The first one is to install a resistor under the speaker to do current sampling. In fact, the feedback is still the voltage signal. It is the analog current feedback. It is the most people, but this circuit is defective. There are two reasons. 1 is that his output gain will change with the change of impedance (the speaker impedance rises and the output amplitude increases). . . As a result, it is not constant voltage applied to the end of the horn 2, and (refers to the equal-amplitude input) as if the power applied to the horn is constant. . But don't forget, the sound pressure characteristic curve of the speaker is TEST under the constant voltage output, so the simple circuit is not good, the listening is not good, it is fun, but there is an improved circuit with a negative voltage. The feedback is mainly based on adding this kind of current negative feedback, so that a good sound can be made, but the effect of current negative feedback is to change the damping coefficient of the power amplifier, which has little effect on the amplitude-frequency characteristics. A detailed introduction to this circuit is given in the book "High-Fidelity Power Amplifier" by Reader. 2 is the sampling point below the speaker, huh, horn is an inductor, the current flowing through the inductor can change its phase, the low frequency is good, the high frequency can be phase shifted by 90 degrees. . Oh, the phase characteristics are extremely poor. . .
The second type: negative resistance amplifier, except for some special occasions, the first one to be used for acoustics and succeeded is YAMAHA, whose main function is to improve the low frequency extension, but for 200Hz or above. The frequency will degrade the sound quality, so it is generally used on ultra-low frequency active speakers. . In fact, this circuit is used in conjunction with the speaker, and there is no practical use of it alone. The working principle is: If the speaker is a rigid body, then add a tube, it can become an ideal Holmo resonance box, then, regardless of the size of the box, the thickness of the tube, as long as it meets Holmo滋 resonance calculation formula. . . . Even if the 20Hz resonance point can be done, the size of the box is only the efficiency. Because there is a speaker on the speaker, the speaker is moving when the sound is sounding, the speaker is not a rigid body, then the box is There will be no resonance with Holmó. . Therefore, if the horn of the horn is stationary when it is sounding. . Then, the box is close to the rigid body, and it can satisfy the conditions of Holmó resonance, and the resonance point of the box can be arbitrarily designed. When the sound is made, the work of letting the speaker not move is the task of negative resistance power amplifier. The working principle of the negative-resistance power amplifier is that when the horn is working in the low frequency band, its impedance characteristic changes abruptly. The amplifying circuit takes out the change and feeds it back to the power amplifier through current sampling, so that the power amplifier controls the horn in the form of current. The amplifier circuit performs an equivalent analysis, and it can be found that the internal resistance of the power amplifier is negatively calculated in calculation. . During dynamic amplification, the internal resistance of the horn plus amplifier is close to zero. . . As a result, this circuit allows the horn to be strongly damped no matter which direction it is facing. . As soon as the sound is over, the horn will not move, and the box will become rigid. . The so-called negative resistance, to put it plainly, is positive feedback. . Oh, know why it is only used in ultra low frequency applications. . . A detailed introduction was made in the "Radio and TV" in 1995. . Can go and see
The third type: current mode feedback amplifying circuit, this is a practical current amplifying circuit, which is also a true current type negative feedback. The feedback signal is current, not voltage, that is, it is not added at the negative feedback end, but is added. There is a current flowing in. . . This circuit was first used in video transmission, or in the use of instrumentation like an oscilloscope. . Because of the low-resistance and negative-current feedback input, the high-frequency characteristics of this circuit are excellent, and the capacitive load has a strong driving capability. As long as it has been improved, it is found to be a good power amplifier, which can make up for some congenital deficiency of the voltage amplifier. Low open-loop frequency response, closed-loop transient frequency response distortion, and extremely weak capacitive load drive capability. The disadvantage is that the open-loop gain of this circuit is relatively low, and the distortion after the closed loop is an order of magnitude higher than that of the voltage amplifier. However, the total distortion will not be over 0.01%, huh, huh, know, the Keller amplifier is the current mode, because the color of this circuit is relatively new, introduce a lot of articles, go find it yourself
Right, there is another kind, called current dumping power amplifier, whose working principle is that there are 2 amplifiers, 1 class A and 1 class B in the power amplifier.
The current is input to the output of the class B amplifier by the class A amplifier to correct various distortions of the amplifier. At the same time, the LC compensation bridge is connected to the negative feedback loop. Through precise adjustment, the total distortion can be less than 0.0001%, exaggerated. Going, it has been popular for a while, but the debugging is too complicated, the requirements of the required instruments are too high (you can test the distortion of 0.0001% below the distortion of the instrument is more than 1 million yuan, huh, huh), good voice It's easy to make it, and there are fewer people playing now. . . This is also one, because the distortion is the current. . . China United has done several models and now seems to have stopped production.
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