How to distinguish voltage series negative feedback circuit and current series negative feedback circuit

The sampling mode of negative feedback amplification circuit can be divided into series feedback and parallel feedback by the way of voltage feedback and current feedback from the input terminal. The simplest way to differentiate is : if the output of the feedback sampling point and output at the same point is the voltage feedback , not the same point is the current feedback ; at the input, if the feedback signal and the input signal is connected to the same input, it is in the form of current to participate in the calculation, is the negative current feedback , if the feedback signal and the input signal is connected to the different terminals of the amplification circuit, then is the voltage in the form of participation in the operation, is the negative voltage feedback. 

Short-circuit the load, that is, RL short-circuit, if the feedback signal is still present, is the negative current feedback, if the feedback signal is 0, is the voltage negative feedback.

In the op amp negative feedback circuit, the feedback is returned to the other end of the input is a series of feedback 4, the diagram of uD and uF Series connection; If the back of the input to the other end of the parallel feedback  5, the diagram iD and iF in parallel connection. 


（2) Determination of voltage and current
Voltage-current feedback refers to the form of the feedback signal taken from the output signal (voltage or current). Voltage feedback with Diagram4 As an example, the feedback voltage uF is a voltage divider composed of R1,R2 by the output pressure uO sampling is obtained。The feedback voltage is part of the output voltage, so the voltage feedback。in judging the voltage feedback, it is possible to use a simple method, that is, according to the definition of voltage feedback-the feedback signal and the output voltage ratio, it is envisaged to amplify the load of the circuit RL Short Circuit, after short circuit, such as the uF=0 (or IF=0), is the voltage feedback. 



current Feedback in Figure 5 For example , the feedback current iF is the Resistance R1 and R2 to the output current I O    the shunt, so is the current feedback. Another easy way is to load R L Open Circuit (Rl=∞), resulting in iO= 0, so that iF  = 0, that is, the feedback signal caused by the output disappears, thereby determining the current feedback. 

configuration analysis of negative feedback circuit of op amp
The following four ways to observe the OP amp negative feedback circuit:
1, shunt voltage negative feedback
Figure1 (a) is an inverse proportional operation circuit.from the feedback type, the feedback circuit is derived from the output end and is connected to the inverting input. Set input voltageIf the μi is positive, the output voltage μo is negative. At this point the potential of the inverting input is higher than the potential at the output terminal. The actual direction of the input current and the feedback current is shown in 1 (a). The differential current weakens the net input current (differential current ) and is therefore negative feedback. 
The feedback current is taken from the output voltage (i.e. the load voltage ),   and it is proportional to the voltage feedback. The feedback signal is compared with the input signal in the form of current in the transmission terminal.,The two parallel, so the parallel feedback。so,Inverse-phase proportional operation Circuit is a circuit that introduces negative feedback of shunt voltage。 From the previous discussion,the effect of voltage negative feedback is to stabilize the output voltage, and the shunt feedback circuit can reduce the input resistance. Feedback coefficientF is derived from the definition:The XF is the feedback current, so the feedback factor. It can be seen that the feedback coefficient has the dimension of conductance (the reciprocal of resistance), called the reciprocal feedback coefficient.


2, Series voltage negative feedback
from 1 (b) is the same-phase proportional computing circuit。 From the feedback type, the feedback circuit is derived from the output terminal to the reverse phase of the transmission of the end, after the surface through the resistorRL Connect"Ground”。 is set to positive, it is also positive. At this point the potential of the inverting input is lower than the potential at the output end, but higher than"Ground"Potential, and the actual direction of the circuit in the opposite direction of reference." ByRF andR1 after the split pressure. Feedback voltage=-R1 It is part of it. The difference voltage, which weakens the net input voltage, can be obtained from the transmission terminal circuit.(Differential voltage), it is negative feedback.feedback voltage derived from output voltage , and it is proportional to the voltage feedback. The feedback signal is compared with the input signal in the form of a voltage at the input terminal. The two are in series, so the feedback is in series. Therefore, the parallel-phase proportional operation circuit is a circuit which introduces the negative feedback of series voltage.
feedback coefficient f by the definition of the voltage negative feedback function is to stabilize the output voltage, series feedback circuit has a high input resistance. 



3, series current negative feedback
First analysis diagramThe function of the circuit shown in 1 (C). From the circuit structure to see it is the same proportional operation circuit, so the output current from the above two-type derived
visible output current vs. load RL irrelevant，so the figure1 (C) is the input of constant%@5? source circuit, or voltage -  Current conversion Circuit。Change Resistancer resistance, you can change the size. 
Second, analyze the feedback type. Referring to the same-phase proportional operation Circuit, the figure1 (c) The circuit also introduces negative feedback.Feedback Voltage it is derived from the output current (i.e. load current) and is proportional to the current feedback. The feedback signal is compared with the input signal at the input voltage ( ), both in series, so the series feedback。 Therefore, the same-phase input constant current source circuit is a circuit that introduces negative feedback of series currents.
It can be seen that the feedback coefficient F has the dimension of resistance, called the mutual resistance feedback coefficient. 

4, shunt current negative feedback
First analysis diagram1 (D) The function of the circuit shown. It can be concluded from the graph that
Output current is set.
Visible output current and loadRL is irrelevant, because the diagram1 (D) is an inverting input constant current source circuit. Change resistanceRF orR resistance, you can change the size.
Second, analyze the feedback type. is set to positive, that is, the potential of the inverting input is positive, the output terminal is negative. At this point, and the actual direction is shown in, the difference current, that is, weaken the net input current, it is negative feedback. The feedback current is derived from the output current and is proportional to the current feedback. The feedback signal and the input signal are compared in the form of current in the input terminal (), the two parallel, therefore is the parallel feedback, so, the inverting input constant current source circuit is the circuit which introduces the negative feedback of the shunt currents.
Feedback coefficient

In summary, from the four op amp circuits above, it can be seen that:
(1) The feedback circuit directly from the output, is the voltage feedback, from the load resistance near the ground end. is the current feedback;
(2) input signal and feedback signal are added to two input terminals respectively(Same-phase and reversed-phase) is a tandem feedback, and a parallel feedback is added to the same input (in the same or inverting phase);
(3) The feedback signal reduces the net input signal, is negative feedback.
As for the effect of negative feedback on the performance of amplifying circuit, such as reducing magnification, increasing the stability of magnification, improving waveform distortion, broadening the band and the effect on input resistance and output resistance of amplifying circuit, as described in discrete component amplification circuit.



5, Example:
Cases1: Trial discriminant diagram2 (A) and (b) from op amps in two two-stage amplification circuitsA2 output End toThe A1 input is the type of feedback circuit.
Solution:（1) in the figure2 (A) from the op ampA2 output End toThe A1 phase input is a series voltage negative feedback:
A. Feedback circuitry fromA2 output, it is voltage feedback;
B. Feedback voltage and input voltage are respectively added to theA1 two input terminals of the same phase and reversed phase, so it is a series feedback;
C. Set to positive, then negative, positive. Feedback voltage%@d? The net input voltage is reduced, so it is negative feedback.
（2) in figure (B),   A shunt current negative feedback circuit is introduced from the near "ground" end of the load resistor RL to the A1 co-phase input:
① feedback circuit from the RL near the "ground " end, it is the current feedback;
② feedback current and input current are added to the same input end of A1, so the feedback is parallel.
③ is set to positive, then negative, positive. the potential of the A1 at the same phase input is higher than the a point, and the actual direction of the feedback current is shown in the figure, which reduces the net input current, hence the negative feedback.

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How to differentiate the voltage series negative feedback circuit and the current series negative feedback circuit (RPM)