[SOLVED] problem with positive and negative feed back of opamp at a time?

hi!!
I got stuck up with the op amp circuit which has both the positive and negative feed backs? What is the use of using op amp with both feed backs at a time? How to analyze the circuit? Please suggest me the solution with the required formulas.i have attached the circuit related to this..
Thank u..

search in wikipedia or google you ll get

i have tried, but useless.. So i have posted here.

chaitanya2k6er said:

hi!!
I got stuck up with the op amp circuit which has both the positive and negative feed backs? What is the use of using op amp with both feed backs at a time? How to analyze the circuit? Please suggest me the solution with the required formulas.i have attached the circuit related to this..
Thank u..

Click to expand...

1.) For analyzing the circuit you have to realize that there are two feedback pathes - positive and negative:

Hr=Hp+Hn with Hp=R3/(R3+R4) and Hn=-R2/(R1+R2).

Then, according to Black's classical formula the closed loop transfer function is (for ideal opamp)

T=-Hf/Hr with the forward function Hf=R4/(R3+R4)

2.) It is important to know that for stability reasons the denumerator must always be negative (therefore: Hp<|Hn|)
yielding an overall plus sign for T(s) (in agreement with the visual inspection of the circuit).

3.) Advantages: This circuit is of special importance for opamps which are NOT universal compensated.
If you are familiar with BODE diagrams, you can proove that it is possible to make the loop gain as low as necessary to ensure stability - without any restriction to the overall gain value which can be chosen beween 0 db (unity gain) and any other large value.
In contrast, for simple negative feedback circuits the closed-loop gain cannot be chosen independent on the loop gain.

4.) However, the circuit is used also also for universal-compensated opamps because the stability margin is improved, for example with the benefit of a better step response (less ringing) for unity gain applications.

LvW

chaitanya2k6er said:

hi!!
I got stuck up with the op amp circuit which has both the positive and negative feed backs? What is the use of using op amp with both feed backs at a time? How to analyze the circuit? Please suggest me the solution with the required formulas.i have attached the circuit related to this..
Thank u..

Click to expand...

i hope it will help you to analyse

It's a differential amplifier, you can find a lot of results if you google with that name

Differential Amplifier (Voltage Subtractor)
Op-amp Varieties

Alex

alexan_e said:

It's a differential amplifier, you can find a lot of results if you google with that name

Differential Amplifier (Voltage Subtractor)
Op-amp Varieties

Alex

Click to expand...

No, it's not!
A differential amplifier has two inputs and - normally - no positive feedback.
But the question was: why negative AND positive feedback?

LvW said:

No, it's not!
A differential amplifier has two inputs

Click to expand...

I may be wrong but I wouldn't agree on that, i use this king of amplifier (negative feedback only) to amplify the voltage drop over a small resistor (0,02 ohm) connected between the gnd and the load,
it amplifies the difference between gnd and positive input, what stops a differential amplifier from being one when it amplifies a voltage where one end is the gnd.

LvW said:

A differential amplifier has two inputs and - normally - no positive feedback.
But the question was: why negative AND positive feedback?

Click to expand...

You are right that i missed that part about the positive feedback, so this isn't the differential amplifier i thought

As i said, i may be wrong.
Alex