Understand the knowledge of the amplifier circuit (I)

The amplifying circuit, also called an amplifier, is the basic circuit that constitutes other electronic circuits, and is a circuit formed to amplify a weak signal. The amplifying circuit can be divided into low frequency, intermediate frequency and frequency modulation according to the frequency; according to the strength of the output signal, it can also be divided voltage amplification, power amplification and the like. It is the most complex and variable circuit in the circuit, so beginners should understand and master the following knowledge.

(1) The amplifying circuit is an energy converter that cannot create energy. The transistor uses a small change in the base current to control the collector current to change greatly. The tube and the field effect transistor control the screen current with a small change in the gate voltage. Therefore, the field effect transistor and the electron tube It is a voltage control device and the transistor is a current control device. The magnifying circuit does not magnify the viewed text or object directly like a magnifying glass. The amplifying circuit superimposes the alternating current signal on the direct current signal, causes the change of the direct current signal by the change of the alternating current signal, and converts the change of the direct current signal into the change of the alternating current signal through the load resistor. The transistor in the amplifying circuit acts as such a conversion, and a small change in the collector current is controlled by a slight change in the base current, which is equivalent to amplifying the base current.

(2) In the amplifier, there are both a DC component and an AC component. To facilitate the analysis of the circuit, the path through which the DC component passes is often referred to as a DC path, and the path through which the AC signal passes is referred to as an AC path. Since the capacitor has the function of blocking DC communication, when drawing the DC equivalent circuit, the capacitor should be regarded as an open circuit, and the others are unchanged. When analyzing the DC path, it is necessary to return from the positive pole of the power supply to the negative pole of the power supply to form a closed path; when drawing the AC equivalent circuit, the capacitor should be regarded as a short circuit, and the DC power supply will not change due to the voltage across it, no AC The pressure drop is also generated as a short circuit, and the others are unchanged. When analyzing the communication path, it is not necessary to repeat the analysis at each level, but to know where the whole signal is coming from, what components have passed, what changes have occurred, and where it ultimately arrives.