
I notice that they have the polarity of the LED backwards because Vcc is a positive supply voltage (Vee is negative).
 

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dl324

Joined Mar 30, 2015
9,984
I completely agree and sorry about that. I made a quick screenshot (of gEDA schematic) and since I am used to it and on my screen it is visible, I did not bother turning it to white background. But next time I will do it.
Reversed colors and converted to grayscale for easier reading.
1578562927773.png
 

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OBW0549

Joined Mar 2, 2015
3,203
Thanks OBW0549 for pointing out what I completely ignored in the datasheet. I believe the most difficult part in learning op-amps is accepting that they are not ideal at all.
While I can agree that accepting that op amps (as well as all other real-world components) are not ideal can be difficult, I believe the greater difficulty lies in developing a good understanding of how their non-idealities can affect the performance of a circuit, and the techniques needed for evaluating those effects.

That can take years of practice and study.
 
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