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operational amplifier feedback

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preethi19

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Hi all i am trying to learn the basics of amplifier. I understood the basic principle is for a small input we get an amplified output. The output gets the amplification with the help of power supply. So my question is why do we need feedback. We just give an input and get an amplified output ( which is what we want). I read feedback helps in controlling the gain which is infinite in an ideal op-amp. In real the gain is really huge. Why do we need a feedback??? And i read it is to control the gain. And if it does why do we need to control the gain. If the gain is high then it is good to have an high amplified output right. Could anyone pls explain it to me. Thank you!!!! :)
 

We use feedback to control the gain because, either the gain can vary significantly due to component variations (such as a simple transistor amplifier) or the gain is too high to be useful (such as an op amp).

For example, an op amp with an open-loop gain of a million would take a signal of only a few microvolts before the output would saturate at it's maximum output voltage.
For most practical amplifier requirements you don't need a gain of more than a few hundred.
Also the op amp usually has in input offset of at least a few hundred microvolts so, without feedback, the output would be saturated even if the inputs were grounded.

Another factor is that an amplifier without feedback may have unacceptable levels of distortion due to the non-linear gain of the open-loop amplifier since negative feedback also reduces distortion.

You should read some articles on negative feedback, such as this.
 

Hi,

Gain is a value that has to be carefully considered.

High gain is by way not always good.

Imagine an audio amplifier. Extremely loud music will be distorted. And your neighbours won't be very excited.

Or with high gain you TV has no no colors, only black or white, no grey. Not nice.

Klaus
 

If the gain is high then it is good to have an high amplified output right.
You will have high output only if the power supply voltage to the amplifier is high because with high gain then an ordinary input will cause the output to SATURATE at the limits of the power supply voltage. The saturation of the output of an amplifier is called "clipping" where the tops and bottoms of the waveform are clipped off and it is severely distorted.
 

Hi all i am trying to learn the basics of amplifier. I understood the basic principle is for a small input we get an amplified output. The output gets the amplification with the help of power supply. So my question is why do we need feedback. We just give an input and get an amplified output ( which is what we want). I read feedback helps in controlling the gain which is infinite in an ideal op-amp. In real the gain is really huge. Why do we need a feedback??? And i read it is to control the gain. And if it does why do we need to control the gain. If the gain is high then it is good to have an high amplified output right. Could anyone pls explain it to me. Thank you!!!! :)

You can try to make a device with a fixed gain value but in real life there are few good ways to do this in a controlled fashion which doesn't drift between devices, over temperature, over load, over time, over frequency etc.

On the other hand you can make devices quite easily with a very high gain (but not too well controlled) and then use feedback through well controlled components (like resistors) to get a precise and well controlled result.

Note that I think feedback is often presented in a confusing way. It's just a comparison. If the feedback signal (- pin) goes too low (lower than the + pin) the output goes higher (due to the high gain) which, if the circuit/application is properly designed causes the feedback signal (- pin) to increase until it matches the + pin at which point the system is stable.
 

Negative feedback does not make a comparison. Instead it feeds back some of the output to cancel some of the input to reduce the gain and at the same time it cancels and reduces some of the distortion.
Negative feedback also reduces the output impedance of an amplifier.
 

Negative feedback does not make a comparison. Instead it feeds back some of the output to cancel some of the input to reduce the gain and at the same time it cancels and reduces some of the distortion.
Negative feedback also reduces the output impedance of an amplifier.

Sorry but this is exactly what I meant when I said "presented in a confusing way". We don't need words like distortion and impedance to describe a process that anyone can understand in the context of, say, a car's cruise control which compares current speed to requested speed and adjust accordingly (where the current speed is like the - input and the requested speed is the + reference).

There is also a reason why comparators and op-amps are variations of the same thing.
 

Negative feedback trades the open loop (no feedback) gain of the amplifier for:
Reduced distortion
Increased bandwidth
Lower output impedance
More stable gain
More predicable gain (as determined by passive devices)
 

Hi all thank you so much for all the response!!! I get the part why feedback is needed and how it stabilizes the system and how high gain is not useful and is needed to be controlled
but i was reading online and found this
"Let’s say that our op-amp has a differential voltage gain of 200,000. If Vin equals 6 volts, the output voltage will be 5.999970000149999 volts. This creates just enough differential voltage (6 volts - 5.999970000149999 volts = 29.99985 µV) to cause 5.999970000149999 volts to be manifested at the output terminal, and the system holds there in balance. "
So my doubt is suppose say i want 29.99985uV at the output. Why don't i just give 6 and 5.9999 volts at the output so i get the desired output. From the example i can see op-amp is nothing but like a comparator just giving the amplified output from the difference of two inputs (- and +) terminals. so if i know the input voltages and the amplifier gain( that i can build the op-amp to be) So why is feedback required if i can just do this directly? I mean if the op-amp has a gain of 20,000 then how come a small difference gives a large output? So can't i just control the input signals so their difference is amplified by the op-amp gain and then desired output is obtained and thereby controlling the system. And i understand feedback helps preventing saturation of the output. But first of all how will the output saturate if its just taking difference from a fixed steady input and steady gain of an op-amp? also say V+ is fixed reference and V- is varying. but still its like the op-amp is just supposed to take their difference and multiply to get amplified output. And i am guessing this multiplication is with the power supply which it can amplify only according to the gain of the op-amp. suppose we built an op-amp with less gain and so if we amplify now it won't saturate right??? sorry but still little confusing to me!!! Would be really great if someone could help. thank you!!!!
 
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I theory you could do that but in practice it would be extremely difficult to maintain such a small voltage difference.
Also that only works for DC. For most op amps the internal compensation causes the gain to start to roll off at a very low frequency.
So it makes little sense to do that in a real circuit.
Give it a shot and you'll see what we mean.
 

The output voltage of an opamp can be any voltage within its output voltage range, determined by the input and negative feedback voltages.
A comparator almost always does not use negative feedback. Its output cannot be any voltage, it is a switch that is near ground or it is floating and an output resistor to the positive supply pulls it up.
 

Preethi19, this is not the way to use an op-amp, not what they are designed to do.

If you want a certain low Vout, or Vref, use a volt reference and divide down, op-amps are used to amplify, sometimes by < 1, e.g. a differential amp, but mostly for useful amplification and level shifting, using the neg feedback creates a very stable amplifier over Vcc range and temp - which is usually very desirable...
 

Suppose you have a signal with 500mV swing and you want amplify it to get 1V swing signal. How would you achieve this precise gain of 2 with opamps with high gain, without employing negative feedback?. This tells the necessity for feedback.

If the gain is slightly more than 2, the output, feeding back negatively, tells the circuit to reduce the gain to go back the desired value. Similarly if the gain is less than 2, the feed back tells the circuit to increase till the gain reaches the desired value.

You want to drive at 60KMPH, you keep looking at the meter once in a while(feedback). If the speed is say 65kmph, you reduce the acceleration to reach back 60kmph. In that process if the speed goes below 60kmph, say 55kmph, you give feedback to yourself to accelerate more to reach the desired speed. This is what feedback does.
 
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