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..
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