Necessity of DC feedback in op-amp

[mixedsignal]

Please explain the necessity of dc feedback in op-amp.

 

[srizbf]

Dear mixedsignal,

DC feedback provides the bias point voltage at the input(eg.noninv).
from the o/p to the i/p.

if the bias points at both the inputs are not equal ,
u get the output going to +V or -V due to very high gain of the opamp.
srizbf
23rdapril2010

 

[LvW]

DC feedback provides the bias point voltage at the input(eg.noninv).
from the o/p to the i/p.
......

That's correct - however, the answer is for my opinion not precise enough, because the designed bias point is zero (which could be realized by grounding the input).
But now comes the offset voltage into the game.
The input offset voltage is multiplied by the amplifier gain. Without feedback this gain is app. 90..110 dB - and the output stage will go into saturation.
Therefore, the dc gain has to be reduced considerably using negative feedback.
Example: Offset 1mV, gain with feedback: 100 (40 dB), dc output: 0.1 volts (acceptable).
Summary: The main task is to maintain the dc output voltage app. in the middle of the voltage range determined by the supply voltages. Thus, signal voltage excursions are possible with both signs.