Non inverting opamp not working

student_897

Joined Nov 2, 2023
12
Good morning to everyone,
I am really new to this forum, thus I am sorry if I bother you with this stupid issue.
I just realized a very very simple non inverting amplifier, using a uA741 opamp.
The only two resistances are connected in the very classical way, being R_feedback = 10kOhm and R_noninvinput = 5kOhm.
It does not work, but I can't understand why.
Input = 2V (unfortunately, it is the minimum voltage my supplier is able to deliver) and Output = -10.6V.

Clearly, the input is applied to the non inverting V+ input.

Can someone help me?

Thank you very much in advance.

ericgibbs

Joined Jan 29, 2010
18,656
Hi 897,
Welcome to AAC.
Do you have a circuit diagram to post?
E

LesJones

Joined Jan 8, 2017
4,173
IF I have GUESSED correctly as to your schematic the output is about what I would expect. Post the schematic including power supply voltages. (Hand drawn on paper and photographed is OK)

Les.

student_897

Joined Nov 2, 2023
12
You are precious, thank you for the answers.
I attach the photograph and hand drawn schematic, I am sorry if I did not post it previously.

For the photograph: on the top left part you have the +-12V supply voltage. On the right part, instead, the GND connections.

Thank you again for your support.

ericgibbs

Joined Jan 29, 2010
18,656
Hi,
Are you sure it is a 741, I do not see any lettering on the OPA?
Do you know the gain equation for a NON INV OPA?
E

student_897

Joined Nov 2, 2023
12
Hi,
Are you sure it is a 741, I do not see any lettering on the OPA?
Do you know the gain equation for a NON INV OPA?
E
Hi, yes I re-checked and it is a ua741.
The gain is G = 1 + Rf/R1

Thank you again

ericgibbs

Joined Jan 29, 2010
18,656
Hi,
So from that equation you should get a Gain of 3.. ie =6v
Re-Check the Voltage on the NON INV input.

E

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ericgibbs

Joined Jan 29, 2010
18,656
Hi,
Do you have the 0v/Gnd line from your external -2V voltage source connected to the 0V of the bread line?

E

Alec_t

Joined Sep 17, 2013
14,222
on the top left part you have the +-12V supply voltage
If the red wire is +V (as is conventional) then you have the supply wires transposed.

BobTPH

Joined Jun 5, 2013
8,665
The resistor to ground looks like brown blue red, 1.6K. If so, the gain is -7.25 output should be -14.5. Outside the supply range. -100.6 is likely the lowest voltage you can get with ±12V supply.

MrAl

Joined Jun 17, 2014
11,271
The resistor to ground looks like brown blue red, 1.6K. If so, the gain is -7.25 output should be -14.5. Outside the supply range. -100.6 is likely the lowest voltage you can get with ±12V supply.
Hi,

-100.6v ??
Did you mean -10.6v or something?

MrAl

Joined Jun 17, 2014
11,271
Here is a lighter image and the package pinout for the 741. The pinout orientation is the same as the chip placement in the image on the breadboard with pin 1 at the top left corner.
The connections look correct assuming V+ is the red connecting wire.

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LesJones

Joined Jan 8, 2017
4,173
In your first post you say that the 5K resistor is connected to the non-inverting input whiich is wrong and is not what your schematic shows. Your schematic in post #4 is correct. Your bredboard layout looks correct but you have not labled the wires coming into it.
BobTPH has spotted the problem that the resistor that you say is 5K is in fact a 1.6 K resistor. ( I had been trying to work out what the first two colours were as they both looked black.)

Les.

MrAl

Joined Jun 17, 2014
11,271
Hi,

Actually that 5k resistor looks more like a 5.6k resistor.
Here is a close up of the chip and resistors after adjusting the gamma.
A visual inspection shows that the brown on the 10k resistor is very much brown while the first band on the other resistor is clearly, at the very least, not brown.
Using a color picker, the first band on the other resistor shows a lot of green tones while the 10k brown band does not.

Thus, the reasonable conclusion is that the 5k resistor is really a 5.6k resistor (the second band is blue of course). That puts the gain at somewhat less than 3 for the non inverting amplifier.

Note that different computer screens may render the color as a slightly different tone so the best bet is to use a color picker and examine the color components.
Also note that converting to .jpg can also skew the colors and make green look darker or maybe even more toward brown. That could also make red look darker.

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WBahn

Joined Mar 31, 2012
29,867

Is the top-left one the -12 V, the one right next to it +12 V, and the far right one 0 V?

Are you SURE that the +/-12 V rails are actually referred to the 0 V? Or is your supply floating?

This looks like Green-Blue-Red, making it a 5.6 kΩ resistor, not the 5 kΩ you mentioned.

The other one looks like Brown-Black-Orange, making it the 10 kΩ resistor you mentioned.

I assume the black lead going into Pin 3 is your 2 V input signal? Have you confirmed that it is actually 2 V?

The Pin 1 designator isn't too obvious in the picture. I think I see a circle in the upper-left, but I'm used to seeing the designators closer to the actual pin than that one is. So double check and make sure you have the IC oriented correctly.

What is the output if you tied the input to 0 V?

Have you tried putting a 10k pot between the offset null pins, with the wiper going to -12 V, to see if you can null out the offset voltage?

BobTPH

Joined Jun 5, 2013
8,665
Hi,

-100.6v ??
Did you mean -10.6v or something?
Yes, of course I meant -10.6V, which is what he measured.

But now it appears that the resistor is really 5.6K so the output should be 5.6V which is within range.