# Need help on amplifier feedback

#### Richardysy

Joined Jun 25, 2015
6
Hi,

I am currently working on a school project that involves the designing of an op-amp with 0 to 10V supply. Currently I am able to get a large open loop gain, (around 200,000 V/V). However, the DC offset of input and output are different, and they are clearly not zero. Because of this, I find it hard to implement feedback with just resisters, like the "inverting" or "non-inverting" configurations of ideal op-amp.

My question is whether there is a way to design feedback without using any voltage conditioning circuit.

I appreciated the help.

Richard

#### Richardysy

Joined Jun 25, 2015
6
This is the circuit. Thanks.

#### shteii01

Joined Feb 19, 2010
4,644
You fail to present information in clear and organized manner.

#### crutschow

Joined Mar 14, 2008
27,237
What do you mean "DC offset of input and output are different"?
The output open-loop offset will obviously be very large because of the high gain.
That offset is reduced when the loop is closed so that it equals the input offset times the close-loop gain.

What is a "voltage conditioning circuit"?

#### Richardysy

Joined Jun 25, 2015
6
What do you mean "DC offset of input and output are different"?
The output open-loop offset will obviously be very large because of the high gain.
That offset is reduced when the loop is closed so that it equals the input offset times the close-loop gain.

What is a "voltage conditioning circuit"?
For example, my input might be vi = Vin + 1e-6sin(wt), and my output is vo = Vout + A*1e-6sin(wt). Usually, Vout not equal to Vin.
As for "voltage conditioning circuit", I mean a voltage adder or subtractor using op-amp.

Thanks again!

#### crutschow

Joined Mar 14, 2008
27,237
Of course Vout does not equal Vin.
Vout = Vin * A.

#### Bordodynov

Joined May 20, 2015
2,871
I want to warn that such a circuit simulation (without the parasitic capacitance of transistors) are very far from reality.
When you add the necessary parameters in the scheme, the scheme is not likely to be stable (with negative feedback). For the stability required correction schemes.

#### dtekumse

Joined Dec 15, 2015
9
I agree with other participants of this discussion - the problem is not given in a clear manner, but after clarification by Richardysy, I believe I understand his/her confusion.

This difference in offsets is something expected, as somebody else already said. The reason why it is still fine to use resistive feedback network (not considering stability at this point, whether it is negative or positive, parallel or series or other details related to its implementation) is that resistive network is actually voltage divider. So within your resistive network there will be different voltage levels (i.e. "offsets") anyway. Now, your task is to design that resistive network in the appropriate manner - so that all voltages are what they should be.

If I did not get the idea - I apologize for off-topic.

#### Richardysy

Joined Jun 25, 2015
6
I agree with other participants of this discussion - the problem is not given in a clear manner, but after clarification by Richardysy, I believe I understand his/her confusion.

This difference in offsets is something expected, as somebody else already said. The reason why it is still fine to use resistive feedback network (not considering stability at this point, whether it is negative or positive, parallel or series or other details related to its implementation) is that resistive network is actually voltage divider. So within your resistive network there will be different voltage levels (i.e. "offsets") anyway. Now, your task is to design that resistive network in the appropriate manner - so that all voltages are what they should be.

If I did not get the idea - I apologize for off-topic.

Thanks