# Calculate the transfer function of an OPAMP circuit

#### Mahdi16999

Joined Mar 31, 2024
5
I have an OPAMP circuit that its output increase steadily without cahnge in the input. I guess the issue is in the transfer function of the circuit and its stability. I want to calculate the transfer function of my circuit but I do not have any idea about that. Could you please help me to calculte the transfer function?
I would be grateful if anyone knows the reason for this problem

#### Ian0

Joined Aug 7, 2020
10,076
Some figures would be useful.
It can't keep increasing indefinitely, so how long does it spend increasing? What voltage does it stop at?
What values do you have for the various resistors and capacitors?

#### MrChips

Joined Oct 2, 2009
30,987
Is this related to school work?

#### crutschow

Joined Mar 14, 2008
34,704
if anyone knows the reason for this problem
What problem?
Calculating the transfer function?

You circuit is a 3-pole underdamped, low-pass filter.
You combine the transfer function of the three poles.

#### Mahdi16999

Joined Mar 31, 2024
5
ok
5 volt is generated by a switching regulator called MC34063 and the opamp is a LM358 and I use a UGN3503 Hall Effect sensor

#### Mahdi16999

Joined Mar 31, 2024
5
here is the components values

#### Mahdi16999

Joined Mar 31, 2024
5
And what happens when you switch it on?
its output increase steadily without cahnge in the input. I think it happens because of unstability of transfer function of OPAMP.
Do you have any different idea?

#### Ian0

Joined Aug 7, 2020
10,076
its output increase steadily without cahnge in the input. I think it happens because of unstability of transfer function of OPAMP.
Do you have any different idea?
What is the output? A DC voltage? An AC voltage? what frequency?

#### WBahn

Joined Mar 31, 2012
30,252
Have you done some basic testing, such as remove the sensor and replace it with an adjustable supply that you can vary from 0 V to 5 V and map the output voltage and see how stable it is?

As others have asked, you really need to provide more details. Saying that it's output increases steadily without a change in the input is way to vague to be useful.

What is the input signal? How stable is it?

What does the output signal start at?

What does the output signal stop at?

How long does it take for that to happen? Are we talking about seconds, milliseconds, or microseconds?

What do you expect it to be, based on your circuit component choices?

Is it a smooth increase, or is it erratic?

Does it oscillate at all? If so, what frequency?

Are you honoring the LM358's basic limitations?

You are powering it from a single 5 V supply. That means you can expect the output to be able to swing from about 0.1 V to about 3.6 V.

Your inputs need to honor the common-mode input range of about 0 V to 3.0 V (or perhaps 3.5 V if the junction is saying very close to 25°C).

Your reference voltage at the non-inverting input of the opamp is nominally 2.5 V.

You have a nominal DC gain of 47.

Given all of that, what is the range of voltages at Vin that are consistent with the these constraints?

I think you might be surprised at the results.

What makes you believe that your sensor output happens to fall within that range of voltages?

#### Mahdi16999

Joined Mar 31, 2024
5
Have you done some basic testing, such as remove the sensor and replace it with an adjustable supply that you can vary from 0 V to 5 V and map the output voltage and see how stable it is?

As others have asked, you really need to provide more details. Saying that it's output increases steadily without a change in the input is way to vague to be useful.

What is the input signal? How stable is it?

What does the output signal start at?

What does the output signal stop at?

How long does it take for that to happen? Are we talking about seconds, milliseconds, or microseconds?

What do you expect it to be, based on your circuit component choices?

Is it a smooth increase, or is it erratic?

Does it oscillate at all? If so, what frequency?

Are you honoring the LM358's basic limitations?

You are powering it from a single 5 V supply. That means you can expect the output to be able to swing from about 0.1 V to about 3.6 V.

Your inputs need to honor the common-mode input range of about 0 V to 3.0 V (or perhaps 3.5 V if the junction is saying very close to 25°C).

Your reference voltage at the non-inverting input of the opamp is nominally 2.5 V.

You have a nominal DC gain of 47.

Given all of that, what is the range of voltages at Vin that are consistent with the these constraints?

I think you might be surprised at the results.

What makes you believe that your sensor output happens to fall within that range of voltages?
i use a hall effect sensor and a magnet that mounted in a constant distance with the hall effect sensor called ugn3503.
the input is constant. I measure the voltage that generated by signal and it is also constant. The output voltage remains constant for even one or two days. but it starts to increasing after that and maybe increase 0.05 V after a day. I expext the output voltage remains constant because thedistance between sensor and magnet is constant.

#### Ian0

Joined Aug 7, 2020
10,076
I note that the UGN3503 has an output voltage in zero magnetic field which can vary from 2.25V to 2.75V, yet it would only take a variation from 2.44V to 2.55V to saturate the op-amp output.
I also note that the UGN3503 output varies with temperature.

#### WBahn

Joined Mar 31, 2012
30,252
i use a hall effect sensor and a magnet that mounted in a constant distance with the hall effect sensor called ugn3503.
the input is constant. I measure the voltage that generated by signal and it is also constant. The output voltage remains constant for even one or two days. but it starts to increasing after that and maybe increase 0.05 V after a day. I expext the output voltage remains constant because thedistance between sensor and magnet is constant.
What change in distance does a 50 mV change in output correspond to?

What change in temperature does a 50 mV change in output correspond to?

It's not sufficient to expect the output voltage of the sensor to be constant. Measure it. What is it after the output increases? Is it really the same that it was before?

Decouple some of the potential issues. Remove the sensor and replace it with a suitable potentiometer adjusted to give the same initial reading. Does it also increase after a couple days? If so, the problem isn't the sensor. If not, the problem is with the sensor.