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well positive feedback is when a part of the output is given back to the input so that the output increases more. this is done to produce oscillations. and negative feedback is when a part of the output is given back to the inputs so that the output decreases more. this is done to stabilize the gain of an amplifier.
positive feedback is also known as regenerative feedback and negative feedback is also known as degenerative feedback.
a great example of the usefullness of negative feedback is the opamp. before the days of the opamp, amplifiers were built to have a large gain. but every time the gain got instable because of temperature effects. the idea in the opamp was to produce an amplifier that had lots of gain and with using negative feedback the gain could be brought down. it was seen as the perpetual motoin machine. with producing a large gain and then using negative feedback the amplifier becomes independent of the characteristics of the opamp. it just depends on the external components which are resistors. so now the gain is not dependent on temperature.
Well instead of going in to any applications, in theory positive feed back is when the output is fed back to the input and it adds to the input or called in phase with input. So generally this leads to increasing input to the system as time progresses. With negetive feed back you have the same feed back as before but intead the feedback is out of phase with the input, so it subtracts from the input or reduces the input to the system. Now this feed back can be either voltage(more common) or current or any quantity as such where input is converted to output.
Opamp is one of typical examples of the feedback system. But it can be any sytstem other than electrical also, even biological.
Positive feedback is feedback that is used in ossilators to maintain ossilation, a portion of the output is feed backto the input . Negative feedback is used to improve the performance of an amplifiers the output is feedback but inverted to produce a reduction in gain and therefor reduce distortion.
This is a quick and dirty explination . There are many vereations on these theams and you would be well advised to reserch this as the theory can become long and heavy.
Positive feedback is mean that: we use some part of output signal which have same phase with input signal, as input signal. Positive feedback is usually in oscilator circuit.
Negative feedback is mean that: we use some part of output signal which have opposite phase with input signal, as input signal. Negative feedback is usually in amplifier circuit ... Negative feedback help the transitor circuit independently with temperature.