Amplification is the most basic analog signal processing function. It is implemented by an amplifying circuit. Different types of amplifying circuits are used in most analog electronic systems. The amplifying circuit is also a basic unit circuit that constitutes other analog circuits, such as filtering, oscillation, and voltage regulation.
The analog amplifying circuit mainly includes a common-emitter, common-collector and common-base amplifier which is a single-tube amplifier, a RC coupling, a transformer coupling and a direct-coupled multi-stage amplifying circuit, a field effect transistor amplifying circuit, a tube amplifying circuit, a feedback amplifying circuit and a power amplifying circuit. . The amplifying circuit is further divided into a low frequency amplifying circuit and a high frequency amplifying circuit. Regardless of the amplification circuit, they have the following characteristics.
(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.
(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 restricted 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 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.
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