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

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How to distinguish between voltage series negative feedback circuit and current series negative feedback circuit [Copy link]

The negative feedback amplifier circuit can be divided into voltage feedback and current feedback from the sampling method of the output end, and can be divided into series feedback and parallel feedback from the way of connecting the circuit from the input end. The simplest way to distinguish is: if the feedback sampling point at the output end is at the same point as the output, it is voltage feedback, if it is not at the same point, it is current feedback; at the input end, if the feedback signal and the input signal are connected to the same input end, it participates in the calculation in the form of current, which is current negative feedback. If the feedback signal and the input signal are connected to different terminals of the amplifier circuit, then it participates in the calculation in the form of voltage, which is voltage negative feedback.

Short-circuit the load, that is, short-circuit RL. If the feedback signal still exists, it is current negative feedback; if the feedback signal is 0, it is voltage negative feedback.

In the negative feedback circuit of the operational amplifier, if the feedback is led back to the other end of the input, it is series feedback as shown in Figure 4, in which uD and uF are connected in series; if it is led back to the other end of the input, it is parallel feedback as shown in Figure 5, in which iD and iF are connected in parallel.

(2) Determination of voltage and current

Voltage and current feedback refers to the feedback signal being taken from the output signal (voltage or current). Taking Figure 4 as an example of voltage feedback, the feedback voltage uF is sampled from the output voltage uO through a voltage divider composed of R1 and R2. The feedback voltage is a part of the output voltage, so it is voltage feedback. When judging voltage feedback, a simple method can be used, that is, according to the definition of voltage feedback - the feedback signal is proportional to the output voltage. Imagine that the load RL of the amplifier circuit is short-circuited. If uF=0 (or IF=0) after the short circuit, it is voltage feedback.

Taking Figure 5 as an example of current feedback, the feedback current iF in the figure is the shunt of the output current iO by resistors R1 and R2, so it is current feedback. Another simple method is to open the load RL (RL=∞), causing iO=0, and thus iF=0, that is, the feedback signal caused by the output disappears, thus determining it as current feedback.

Configuration Analysis of Negative Feedback Circuit of Operational Amplifier

The following are four ways to use negative feedback circuits in operational amplifiers:

1. Parallel voltage negative feedback

Figure 1(a) is an inverting proportional operational circuit. From the feedback type, the feedback circuit is connected from the output terminal to the inverting input terminal. If the input voltage μi is positive, the output voltage μo is negative. At this time, the potential of the inverting input terminal is higher than the potential of the output terminal. The actual direction of the input current and the feedback current is as shown in Figure 1(a). The difference current weakens the net input current (difference current), so it is negative feedback.

The feedback current is taken from the output voltage (i.e., the load voltage) and is proportional to it, so it is voltage feedback. The feedback signal is compared with the input signal in the form of current at the input end, and the two are connected in parallel, so it is parallel feedback. Therefore, the inverting proportional operation circuit is a circuit that introduces parallel voltage negative feedback. From the previous discussion, it can be seen that the role of voltage negative feedback is to stabilize the output voltage, and the parallel feedback circuit reduces the input resistance. The feedback coefficient F is derived from the definition formula: where XF is the feedback current, so the feedback coefficient. It can be seen that the feedback coefficient has the dimension of conductance (the inverse of resistance), which is called the mutual conductance feedback coefficient.

2. Series voltage negative feedback

1(b) is a common-mode proportional operational circuit. From the feedback type, the feedback circuit is connected from the output end to the inverting input end, and then connected to the "ground" through the resistor RL. If is set to positive, then it is also positive. At this time, the potential of the inverting input end is lower than the potential of the output end, but higher than the "ground" potential, and the actual direction of and is opposite to the reference direction in the circuit. After voltage division by RF and R1, the feedback voltage = —R1, which is a part of. From the input end circuit, it can be obtained that the difference voltage, that is, the net input voltage (difference voltage) is weakened, so it is negative feedback. The feedback voltage is taken from the output voltage and is proportional to it, so it is voltage feedback. The feedback signal and the input signal are compared in the form of voltage at the input end. The two are connected in series, so it is series feedback. Therefore, the common-mode proportional operational circuit is a circuit that introduces series voltage negative feedback.

The feedback coefficient F is defined by the formula that the function of voltage negative feedback is to stabilize the output voltage, and the series feedback circuit has a very high input resistance.

3. Series current negative feedback

First, let's analyze the function of the circuit shown in Figure 1 (C). From the circuit structure, it is a proportional operation circuit, so the output current is obtained from the above two equations. It can be seen that the output current has nothing to do with the load RL. Therefore, Figure 1 (C) is a constant current source circuit with a common input, or called a voltage-current conversion circuit. By changing the resistance value of resistor R, the size can be changed.

Next, we analyze the feedback type. Referring to the above-mentioned in-phase proportional operational circuit, it can be seen that the circuit in Figure 1(c) also introduces negative feedback. The feedback voltage is taken from the output current (i.e., the load current) and is proportional to it, so it is current feedback. The feedback signal is compared with the input signal in the form of voltage at the input end (), and the two are connected in series, so it is series feedback. Therefore, the in-phase input constant current source circuit is a circuit that introduces series current negative feedback.

It can be seen that the feedback coefficient F has the dimension of resistance and is called the mutual resistance feedback coefficient.

4. Parallel current negative feedback

First, analyze the function of the circuit shown in Figure 1(d). It can be concluded from the figure that, if, then the output current is obtained. It can be seen that the output current is independent of the load RL, because Figure 1(d) is an inverting input constant current source circuit. By changing the resistance value of resistor RF or R, the size of can be changed. Secondly, analyze the feedback type. Set to positive, that is, the potential of the inverting input terminal is positive, and the potential of the output terminal is negative. At this time, the actual direction of and is as shown in the figure, the difference current, that is, the net input current is weakened, so it is negative feedback. The feedback current is taken from the output current and is proportional to it, so it is current feedback. The feedback signal is compared with the input signal in the form of current at the input end (), and the two are connected in parallel, so it is parallel feedback. Therefore, the inverting input constant current source circuit is a circuit that introduces parallel current negative feedback.

Feedback Factor

In summary, from the four operational amplifier circuits above, we can see that:

(1) The feedback circuit is directly derived from the output terminal, which is voltage feedback; the feedback circuit is derived from the load resistor close to the ground terminal, which is current feedback;

(2) When the input signal and feedback signal are added to two input terminals (in phase and out phase) respectively, it is series feedback; when they are added to the same input terminal (in phase or out phase), it is parallel feedback.

(3) When the feedback signal reduces the net input signal, it is negative feedback.

As for the influence of negative feedback on the working performance of the amplifier circuit, such as reducing the amplification factor, improving the stability of the amplification factor, improving waveform distortion, widening the passband, and the influence on the input resistance and output resistance of the amplifier circuit, it is the same as described in the discrete component amplifier circuit.

5. Example:

Example 1: Try to determine what type of feedback circuit is connected from the output of operational amplifier A2 to the input of A1 in the two-stage amplifier circuits of Figures 2(a) and (b).

Solution: (1) In Figure 2(a), the circuit from the output of operational amplifier A2 to the non-inverting input of A1 is a series voltage negative feedback: a. The feedback circuit is derived from the output of A2, so it is a voltage feedback; b. The feedback voltage and input voltage are respectively applied to the non-inverting and inverting inputs of A1, so it is a series feedback; c. If is set to be positive, then it is negative, and if is positive. The feedback voltage reduces the net input voltage, so it is a negative feedback. (2) In Figure (b), the circuit from the end of the load resistor RL close to the "ground" to the non-inverting input of A1 is a parallel current negative feedback circuit: ① The feedback circuit is derived from the end of RL close to the "ground", so it is a current feedback; ② The feedback current and the input current are applied to the same input of A1, so it is a parallel feedback; ③ If is set to be positive, then it is negative, and if is positive. The potential of the non-inverting input of A1 is higher than point a. The actual direction of the feedback current is as shown in the figure, which reduces the net input current, so it is a negative feedback.