The difference between positive and negative feedback

[vijay_nag]

Opamp

HI

WHAT IS DIFFERENCE BETWEEN POSITIVE FEEDBACK AND NEGATIVE FEEDBACK AND WHY WE NEGATIVE FEEDBACK IN OPAMP

 

[vicky]

Re: Opamp

HI
HAVE A NICE DAY AND BE HAPPY

NEGATIVE FEED BACK
Connecting the output of an op-amp to its inverting (-) input is called negative feedback. This term can be broadly applied to any dynamic system where the output signal is "fed back" to the input somehow so as to reach a point of equilibrium (balance).
When the output of an op-amp is directly connected to its inverting (-) input, a voltage follower will be created. Whatever signal voltage is impressed upon the noninverting (+) input will be seen on the output.
An op-amp with negative feedback will try to drive its output voltage to whatever level necessary so that the differential voltage between the two inputs is practically zero. The higher the op-amp differential gain, the closer that differential voltage will be to zero.
Some op-amps cannot produce an output voltage equal to their supply voltage when saturated. The model 741 is one of these. The upper and lower limits of an op-amp's output voltage swing are known as positive saturation voltage and negative saturation voltage, respectively.

POSITIVE FEED BACK

Another type of feedback, namely positive feedback, also finds application in op-amp circuits. Unlike negative feedback, where the output voltage is "fed back" to the inverting (-) input, with positive feedback the output voltage is somehow routed back to the noninverting (+) input.The inverting input remains disconnected from the feedback loop, and is free to receive an external voltage.

With the inverting input grounded (maintained at zero volts), the output voltage will be dictated by the magnitude and polarity of the voltage at the noninverting input. If that voltage happens to be positive, the op-amp will drive its output positive as well, feeding that positive voltage back to the noninverting input, which will result in full positive output saturation. On the other hand, if the voltage on the noninverting input happens to start out negative, the op-amp's output will drive in the negative direction, feeding back to the noninverting input and resulting in full negative saturation.

What we have here is a circuit whose output is bistable: stable in one of two states (saturated positive or saturated negative). Once it has reached one of those saturated states, it will tend to remain in that state, unchanging. What is necessary to get it to switch states is a voltage placed upon the inverting (-) input of the same polarity, but of a slightly greater magnitude. For example, if our circuit is saturated at an output voltage of +12 volts, it will take an input voltage at the inverting input of at least +12 volts to get the output to change. When it changes, it will saturate fully negative.

So, an op-amp with positive feedback tends to stay in whatever output state it's already in. It "latches" between one of two states, saturated positive or saturated negative. Technically, this is known as hysteresis

 

[sina25]

Re: Opamp

vicky said:

Connecting the output of an op-amp to its inverting (-) input is called negative feedback. This term can be broadly applied to any dynamic system where the output signal is "fed back" to the input somehow so as to reach a point of equilibrium (balance).


Does the point of equilibrium set an output voltage and a differential input voltage according to the opamp's gain?

Does that point of equilibrium depend from the amplitude of the differential input voltage?

How could i obtain that point of equilibrium as a function of the amplitude of the diferential input voltage and the opamp's gain?

thanks

[/quote]

 

[mm_naghsh]

Re: Opamp

HI

WHAT IS DIFFERENCE BETWEEN POSITIVE FEEDBACK AND NEGATIVE FEEDBACK AND WHY WE NEGATIVE FEEDBACK IN OPAMP

negative feedback cause to unstability and use for oscillator biuld.(-)
positive feedback is more usual and useing for amplifire.(+)
you can see lm741 datasheet and astable circiut .

 

[electronics_kumar]

Re: Opamp

HI

WHAT IS DIFFERENCE BETWEEN POSITIVE FEEDBACK AND NEGATIVE FEEDBACK AND WHY WE NEGATIVE FEEDBACK IN OPAMP

negative feedback cause to unstability and use for oscillator biuld.(-)
positive feedback is more usual and useing for amplifire.(+)
you can see lm741 datasheet and astable circiut .

complement of passage is perfect definition for oscilltor and amplifier

 

[Miguel Gaspar]

Re: Opamp

Negative Feedback in op amps result in

Vo/Vi (s)= A/(1+Aβ)

but in the case A, named op-amp gain is very big the term Aβ much bigger than 1 an aproximation occurs

Vo/Vi(s)= A/(1+Aβ) ≈A/(Aβ) = 1/β
where β is the loop gain and depends fo the loop components like resistors, capacitors, indutors, etc. But it doesn't deppends ont the characteristics of the loop.
And we can obtain circuit that we can adapt by changing components. The bigger the gain A of the opamp, the better the approximation. We can change the opamp with theonly condition that Aβ must be bigger than 1 and the circuits work the same way.
This kind of applications includes such as analog computers, perform operations, instrumentation, music, filters, ADC and DAC etc.

Positive feedback are user for oscillator and Histeresis circuits (circuits with memory) like hysteresis comparators (or smitt triggers).

I hope this can help you

 

[ee484]

Re: Opamp

Positive feedback adds output energy with input signal. --> Oscialltion
Negative feedback subtract output feedbacked signal from the input --> input and the feedbacked signal are almost identical so that the output of the negative feedback can have a desired value (smaller)

Other negative feedback advantages:
1) Gain insenstive - open-loop gain is not well controlled, but feedback is.
2) BW extension - by trading with gain, BW is now much higher
3) Output and input terminal impedance modification - change is always good direction..
4) Low distortion

down side is....stability might cause a problem.

 

[Andrew8611]

Re: Opamp

The only diference is the pin that conects the entrance of the opamp with the output, if the pin is the negative one, it's negative feedback and if it's the positive one, it's positive feedback.

Added after 4 minutes:

you need negative feedback on opamps because it has an important efect on the bandwidth, and on the stability, and on the output and input resistances, depending on the feedback network you can improve all these cahracteristics of the opamp, but it has a terrible effect on the gain because it diminishes according to the feedback network

 

[10kangstroms]

Re: Opamp

...Vo/Vi(s)= A/(1+Aβ) ≈A/(Aβ) = 1/β
where β is the loop gain...

Actually, Aβ is the loop gain. β is the feedback ratio. Not a big deal, but for someone trying to understand op amps, correct terminology is important.

 

[tjr]

Re: Opamp

positive feedback for oscillations and negative feedback for stability

 

[m.mokhtar]

Re: Opamp

Hi
positive feed back simply means asmall change in input will be amplified in the output and fed back to the input and so on which will lead to saturation even transients and noise can cause saturation that is why it is not stable, and vice versa for negative feed back.\][/url][/list][/list][/code][/quote][/i][/b]

Added after 2 minutes:

I m sorry for the lettersat the end of my reply it is accidental mistake.

 

[safwatonline]

Re: Opamp

positive feedback for oscillations and negative feedback for stability

hello,
i want to know what do u mean by stability, i dont get how using a negative feedback increase stability.
regards,
a.safwat

 

[raybo]

Re: Opamp

amps have a definite A=GAIN of open loop no feddback at all. if the A is large enough they actualy saturate to either power supply rail. by providing some feedback {neg} it reduces this gain to a stable gain. positive feedback is just the opposite of course it will saturate the amp as described above. the thing is if phase is introduced in the F/B then oscillation will occur and the freq wil be dependended on external components