[Photo] Two kinds of current feedback in Hi-Fi circuit

In recent years, the term "current feedback" is often used in articles related to audio technology, but it has two very different meanings, which confuses many readers.
The composition of the first "current feedback" circuit is shown in Figure 1. Its characteristics are: the feedback network of the negative feedback amplifier is connected in series with the load resistor RL, and the output current flows through the sampling resistor R to form a feedback signal, so the feedback signal is taken from the amplifier's The output current is named "current feedback". Current negative feedback can stabilize the current gain of the amplifier, and when the load characteristics change, it can play a role in stabilizing the output current, so the current negative feedback amplifier is also called a constant current output amplifier.
As we all know, moving-coil speakers are inductive devices that exhibit different impedance characteristics “RL (f)” for different frequencies. If a traditional constant-voltage output power amplifier is used to drive the speaker, the actual power obtained by the speaker is: PO = VO2 / RL ( f), where "VO" is the effective value of the output voltage, that is, "PO" is inversely proportional to the load impedance; when driven by a constant current output power amplifier, the actual power obtained is: PO = IO2RL (f), where " "IO" is the effective value of the output current, that is, "PO" is proportional to the load impedance. Obviously, the frequency characteristics of the entire system under these two driving modes are different. In addition, the driving mode of constant current output can also minimize the degradation of sound quality caused by the distortion of the frequency characteristics of the speaker feeder and the output resistance of the amplifier.
Practice has proved that a reasonably designed constant current output power amplifier can obtain better sound quality, and the timbre has the charm of a tube power amplifier.
The second kind of "current feedback" is a concept in the current mode circuit, which actually refers to the circuit structure of a high-speed operational amplifier. The simplified model is shown in Figure 2. Its characteristics are: the voltage at the inverting input terminal of the amplifier u-follows the same phase The input voltage u + changes, and the inverting input is a low impedance node. Because the gain Ar = u. / I- → ∞, so when the limited output voltage u0, i- → 0, then Il + if = 0 (see Figure 2). It can be seen that the working principle of the current feedback amplifier is different from the ordinary (voltage feedback) amplifier. Figure 3 is a simplified schematic diagram of an example circuit. The main advantage of this circuit is that the nodes in the circuit are mostly low-impedance nodes (only the node Z in the circuit of Figure 3 is a high-impedance node), so it has good high-frequency characteristics and transient response. The circuit model of current feedback was first used in the design of high-speed video operational amplifiers, and was later cited in the design of Hi-Fi audio amplifiers. The main purpose of using the current feedback circuit structure is to increase the gain-bandwidth product and slew rate of the amplifier .





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