what is the difference between positive feedback and negative feedback?i have attached the image below.

Hello Vishwas,

The "All About Circuits" web site has some good, fundamental information about feedback in op-amp circuits. Both negative and positive feedback are discussed. It might be a good starting place:

http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

http://www.allaboutcircuits.com/vol_3/chpt_8/4.html

Regards, Thomas

PA - Linear Applications Engineering

hello sir 

in this both circuit

As we know that vout=A (Vin+ - Vin-)

for negative feedback  what we will get is vout=(Vin+)   [A/(1+A)]

 for positive  feedback  what we will get is vout=(Vin-)   [A/(A-1)]

when A is large both the circuit will be the same ryt sir??

how in the above positive feedback circuit output is going  rail to rail  can u tell me please?

i am unable to figure out.

regards

vishwas

Vishwas,

Instead of trying to understand the concept of the negative vs positive feedback from a mathematical point of view, please read below what physically happens in each case. A negative feedback tends to drive the inputs toward one another, reaching equilibrium only when the voltage at the inputs become the same (virtual short), while the positive feedback tends to drive the inputs away from each other, stopping only when the output (same as non-inverting input) becomes fully saturated and cannot move any further from the voltage potential on the inverting input terminal.  Negative feedback results in predictable outcome (output follows the input) while positive feedback results in bi-stable operation where the output collapses on one of the rails. 

hello marek

while designing op-amp,i have tried  to maintain the minimum  phase margin of 60 deg in open loop configuration as per spec.
how much  should i expect the phase margin of loop gain when configured in unity gain follower for performimg stability analysis?

with regards

vishwas