Understand the knowledge of the amplifier circuit (II)

(3) The amplifying circuit usually has two working states, static and dynamic. Static means that when the input signal is zero, the DC power supply supplies a suitable DC operating voltage to each electrode of the triode, so that the triode operates in the amplification region, that is, the external condition of the triode amplification is the positive junction of the emitter junction, and the collector junction is reverse biased. . Dynamic refers to the ability to amplify the signal of the amplifier circuit after adding the input signal to the input of the amplifier circuit.

Therefore, when analyzing the amplifying circuit, first static and dynamic, that is, analyze the static DC path first, to see whether the working voltage of the transistor, the electron tube and the FET is normal, and then analyze the dynamic communication path after the static working point is normal. The AC path and the DC path coexist in the same circuit, and they are connected to each other and different from each other. The AC signal voltage is superimposed on the DC operating voltage, and the AC performance of the circuit is affected and constrained by the DC operating point. If the DC bias voltage is unstable or faulty, the AC path will be affected and malfunction.

(4) The amplitude of the output signal increases in the negative feedback circuit. The main cause of the distortion fault phenomenon is that the negative feedback component in the amplifier circuit is damaged, and the negative feedback action disappears, which makes the gain of the amplifier become larger, resulting in an increase in the amplitude of the output signal. At this point, it is important to check whether the negative feedback component in the circuit has open circuit, virtual soldering, resistance variable value and so on. If the output signal is distorted, the amplifier has been operating in a non-linear region (saturated or cut-off state). The voltage at the operating point of the amplifier should be measured to find out if the resistance in the circuit is normal and the parameters of the amplifier tube have changed.