OPAMP output is high even if the two inputs are low

[deepakdkt]

I have problem in a circuit , where I have OPAMP with inputs low . But at output I'm getting 20volts ... I have attached the circuit image below. OPAMP has biasing voltage of 20 volts

 

[Dominik Przyborowski]

Your opamp works without negative feedback so now works like voltage comparator. Remember that open loop gain is around 100-120dB so very small difference between inputs causes saturation of opamp output.

 

[LvW]

Your opamp works without negative feedback so now works like voltage comparator. Remember that open loop gain is around 100-120dB so very small difference between inputs causes saturation of opamp output.

I don´t know - perhaps there is some feedback (it is not clear where some connection lines are connected to).
However, no power supply is shown.
Therefore my question: Single or double supply voltages?

 

[D.A.(Tony)Stewart]

If you lower (+) input voltage with a pull down resistor to VEE and output drops, something else is wrong. If the Op Amp does not obey spec, replace it.

 

[deepakdkt]

@Dominik Przyborowski There is no difference between inputs.. both the inputs are Zero!!!

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@LvW There is no feed back to the input. its just output of another opamp to the D15 cathode. D15 blocks the voltage . It is a Single supply voltage.

 

[crutschow]

You appear to have a fundamental misunderstanding of an op amp's operation. It responds to the difference voltage on the inputs, not the absolute value. If you tie the inputs together (both have the same voltage) than the intrinsic input offset voltage will drive the output to the rail, either high or low depending upon the polarity of the offset. To drive the output low the (-) input must be slightly more positive than the (+) input (by a value greater than the offset value as shown in the data sheet).

 

[deepakdkt]

If you lower (+) input voltage with a pull down resistor to VEE and output drops, something else is wrong. If the Op Amp does not obey spec, replace it.

@SunnySkyguy I have replaced the opamp with the new one.
i checked the circuit by isolating output circuit, still i am getting 20 V at output.

 

[LvW]

@SunnySkyguy I have replaced the opamp with the new one.
i checked the circuit by isolating output circuit, still i am getting 20 V at output.

If you want linear amplification you NEED negative feedback.
More than that, do you know the restrictions connected with single supply?

 

[KlausST]

Hi,

Tell us more: what supply voltage do you use? What OPAMP do you use?
What idea is behind the circuit?

The posts before are right. With this configuration nobody can predict what the output voltage is. With one OPAMP it is at positive rail, with another OPAMP it is at negative rail, even oscillation can appear.
The 0V input is meaningless in your configuration. Even if you solder both inputs together.


Without negative feedback this OPAMP circuit acts like a comparator. If this is what you want, then use a comparator instead of an OPAMP.
If you want to act it like an amplifier, then add a negative feedback.


You have to take into account: Input offset voltage, input offset current, common mode input range...

Good luck
Klaus

 

[deepakdkt]

You appear to have a fundamental misunderstanding of an op amp's operation. It responds to the difference voltage on the inputs, not the absolute value. If you tie the inputs together (both have the same voltage) than the intrinsic input offset voltage will drive the output to the rail, either high or low depending upon the polarity of the offset. To drive the output low the (-) input must be slightly more positive than the (+) input (by a value greater than the offset value as shown in the data sheet).

Yes. i checked with it. whenever i give higher voltage to the (+) circuit works properly.
but the problem is i cant change the value of resistors, i have another unit with the same circuit and it works properly .

 

[KlausST]

Hi,

i have another unit with the same circuit and it works properly .

Maybe we don't have full information on the circuit, or it is simply good fortune..

Tell us more about the function, supplies, partnames and so on...

Klaus

 

[Audioguru]

Yes. i checked with it. whenever i give higher voltage to the (+) circuit works properly.

No. If the (+) input is more positive than the (-) input then the output will remain high. The (-) input must be more positive then the (+) input for the output to go low.

With a single positive supply then the opamp must be designed so that its inputs work at the negative supply (VEE) which is 0V. What opamp number is it? Many ordinary opamps like a 741 will not work.

 

[Borber]

I am wondering if you heared about comprators.

 

[LvW]

Many ordinary opamps like a 741 will not work.

What does this mean (...will not work) ? Under which condition?

 

[Audioguru]

What does this mean (...will not work) ? Under which condition?

The inputs of a 741 opamp have a useable common-mode voltage range of 3V more positive than VEE and 3V more negative than VDD. If they are closer to the supply voltages then the inputs are cutoff or are saturated, then they don't work properly. That is why they usually use a dual-polarity power supply and keep their input voltages away from its supply voltages.

We still do not know which opamp Deepak is talking about. An LM358 opamp has inputs that work perfectly at VEE. If the (+) input is at VEE (ground) and the (-) input is slightly positive then the output will go low.

 

[LvW]

Audioguru - thank you for explaining. I think, such an explanation/justification is helpful for the questioner.

 

[D.A.(Tony)Stewart]

@SunnySkyguy I have replaced the opamp with the new one. i checked the circuit by isolating output circuit, still i am getting 20 V at output.

THen the fault is somewhere else in the circuit or in the design. WIthout details on the overall circuit, there is insufficient info.

 

[crutschow]

..............................
but the problem is i cant change the value of resistors, i have another unit with the same circuit and it works properly .

Just because one marginally designed circuit will work doesn't mean the second one you build will. If you can't change the circuit then I don't see how you can fix it. Reminds me of the tongue-in-cheek rule where I used to work "Fix it but don't change it".

 

[KlausST]

Hi,

@crutschow

I'm right there with you.

Klaus

 

[schmitt trigger]

Just because one marginally designed circuit will work doesn't mean the second one you build will. If you can't change the circuit then I don't see how you can fix it. .

+1
I'm going to engrave your post in golden letters.

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+1
I'm going to engrave your post in golden letters.

I collect engineering truisms. You know, those simple phrases that have a deep meaning.

Here we have two.
Just because one marginally designed circuit will work doesn't mean the second one you build will
Just because one is lucky enough to make the first circuit to work, if such circuit is not properly analyzed and understood, and the normal component tolerances and/or parasitics are not taken into account, there is no guarantee that your next replica will work as intended.

If you can't change the circuit then I don't see how you can fix it.
This truism can be applied to many things in life. One cannot expect to have a different outcome, if one is not allowed to perform the required changes.