The function of capacitive coupling is to pass the AC signal from one stage to the next. There are also direct coupling and transformer coupling methods. The direct coupling has the highest efficiency and the signal is not distorted, but the adjustment of the front and back working points is more complicated and implicated. In order that the operating point of the latter stage is not affected by the former stage, it is necessary to separate the former and the latter stage in terms of direct current.
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At the same time, the AC signal can be smoothly transmitted from the previous level to the next level, and the way to accomplish this task is to use capacitor transmission or transformer transmission to achieve. They can transmit AC signals and cut off DC, so that the front and rear operating points are not involved. However, the difference is that the phase of the signal is delayed when the capacitor is transmitted, and the high-frequency component of the signal is lost when the transformer is transmitted. In general, when a small signal is transmitted, a capacitor is commonly used as a coupling element, and when a large signal or a strong signal is transmitted, a transformer is commonly used as a coupling element.