The core component of the amplification circuit is the triode, so we should have a certain understanding of the triode. There are many kinds of amplification circuits composed of triodes. Let's explain them with several commonly used ones (as shown in Figure 1). Figure 1 is the basic amplifying circuit of common radiation. What should we know about the amplification path? (1) Analyze the function of each component in the circuit; (2) Emancipate the amplification principle of large circuit;
(3) Be able to analyze and calculate the static working point of the circuit;
(4) Understand the purpose and method of setting static working point;
Of the above four items, the last one is more important.
In Figure 1, C1 and C2 are coupling capacitors, and coupling is used to transmit signals. The capacitor can couple signal signals from the front stage to the rear stage, because the voltage at both ends of the capacitor cannot change suddenly. After the input of the AC signal at the input end, because the voltage at both ends cannot change suddenly, the voltage at the output end will change along with the AC signal input at the input end, thus coupling the signal from the input end to the output end. However, it should be noted that the voltage at both ends of the capacitor cannot change suddenly, but it cannot. R1 and R2 are DC bias resistors of triode V1. What is DC bias? In short, work requires food. If the triode is required to work, certain working conditions must be provided first. The electronic components must have power supply, otherwise they will not be called circuits.
In the working requirements of the circuit, the first condition is to be stable. Therefore, the power must be DC power, so it is called DC bias. Why is power supplied through resistance? Resistance is like the tap in the water supply system, which is used to adjust the current. Therefore, the three working states of the triode, "load stop, saturation, amplification", are determined by the DC bias. In Figure 1, they are determined by R1 and R2.
First of all, we need to know how to distinguish the three working states of the triode. In a nutshell, we can distinguish the working state according to the size of Uce. If Uce is close to the power supply voltage VCC, the triode will work in the load stop state. The load stop state means that the triode does not work on the base, and the Ic current is small (about zero). Therefore, R2 has no current flow, and the voltage is close to 0V, so Uce is close to the power supply voltage VCC.
If Uce is close to 0V, the triode works in the saturated state. What is the saturated state? That is to say, Ic current reaches the maximum value, even if Ib increases, it cannot increase any more.
The above two states are generally called switching states. In addition to these two states, the third state is amplification state. Generally, Uce is close to half of the power supply voltage. If Uce is biased to VCC, the triode will tend to the load stop state; if Uce is biased to 0V, the triode will tend to the saturation state.
► ► ► ► Understand the purpose and method of setting the static working point Amplification circuit is to amplify the input signal and then output it (there are voltage amplification, current amplification and power amplification, which are not included in this discussion). First, let's talk about the signal we want to amplify. Take the sinusoidal AC signal as an example. In the process of analysis, we can only consider whether the signal size changes are positive or negative. It is mentioned above that in the amplifying circuit of Figure 1, the setting of the static operating point is Uce close to half of the power supply voltage. Why? This is to make the positive and negative signals have a symmetrical change space. When there is no signal input, that is, the signal input is 0. Assuming Uce is half of the power supply voltage, we can use it as a horizontal line as a reference point. When the input signal increases, Ib increases, Ic current increases, and the voltage U2 of resistance R2=Ic × R2 will increase accordingly, Uce=VCC-U2, will decrease. Theoretically, if the maximum U2 is equal to VCC, then the minimum Uce will reach 0V. This means that when the input signal increases, the maximum Uce change is from 1/2 of VCC to 0V.
Similarly, when the input signal decreases, Ib decreases, Ic current decreases, and the voltage U2 of resistance R2=Ic × R2 will decrease accordingly, Uce=VCC-U2, will increase. The maximum change of Uce is from 1/2 of VCC to VCC when the input signal is reduced. In this way, when the positive and negative changes occur within a certain range of the input signal, if Uce is based on 1/2 VCC, there will be a symmetrical positive and negative change range. Therefore, generally, the setting of the static operating point in Figure 1 is that Uce is close to half of the power supply voltage.
It is our goal to design Uce to be close to half of the power supply voltage, but how can we design Uce to be close to half of the power supply voltage? This is the means.
Here we need to know a few things. The first one is Ic and Ib, which are the collector current and base current of the triode. One of their relations is Ic= β× Ib, but when we first learned, the teacher obviously didn't tell us how old Ic and Ib were? This question is difficult to answer, because there are many things involved, but generally speaking, for small power tubes, Ic is generally set at a few tenths of a milliampere to a few milliamperes, for medium power tubes, it is set at a few tenths of a milliampere, and for large power tubes, it is set at a few tenths of a milliampere to a few tenths of a.
In Figure 1, if Ic is set to 2mA, the resistance of resistance R2 can be calculated by R=U/I. If VCC is 12V, then 1/2VCC is 6V, R2 is 6V/2mA, and 3K Ω. If Ic is set to 2mA, Ib can be determined by Ib=Ic/ β The key to launch is β Value of, β Generally, the theoretical value is 100, then Ib=2mA/100=20 # A, then R1=(VCC-0.7V)/Ib=11.3V/20 # A=56.5K Ω, but in fact β The value is far more than 100, between 150 and 400, or higher. Therefore, if we follow the above calculation, the circuit may be in a saturated state. Therefore, sometimes we do not understand. The calculation is correct, but it cannot be used in practice. This is because there is still little practical guidance to point out the difference between theory and practice. This circuit is subject to β The value has a great impact. When everyone calculates the same value, the results are not necessarily the same. In other words, this kind of circuit has poor stability and is rarely used in practice. However, if the partial voltage bias circuit in Figure 2 is changed, the analysis and calculation of the circuit are closer to the actual circuit measurement.
In the partial voltage bias circuit in Figure 2, we also assume Ic is 2mA and Uce is designed to 1/2VCC is 6V. Then how to value R1, R2, R3 and R4. The calculation formula is as follows: Because Uce is designed to 1/2VCC is 6V, Ic × (R3+R4)=6V; Ic≈Ie。 It can be calculated that R3+R4=3K Ω, so what are R3 and R4 respectively? Generally, R4 is 100 Ω, and R3 is 2.9K Ω. In fact, we usually directly take 2.7K Ω for R3, because there is no 2.9K Ω in E24 series resistors, so there is no big difference between 2.7K Ω and 2.9K Ω. Because the voltage at both ends of R2 is equal to Ube+UR4. 0.7V+100Ω × 2mA=0.9V, we set Ic as 2mA, β Generally, the theoretical value is 100, then Ib=2mA/100=20 # A. Here is a current to be estimated, that is, the current flowing through R1, which is generally about 10 times of Ib. If IR1200 # A is taken, then R1=11.1V/200 # A ≈ 56K Ω R2=0.9V (/200-20) # A=5K Ω; Considering the actual β The value may be much greater than 100, so the actual value of R2 is 4.7K Ω. In this way, the values of R1, R2, R3 and R4 are respectively 56K Ω, 4.7K Ω, 2.7K Ω, 100 Ω and 6.4V.
In the above analysis and calculation, assumptions have been put forward many times, which is necessary in practical applications. Many times, a reference value is needed for us to calculate, but it is often not. First, we are unfamiliar with various devices, and second, we forget that we are the ones who use circuits. Some data can be set by ourselves, so that we can avoid detours.