working of negative feedback

[shaiqbashir]

Hi Guys!

welll! taking an example of an OP-AMP, i want to ask you that it it said that a negative feedback is used to

1)stabilize the voltage gain even if there occurs some change in the input signal voltage.

2) to decrease the gain of the OP-AMP so that it cannot go into the saturation regions.

2) point is clear to me as i know the formula of close loop voltage gain

Vcl=1+ Rf/Ri

.

This function i can understand that a part of output voltage is given to inverting terminal and it gets subtracted from the input and the gain decreases.

but what i didnt get is how a negative feedback works to stabilize the voltage gain by bearing all the unwanted changes in the input signal voltage.

Please explain me this phenomenon. I will appreciate if you would put some detail here.

Thanks a lot in advance

 

[audioguru2]

Hi Shaiq,
Negative feedback decreases the 200,000 gain of an opamp to a useable number. It does not perform an automatic gain control function. If the power supply voltage totals 12V and an opamp's gain is set to 10 by its negative feedback voltage divider resistors, then with a 1V input signal its output signal is 10V or 11V. If you feed it 0.1V then its output is 1.0V or 1.1V. If you feed it about 1.2V or more then its output will be clipped at about 11Vp-p. 

 

[shaiqbashir]

If you feed it about 1.2V or more then its output will be clipped at about 11Vp-p.

Hi Audioguru!

thanks for your help. But im still having some problems in understandin your above quoted line.

what do u think if a change of 1.2V occurs in the input signal., it gets clipped at the output. Now dont u think that negative feedback gets failed here. Or should i say that this will damage our load as well. Where is the stabilized gain then?

is it only for some limits? suddenly a change occurs, and the speaker gets blast. Is it not possible to save it for 1.2V input as well. If yes then how?

Thanks in advance

 

[audioguru2]

Hi Shaiq,
An ordinary amplifier can't have an output more than about 11Vp-p when it has a 12V supply. If it has a gain with negative feedback of 10, then any input level more than 1.1Vp-p will produce a clipped 11Vp-p output. When an amplifier clips, it produces a lot of distortion that can damage a tweeter.
Negative feedback doesn't prevent an amplifier from clipping if it is fed a signal level too high. The gain set with negative feedback doesn't change.

If your input signal frequently exceeds 1.1Vp-p and you don't want to turn down the volume control to prevent the amplifier from clippimg, then you can add an automatic gain control circuit to adjust the volume. At radio and TV stations, an automatic gain contol circuit is called a compressor-limiter.

 

[shaiqbashir]

Thanks a lot audioguru!

As a final conclusion plz check out my following lines whether they are correct or not:

A negative feedback is used to decrease the HIGH GAIN of an Op-AMP to a suitable value. Because the open loop gain of an Op-Amp is so high that even the noise signals can be amplifiied to an extent that can saturate the Op-AMP. It is not possible with a negative feedback to work like autmatic gain control. The output can be clipped off if there would be a drastic change in the input signal beyond the limits provided by the supply and the safety of negative feedback.

Thanks once again!

 

[audioguru2]

Negative feedback is used to decrease the HIGH GAIN of an Op-AMP to a suitable value.
Because the open loop gain of an Op-Amp is so high that even the noise signals can be amplifiied to an extent that can saturate the output of an Op-AMP.
Negative feedback provides the benefits of reduced distortion, increased bandwidth and reduced output impedance.
Negative feedback doesn't work like automatic gain control. The output can be clipped off if the input signal level is too high for the gain and power supply limits of the circuit.

I would write it like that.