Conceptual understanding of what is a feedback circuit, what is a negative feedback, what is a positive feedback, how to judge the positive and negative feedback?

A, feedback amplifier circuit

Basic amplifying circuit, the active components (transistors, etc.) with a one-way signal transmission, the amplified signal from the input end of the input amplifier circuit output, after the input signal to control the output signal of the one-way; If some pathways in the circuit, the output signal is part of the feedback to the amplifier's input, and the external input signal superposition, produce the basic amplifying circuit net input signal, realize the control of the output signal of the input, namely constitutes the feedback,

Here there are a few points:
1, feedback signal and the external input signal, no indication is addition, subtraction, we separate the negative feedback, positive feedback,
2, regardless of feedback polarity, amplifying circuit,
3, the so-called "zoom in" on the face of it, seems to be the signal amplitude changes from small to big, but in "zoom in" electronic technology is so understanding:
A, the essence of the "zoom in" is to realize the control of the energy, use less energy input signal to control output signal energy is larger,
B, "enlarge" action is aimed at variation, variation is the object of
Note:
Amplification circuit without too much emphasis on the input signal and output signal of symbols,

Second, feedback polarity

Positive, negative feedback is for beginners have quite a variety:
1, feedback can be divided into positive feedback and negative feedback, the former makes the input and output have the opposite effect, reduce the error between the system output and the system target, system stability; The latter make the output play a role similar to the input, increasing the system deviation, make the system oscillation, control effect could be amplified,
2, if the feedback signal to make "net input signal", for the negative feedback. If net feedback signal to input signal enhancement, the positive feedback and negative feedback is used to improve the function of the amplifier stage; Positive feedback for oscillation circuit,
3, if increase the incoming signal feedback signal for the positive feedback, otherwise for the negative feedback,
4, feedback is the amplifier output signal (voltage or current), part or all of the feedback to the amplifier input compared with the input signal (addition or subtraction), and compare the effective input signal to control the output of this amplifier is a feedback process. All feedback to the feedback signal at the input amplifier up to strengthen the input of the original input signal and increase the input signal is called positive feedback. The conversely.
5, if the feedback signal by adding injection after the input signal strengthens the original signal, the amplifier amplification caused increase, is a positive feedback, otherwise it is negative feedback,
6, feedback is positive, negative, if feedback are introduced to reduce the net input signal Xid, namely the Xid is smaller than the Xi, is known as the negative feedback, this time the closed-loop magnification is less than the open loop magnification, so negative feedback reduce amplifier circuit gain, is introduced into feedback if increased net input signal Xid, namely the Xid is larger than the Xi, is known as positive feedback, positive feedback amplifier circuit gain increase,
Negative feedback to input and output have the opposite effect, make the system output and error of system target is reduced, the system is stable; Positive feedback output play a role similar to the input, the system bias increases continuously, make the system oscillation, control effect could be amplified,
7, negative feedback is the output signal of the amplifier, in turn, and then send back to the input, negative feedback of the "negative" is the meaning of the turn signal, the signal, in turn, is the input and output signal subtraction, get is the difference between the two, and then through the amplifier amplification of the difference to the output side, because it is a reverse phase signal, so will be difference between output and input offset, reduces the distortion of the amplifier, theoretical electrical signal transmission speed is very fast, so the offset can be instantaneous, difficult to detect, this is the principle of negative feedback can reduce the distortion,

For the above seven, is a meaning, the abstract expression of the concept of positive and negative feedback, 6 is relatively easier to understand,

Three, negative feedback become positive feedback
Negative feedback principle is only in theory, is established on the basis of device half an idealized, which considers only the amplifier gain will decline with frequency, without considering the amplifier output signal and the input signal than there is a delay, namely the instantaneous offset function is actually does not exist, the problem about this delay to speak too much content, later have the opportunity to discuss it later, just know that here,
As you can imagine, when the frequency of the input signal is high to a value, after a delay of the output signal is in turn back to the amplifier's input, and the input signal is in phase, that is, the delay amplifier offset by reverse phase signal, makes the negative feedback into a positive feedback! So what positive feedback can cause? , of course, is to strengthen the input signal, the amplifier amplification and when is greater than 1, this enhancement will strengthen repeatedly, until it reaches an amplifier output limit, this is the so-called "self excited" phenomenon, self-excited phenomenon sometimes be disastrous, not only cause the amplifier doesn't work, and may cause the damage of the device in the amplifier or burning out behind the load, so in the design of the amplifier, must be calculated according to the working performance of amplifier amplifier working frequency range, through a variety of measures to prevent the amplifier self-excited phenomenon,

Note:
By changing the input frequency circuits has become positive feedback,

Four, positive phase, inverting circuit
Electronic circuit of the operational amplifier, in-phase input and inverting input, the polarity at the input and output is of the same polarity is in-phase amplifier, while the polarity at the input and output of opposite polarity is called inverting amplifier,