Opening the feedback of an Op Amp to have more gain

Question

I would like to open the feedback of a difference amplifier to have more gain.

Actually, I would like to increase the gain of the open loop system in order to reduce the error between the reference and the output that I want to regulate. In my application, I would like to regulate the current.

For example if I took a simple model of a linear regulator, the whole system is a closed loop system and so there is a feedback. But locally around the op amp there is no feedback and I think that the op amp which has "no" feedback could be a difference amplifier but the gain would be lower and the regulation would be worse.

So I would like to do the same thing into my system to improve the regulation. Nevertheless this will be the first time that I would do this and I am afraid to let open the feedback. It is probably not as simple as it seems, so I would like to know if I have to take into account some considerations before doing something really bad.

Here is the system with the difference amplifier and "without feedback":

In simulation It works perfectly but it miss a lot of parasitic elements and I do not know how "opening" the feedback will affect the stability.

And as you can see I am currently using resistor of 1 kΩ and 1.21 MΩ for having a gain of 1000 on the difference amplifier and I think it is pretty low and pretty high as resistor of an op amp for setting the gain. This is also why I would like to open the feedback.

Thank you.

Answer 1

Maximum but undefined open-loop gain

At first glance, the idea of ​​negative feedback is absurd because you take a very good amplifier with (almost) infinite gain and make it "bad" with gain on the order of tens and hundreds of times... and sometimes (in the case of a voltage follower) even a unit. Maybe that is what the patent specialists thought when they refused to issue a patent to Harold Black for something like a "perpetual engine".

Now everyone knows the benefit of this "worsening" - from a high but undefined gain we get a relatively low but precisely determined gain. We achieve this by forcing the "undefined" amplifier to act as an "inverted voltage divider" made of two precision resistors.

Therefore, as a rule, in negative feedback circuits, amplifiers operate at their maximum ("open loop") gain that the manufacturer was able to achieve.

Moderate but fixed open-loop gain

In some cases (really rare), we intentionally use "finite (fixed) gain amplifiers". Fortunately, I had to answer such a question yesterday.

Generalization

From this question and the discussion surrounding it, we can derive perhaps the most important principle in analog electronics known as "negative feedback" (I will use a slightly more meaningful and figurative name here):

"Active copying": Make an op-amp with high but undefined gain equalize (a part of) its output voltage to the input voltage to obtain an amplifier with lower but determined gain.

See also

Here are some of my materials revealing in depth the philosophy behind negative feedback circuits.

Can we "reverse" a voltage divider by applying the input voltage to its output and taking the output voltage from its input?

Can we reverse (swap the input and output of) any unidirectional device by applying a negative feedback? If yes, what will be the use of this trick?

What are actually the input and output of a negative feedback system? Can we consider the input quantity as a disturbance and the output quantity - as a reaction?

Can we think of a non-inverting amplifier as of a disturbed voltage follower? Can we generalize this idea for any system with negative feedback?

Answer 2

In simulation It works perfectly but it miss a lot of parasitic elements and I do not know how "opening" the feedback will affect the stability.

So you should add in the relevant parasitic elements to your model, and do a stability analysis.

Crudely you could just do an AC analysis with a perturbation source and check if there are resonances.

For a more sophisticated analysis you can open the loop and find the loop gain characteristic (amplitude and phase) to check the phase margin relative to the Barkhausen criterion. That is, verify that your circuit's loop gain phase remains positive when the amplitude falls to unity.

If you're designing with op-amps, it's common to design for a phase margin of 45 or 60 degrees to be sure of stability as the circuit parameters vary due to temperature, age, part-to-part variations, etc.