Control of Opamp Stabiility with feedback factor

Hello ,
I have some doubt regarding, effect of loop gain control over stability of Opamp.
Normally if we take a correctly designed single ended opamp, it has a one dominant pole, and nondominant pole after unity gan frequeny and a right half zero at twice that non dominant pole.
we ll take two cases:
1)One with worst case feedback factor ie 1. That is our opamp is unity gan buffer. Now our loop transfer function or loop-gain ie G(s) should not have Dc gain very much. That is if G(s)= K(1-bS)/(1+aS)(1+bS), we should not keep on increasing our K which dc gain of our open loop Opamp.
Because if we follow according to root locus diagram, for K=infinite or very high value, our pole has moved to right half plane. That is our sytsem is unstable. So our Dc gain must be in limits.
Is my argument right?
2)Anothe case with feed bach factor of 1/2. Our openloop transfer function is
G(s)= K(1-bS)/2(1+aS)(1+bS) . This must be more stable than the previous case.
Is that right?


Please answer me these question. Please suggest about control of loop gain over stabily.

If your gain K is constant over the whole frequency spectrum, your argument is true. But you can increase the DC gain without make the system unstable. Frequency dependent gain, for example a Lag compensator which is used to increase the low-freq. loop gain, such that the output well regulated at dc, can solve your problem.

Hi ,

In this case u need not worry about the loop gain as the second pole occurs after unity gain freq. First and foremost is that system with two poles (one zero also-so need not worry at all) wont oscillate as the system can oscillate only on theoritical ∞ freq.

Minimum of three poles shd be there to cause a stability concern and thereafter cmpensation .

always remember phase of loopgain shd be 180/-180 in -ve feedback systems and mag. should be greater than or equal to ONE.

Bye