# Offset voltage comes to saturation in coupling amplifier circuit using OpAmp with Gain=100

#### Nhơn Nguyễn

Joined May 21, 2019
15
Hi you guys! I'm new here
I did get in troubles with my circuit below! It's my project which my teacher gave it to me.
My purpose is to amplify difference between two inputs.Specifically, 20mV and 10mV are connected with input to get 1V at the last OpAmp's output.
The problem is when two input are grounded,the output offset voltage of OA1 and OA2 comes to saturation in reality.I attempted to find out what problem is but it's doesn't make sense..
As i figured,I used Potienmeter to adjust offset voltage because I thought the offset voltage is just 20-30 mV..In fact,it doesn't..so anyone here can explain to me the cause of this case?I will really appreciate that!
Here is my circuit on proteus..Sorry if I have dumb question,I have just learned about this component.By the way,English is not my official language so some expression is not really exact.Hope you ignore it..Thank you!

#### ericgibbs

Joined Jan 29, 2010
14,136
hi NN,
Welcome to AAC
What do you calculate the gain of U3, OPA to be for the inputs V01 and V02.?
E

The value of R12 at 100R is too low.

#### Nhơn Nguyễn

Joined May 21, 2019
15
hi NN,
Welcome to AAC
What do you calculate the gain of U3, OPA to be for the inputs V01 and V02.?
E

The value of R12 at 100R is too low.
Gain of OA3=50 because difference was amplified two times at first two OA..
I just want Power at output to be smaller than 100mW so i used 100R

#### ericgibbs

Joined Jan 29, 2010
14,136
hi,
The gain of U3 for the inverting input is R9/R8 = 150k/3k = 50.
For the non inverting input the gain of U3 is 1+[R9/R8] = 51 so there is an in balance
in the gain of the two inputs.

I would suggest R12 is not less than 1k.

Where did you get the circuit design.?

E

#### Nhơn Nguyễn

Joined May 21, 2019
15
I think the problem is not at R12 because i measured output without R12 and the result didn't change..

#### Ylli

Joined Nov 13, 2015
1,053
hi,
The gain of U3 for the inverting input is R9/R8 = 150k/3k = 50.
For the non inverting input the gain of U3 is 1+[R9/R8] = 51
Non-inverting gain at the (+) op-amp input is 1 + (R8/R9) = 51, correct, but there is a voltage divider on the (+) input. So the gain at the non-inverting input port is
51 * [R11/(R10 + R11)] = 51 * (150/153) = 50

#### crutschow

Joined Mar 14, 2008
28,164
Below is the typical differential instrumentation amp circuit.
Why is yours different and more complicated?

#### Nhơn Nguyễn

Joined May 21, 2019
15
Below is the typical differential instrumentation amp circuit.
Why is yours different and more complicated?

Yeah.I know..But That's my teacher request..He want me to build a circuit like that with conditions that i mentioned.I just choose the value of resistors to make the circuit work well..So Have you known what problem is? I think it just because the positive feedback is much larger than negative feedback..But i don't know how to fix it

#### crutschow

Joined Mar 14, 2008
28,164
That's my teacher request..
Are those resistor values also specified by him?

#### Nhơn Nguyễn

Joined May 21, 2019
15
Are those resistor values also specified by him?
No..I chose resistor's value by myself

#### crutschow

Joined Mar 14, 2008
28,164
No..I chose resistor's value by myself
And how were all the values determined?
Did you write the equation for the overall gain of the circuit?

#### Nhơn Nguyễn

Joined May 21, 2019
15
if i choose R2=R7 ,R1=R6 ,R8=R10,R9=R11,R3=R4=R5 (to simplify the equation),the overall gain will be:
Vout=A1.A2.(V1-V2) where:
A1=3R1/(4R2+R1) and A2=R10/R9

#### ericgibbs

Joined Jan 29, 2010
14,136
hi NN,
What tolerance range are you using for the resistor values.? ie: what percent
E

#### Jony130

Joined Feb 17, 2009
5,250
I'm I seeing the positive feedback in this circuit or not?

#### Nhơn Nguyễn

Joined May 21, 2019
15
I'm I seeing the positive feedback in this circuit or not?
both positive and negative feedback..Have you figured out the reason?I think that because positive feedback is stronger than negative feedback so it leads to saturation..but i don't know how to fix it

#### MrAl

Joined Jun 17, 2014
8,548
Hi you guys! I'm new here
I did get in troubles with my circuit below! It's my project which my teacher gave it to me.
My purpose is to amplify difference between two inputs.Specifically, 20mV and 10mV are connected with input to get 1V at the last OpAmp's output.
The problem is when two input are grounded,the output offset voltage of OA1 and OA2 comes to saturation in reality.I attempted to find out what problem is but it's doesn't make sense..
As i figured,I used Potienmeter to adjust offset voltage because I thought the offset voltage is just 20-30 mV..In fact,it doesn't..so anyone here can explain to me the cause of this case?I will really appreciate that!
Here is my circuit on proteus..Sorry if I have dumb question,I have just learned about this component.By the way,English is not my official language so some expression is not really exact.Hope you ignore it..Thank you!
View attachment 177893

Hi,

If you want a 1.000v output with 10mv difference input then the values do not look correct. You might mention how you calculated that and how you chose all the resistors.

A purely linear analysis shows that this circuit provides a bit less than 1.000v (significantly however) and that would not be so bad because it is easily corrected, but because there is positive feedback you have to check for latchup too. Latchup means the output pins to one of the rails. This brings us to how you calculated the three middle resistors all in a row.

10mv or even 20mv on either input should never cause saturation even if that was the input offset so it must be something else.

#### Nhơn Nguyễn

Joined May 21, 2019
15
Hi,

If you want a 1.000v output with 10mv difference input then the values do not look correct. You might mention how you calculated that and how you chose all the resistors.

A purely linear analysis shows that this circuit provides a bit less than 1.000v (significantly however) and that would not be so bad because it is easily corrected, but because there is positive feedback you have to check for latchup too. Latchup means the output pins to one of the rails. This brings us to how you calculated the three middle resistors all in a row.

10mv or even 20mv on either input should never cause saturation even if that was the input offset so it must be something else.

I just realized that this page doesn't support to display equation on Microsoft Word after I finished my answer on it..So i posted screen capture..

#### MrAl

Joined Jun 17, 2014
8,548
View attachment 178119
View attachment 178120
I just realized that this page doesn't support to display equation on Microsoft Word after I finished my answer on it..So i posted screen capture..
Hello again,

Ok good, but i need to ask a couple questions.

First, what are you calling V1 and V2? Is V1 the top input and V2 the bottom input or the other way around?

Second, what level of circuit theory is this for, like what have they taught you so far about op amps?
I have to ask this because we are given a circuit but we are not told what some of the values are doing for us. This means that we have to investigate what some of the components are going to do in order to provide a better circuit than say just using one op amp.
The way to do this would be to look at various advantages a multi op amp circuit has over a single op amp and try to figure out what some of the components are doing to improve over the single op amp circuit. Some examples are CMRR and DC offset.
I suspect R3, R4, and R5 play a role in that, but it could also be that your instructor just threw this together in order to get you to just have to figure out how to get the gain right. You'll have to figure this out, but it depends partly on what level or grade you happen to be in, such as freshman, senior, getting close to Master's Degree, BS, etc.

The simplest of all is to just get the gain right, but i need to know what you are calling V1 and V2, like the top input for V1 and bottom input for V2 for example

#### Nhơn Nguyễn

Joined May 21, 2019
15
Hello again,

Ok good, but i need to ask a couple questions.

First, what are you calling V1 and V2? Is V1 the top input and V2 the bottom input or the other way around?

Second, what level of circuit theory is this for, like what have they taught you so far about op amps?
I have to ask this because we are given a circuit but we are not told what some of the values are doing for us. This means that we have to investigate what some of the components are going to do in order to provide a better circuit than say just using one op amp.
The way to do this would be to look at various advantages a multi op amp circuit has over a single op amp and try to figure out what some of the components are doing to improve over the single op amp circuit. Some examples are CMRR and DC offset.
I suspect R3, R4, and R5 play a role in that, but it could also be that your instructor just threw this together in order to get you to just have to figure out how to get the gain right. You'll have to figure this out, but it depends partly on what level or grade you happen to be in, such as freshman, senior, getting close to Master's Degree, BS, etc.

The simplest of all is to just get the gain right, but i need to know what you are calling V1 and V2, like the top input for V1 and bottom input for V2 for example
Hello again,

Ok good, but i need to ask a couple questions.

First, what are you calling V1 and V2? Is V1 the top input and V2 the bottom input or the other way around?

Second, what level of circuit theory is this for, like what have they taught you so far about op amps?
I have to ask this because we are given a circuit but we are not told what some of the values are doing for us. This means that we have to investigate what some of the components are going to do in order to provide a better circuit than say just using one op amp.
The way to do this would be to look at various advantages a multi op amp circuit has over a single op amp and try to figure out what some of the components are doing to improve over the single op amp circuit. Some examples are CMRR and DC offset.
I suspect R3, R4, and R5 play a role in that, but it could also be that your instructor just threw this together in order to get you to just have to figure out how to get the gain right. You'll have to figure this out, but it depends partly on what level or grade you happen to be in, such as freshman, senior, getting close to Master's Degree, BS, etc.

The simplest of all is to just get the gain right, but i need to know what you are calling V1 and V2, like the top input for V1 and bottom input for V2 for example
V1 is top input and V2 for bottom input..As you see..If i vary R4 and stablize others,the difference between V02 and V01 will change to obtain desired gain....But problem here is to eliminate offset voltage..you know..when i ground V1 and V2 and U1 , U2 comes to saturation..It's weird.That's why I create this post

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