In order to make it easier for everyone to understand the amplification circuit, let’s introduce some basic concepts of amplification circuit.

(1) Input resistance and output resistance

An amplification circuit can usually be equivalent to the circuit shown in Figure 2-4. The basis for such equivalence is that when the amplification circuit works, the input signal is sent to the input end of the amplification circuit. For the input signal, the amplification circuit is equivalent to a load resistance RI, which is called the input resistance of the amplification circuit; After amplifying the input signal, the amplification circuit will output the signal to both ends of the load r L. because the amplification circuit has signal output, for the load R L, the amplification circuit is equivalent to a signal source with internal resistance RO and voltage U2. Here, the internal resistance ro is called the output resistance of the amplification circuit.

Figure 2-4 equivalent diagram of amplification circuit

U 1 in the figure is the signal source voltage, R 1 is the internal resistance of the signal source, and R L is the load. The part in the dotted line frame in the middle of the figure is the equivalent diagram of the amplification circuit, u 2 is the amplified signal voltage, u I is the input voltage of the amplification circuit, u o at both ends of the load R L is the output voltage of the amplification circuit, the current I I flowing into the amplification circuit is called the input current, the current I O flowing out of the amplification circuit is called the output current, and R I is the input resistance of the amplification circuit, R O is the output resistance of the amplification circuit.

From the point of view of reducing the burden of the input signal source and increasing the output voltage of the amplification circuit, it is better to have a larger input resistance RI, because when the internal resistance R1 of the input signal source remains unchanged, on the one hand, a larger input resistance RI will reduce the I I current absorbed by the amplification circuit from the signal source, and at the same time, a higher U I voltage can be obtained at the input end of the amplification circuit, In this way, the output voltage of the amplification circuit is very high. If it is necessary to increase the output current io of the amplification circuit, it is better to have a smaller input resistance RI, because when the input resistance is small, the input current of the amplification circuit is large, and the amplified output current is relatively large.

For the output resistance R O of the amplification circuit, the smaller the better, because when the output resistance is small, the voltage and current consumed on the output resistance are very small, and the load r l can obtain relatively large power, that is, if the output resistance of the amplification circuit is small, the amplification circuit has strong load capacity.

(2) Magnification and gain

There are three kinds of amplification factors of the amplification circuit.

① Voltage magnification. Voltage magnification refers to the ratio of output voltage u o to input voltage U I, expressed in a U

② Current magnification. Current magnification refers to the ratio of output current I O to input current I I, expressed by a I

③ Power magnification. Power magnification refers to the ratio of output power Po to input power PI, expressed in a P

In practical application, in order to facilitate calculation and representation, the logarithm of amplification factor is often used to represent the amplification capacity of amplification circuit. The value obtained in this way is called gain, and the unit of gain is dB. The larger the gain, the stronger the amplification ability of the circuit.

For example, when the voltage magnification of the amplification circuit is 100 times and 10000 times respectively, its voltage gain is 40dB and 80dB respectively.

Three basic connections of amplifying circuits

According to the different connection modes of triode in the circuit, there are three basic connection methods of amplification circuit: common emitter connection, common base connection and common collector connection. The three basic connections of the amplification circuit are shown in Figure 2-5.

Fig. 2-5 three basic connection methods of amplification circuit

The three basic connection circuits of amplification circuit can be analyzed from the following aspects.

(1) Is it capable of amplification

As mentioned earlier, to judge whether the triode circuit has amplification ability, it can generally be judged by analyzing whether the I B, I C and I E currents of the triode in the circuit are complete. If there is a complete way, it indicates that the amplification circuit has amplification ability. In the circuit shown in Figure 2-5, the current analysis of triodes I B, I C and I e of three basic connection circuits is as follows.

I B, I C, I e current path of triode in common emitter connection circuit:

I B, I C, I e current path of triode in common base connection circuit:

I B, I C, I e current path of triode in common collector connection circuit:

It can be seen from the above analysis that the triode I B, I C and I E currents in the three basic connection circuits have complete paths, so they all have amplification ability.

(2) Common electrode form

An amplification circuit shall have input and output terminals. In order to make the AC signals at the input and output terminals have their own circuits, the input and output terminals shall have two poles respectively, while the triode has only three electrodes, so one electrode will be shared by the input and output terminals.

When analyzing the amplification circuit, in order to master the AC signal processing of the amplification circuit, it is necessary to draw its AC equivalent diagram, and DC is not considered when drawing the AC equivalent diagram. To draw an AC equivalent diagram, we should master the following two points.

① The internal resistance of the power supply is very small. For AC signal, it can be regarded as short circuit, that is, for AC signal, the positive and negative poles of the power supply are equivalent to short circuit. Therefore, when drawing AC equivalent diagram, the positive and negative poles of the power supply should be connected with wires.

② The coupling capacitance and bypass capacitance in the circuit have large capacity and little obstruction to the AC signal, which can also be regarded as short circuit. When drawing the AC equivalent diagram, the high-capacity capacitance shall be replaced by wires.

According to the above principles, the AC equivalent diagram of the common emitter connection amplification circuit shown in Fig. 2-5 (a) can be drawn according to the method shown in Fig. 2-6.

Figure 2-6 drawing of AC equivalent diagram of common emitter connection amplification circuit

In the same way, the AC equivalent diagrams of the other two basic connection amplification circuits can be drawn. The AC equivalent diagram of the three basic connection amplification circuits is shown in Figure 2-7.

Fig. 2-7 AC equivalent diagram of three basic connection amplification circuits

In the circuit shown in Fig. 2-7 (a), the base is the input terminal, the collector is the output terminal, and the emitter is the common electrode of the input and output circuits. This amplification circuit is called a common emitter amplification circuit.

In the circuit shown in Fig. 2-7 (b), the emitter is the input end, the collector is the output end, and the base is the common electrode of the input and output circuits. This amplification circuit is called a common base amplification circuit.

In the circuit shown in Fig. 2-7 (c), the base is the input end, the emitter is the output end, and the collector is the common electrode of the input and output circuits. This amplification circuit is called a common collector amplification circuit.

(3) Characteristics of three basic connection amplification circuits

See table 2-1 for the characteristics of three basic connection amplification circuits.

Table 2-1 characteristics of three basic connection amplification circuits