Beginner's World

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	Beginner's Corner Basic electron tubes generally have three poles, a cathode (K) for emitting electrons, an anode (A) for absorbing the electrons emitted by the cathode, and a grid (G) for controlling the flow of electrons to the anode. The basic condition for the cathode to emit electrons is that the cathode itself must have considerable heat. There are two types of cathodes: one is the direct-heated type, in which the current directly passes through the cathode to heat the cathode and emit electrons; the other is called the indirect-heated cathode, which is generally a hollow metal tube with a spiral-wound filament inside the tube. The filament voltage is applied to heat the filament, which in turn heats the cathode and emits electrons. This type of electron tube is mostly used in daily life (as shown in the figure). The electrons emitted by the cathode It passes through the gaps between the grid metal wires and reaches the anode. Since the grid is much closer to the cathode than the anode, the effect of changing the grid potential on the anode current is much greater than changing the anode voltage. This is the amplification effect of the triode. In other words, it is the control effect of the grid voltage on the anode current. We use a parameter called transconductance (S) to represent it. There is also a parameter μ to describe the amplification factor of the electron tube. Its meaning is to explain how many times greater the ability of the grid voltage to control the anode current is than the effect of the anode voltage on the anode current. In order to increase the amplification factor of the electron tube, another grid called the screen grid is added between the anode and the control grid of the triode to form a tetrode. Since the screen grid has a much higher The positive voltage is very strong, so it is also a powerful accelerating electrode, which makes the electrons reach the anode at a higher speed, so that the control effect of the control grid becomes more significant. Therefore, it has a larger amplification factor than the triode. However, due to the acceleration of the electrons by the screen grid, the high-speed electrons hit the anode. The kinetic energy of these high-speed electrons is very large, and they will knock out the so-called secondary electrons from the anode. Some of these secondary electrons will be absorbed by the screen grid to form a screen grid current, which will increase the screen grid current. This will cause the screen grid voltage to drop, thereby causing the anode current to drop. For this reason, the amplification factor of the tetrode is subject to certain restrictions. In order to solve the above contradiction, a pair of cathode and cathode electrodes are added on both sides of the screen grid of the tetrode. The collector-emitter connected to the anode has the same potential as the cathode, so it repels electrons, causing them to move in a certain direction and form a flat beam under the action of the collector-emitter after passing through the screen grid. The electron density of this flat electron beam is very large, thus forming a low-voltage area. The secondary electrons ejected from the anode are repelled by this low-voltage area and pushed back to the anode, thereby greatly reducing the screen grid current and enhancing the amplification ability of the electron tube. This type of electron tube is called a beam tetrode. Not only does the beam tetrode have a higher amplification factor than the triode, but its anode area is larger, allowing a larger current to pass through. Therefore, it is often used as a power amplifier in current power amplifiers . Feedback circuits in electronic circuits Feedback circuits are widely used in various electronic circuits. Feedback is to feed back part or all of the amplifier output signal (voltage or current) to the amplifier input end for comparison with the input signal (addition or subtraction), and use the effective input signal obtained by comparison to control the output. This is the feedback process of the amplifier. Any feedback signal fed back to the amplifier input end that strengthens the original input signal and increases the input signal is called positive feedback. The opposite is true. According to its circuit structure, it is divided into: current feedback circuit and voltage feedback circuit. Positive feedback circuits are mostly used in electronic oscillation circuits, while negative feedback 1. Negative feedback can improve the stability of the amplifier gain. 2. Negative feedback can widen the passband of the amplifier. 3.Negative feedback can reduce amplifier distortion. 4.Negative feedback can improve the signal-to-noise ratio of the amplifier. 5. Negative feedback affects the output and input resistance of the amplifier. Figure F1 is a basic amplifier circuit. This circuit looks very simple, but it actually contains a DC current negative feedback circuit and an AC voltage negative feedback circuit. R1 and R2 in the figure are the DC bias resistors of BG, R3 is the load resistor of the amplifier, R5 is the DC current negative feedback resistor, the branch composed of C2 and R4 is the AC voltage negative feedback branch, and C3 is an AC bypass capacitor, which prevents the generation of AC current negative feedback. 1. DC current negative feedback circuit. The base voltage VB of the transistor BG is the voltage divided by R1 and R2, and the voltage VE of the emitter of BG is Ie*R5, then the voltage between B and E of BG = VB-VE = VB-Ie*R5. When some reason (such as temperature change) causes BG's Ie ↑ then VE↑, the voltage of BG base emitter = VB-VE = VB-Ie*R5↓ so that Ie↓. The DC working point is stabilized. This negative feedback process is caused by Ie↑, so it belongs to the current negative feedback circuit. The emitter capacitor C3 provides the AC path, because if there is no C3, the AC signal will also form a negative feedback effect due to the existence of R5 when the amplifier is working, which greatly reduces the amplifier's amplification factor. 2. AC voltage negative feedback circuit The AC voltage negative feedback branch is composed of R4 and C4, and the output voltage is fed back through this branch Input terminal. Since the signal at the output terminal of the amplifier is in phase with the input signal voltage, the introduction of the feedback signal weakens the effect of the original input signal. Therefore, it is a voltage negative feedback circuit. R4 controls the amount of negative feedback, and C4 plays the role of blocking DC and passing AC. When the amplitude of the input AC signal is too large, if there is no negative feedback branch of R4 and C4, the amplifier will enter a saturation or cut-off state, causing the output signal to be clipped and distorted. Since the introduction of negative feedback controls the amplitude of the input AC signal, distortion is avoided. Thinking draft The basic principle of impedance matching In the right figure, R is the load resistance, r is the internal resistance of the power supply E, and E is the voltage source. Due to the existence of r, when R is very large, the circuit is close to an open circuit state; and when R is very small, it is close to a short circuit state. Obviously, the load cannot obtain the maximum power in both the open circuit and short circuit states. According to the formula: It can be seen from the above formula that when R=r,the value of (Rr) in the denominator of the formula is at least 0, and the power obtained by the load is the largest at this time. Therefore, when the load resistance is equal to the internal resistance of the power supply, the load will obtain the maximum power. This is the basic principle of impedance matching in electronic circuits. Thinking draft Color temperature There is a little-known parameter of color TV - the color temperature of the picture tube. The picture tube with low color temperature has bright and warm colors; the picture tube with high color temperature has fresh and natural images. So what is color temperature? The light emitted by common light sources such as the sun, fluorescent lamps, incandescent lamps, etc. is collectively called white light. But due to different luminous substances, the spectral components vary greatly. How to distinguish the differences between various light sources due to different spectral components? For this reason, a radiation source called a black body is used as a standard in physics. This black body is an ideal thermal radiator, and its radiation level is only related to its temperature. When comparing other light sources with black body radiation, observe its radiation and the radiation characteristics at what temperature the black body is. If the color temperature of a light source is equal (i.e., their spectral components are the same), the temperature (absolute temperature) of the black body at this time is called the color temperature of a light source. In actual use, this is often distinguished by the ratio of the blue spectral component to the red spectral component in the light source. The color temperature of the light source is generally higher when the blue component is high and lower when the red component is high. In daily life, the film used for photography has high and low color temperatures. Daylight film is a high color temperature film, and light film is a low color temperature film. If you use light film to take pictures under daylight or flash, the color of the scene you take will be blue. In addition, color temperature is also a very important parameter when shooting with a camera. If it is not handled well, the color of the image taken will be distorted. Thinking draft Characteristics of series and parallel resonant circuits 1. Series resonant circuit: When an external frequency is applied to a series resonant circuit, it has the following characteristics: 1. When the external frequency is equal to its resonant frequency, its circuit impedance is purely resistive and has a minimum value. This characteristic is called a notch filter in practical applications. 2. When the external frequency is higher than its resonant frequency, the circuit impedance is inductive, equivalent to an inductor. 3. When the external frequency is lower than its resonant frequency, the circuit is capacitive, equivalent to a capacitor. 2. Parallel resonant circuit: When an external frequency is applied to a parallel resonant circuit, it has the following characteristics: 1. When the external frequency is equal to its resonant frequency, its circuit impedance is purely resistive and has a maximum value. This characteristic is called a frequency selection circuit in practical applications. 2. When the external frequency is higher than its resonant frequency, the circuit impedance is capacitive, equivalent to a capacitor. 3. When the external frequency is lower than its resonant frequency, the circuit is inductive, equivalent to an inductor. Therefore, when the series or parallel resonant circuit is not adjusted at the signal frequency point, the signal will produce phase shift when passing through it. (That is, phase distortion) Thinking Draft Electronic constant current source Friends who are interested in electronic technology may often see the term "constant current source" when reading some electronic books and periodicals. So what is a constant current source? As the name suggests, a constant current source is a power supply that can output a constant current. r in Figure 5 is the internal resistance of the power supply E, and RL is the load resistance. According to Ohm's law: the current flowing through RL is I=E/r+R. If r is very large, such as 500K, then when RL changes from 1K to 10K, I will remain basically unchanged (only a slight change) because RL is too insignificant relative to r. At this time, we can think that E is a constant current source. For this reason, we infer that a constant current source is a power supply with a very large internal resistance. In electronic circuits (such as transistor amplifier circuits), we often need some amplifiers with large voltage gains. For this reason, we often design the load resistance of the transistor collector to be as large as possible, but if this resistance is too large, it will easily make the crystal The tube enters the saturation state. At this time, we can use a crystal triode to replace this large resistor. In this way, we can get a large resistance and the DC voltage drop is not large, as shown in Figure 6. The voltage stabilizing circuit composed of the voltage stabilizing tube D and the resistor R2 in the figure is used to bias the working point of BG1 and ensure the stability of the working point (BG2 is an amplifier tube). From the output characteristics of the transistor, it can be seen that when the collector-emitter voltage VEC is greater than 1-2V, the characteristic curve is almost flat, that is, when VEC changes, IC basically remains unchanged, that is, the output resistance of transistor BG1 is very large (more than several hundred kilo-ohms). In the figure, since the current of BG1 is basically constant, BG1 is called the constant current load of BG2. Since the amplifier with a constant current source load has a large load resistance, this amplifier circuit has a huge voltage gain. In fact, this circuit is used in many integrated circuits. Thinking Draft Series voltage regulator circuit is one of the most commonly used electronic circuits. It is widely used in various electronic circuits. It has three forms. 1. As shown in Figure 1,this is the simplest series voltage regulator circuit (some books call it a parallel voltage regulator circuit. I personally always think it should be a series voltage regulator circuit). The resistor RL is the load resistor, R is the voltage regulator adjustment resistor, also called the current limiting resistor, and D is the voltage regulator tube. The voltage regulator value output by this circuit is equal to the nominal voltage regulator value of D. Its working principle is to use the characteristics of the voltage regulator tube working in reverse breakdown. Figure 2 is the volt-ampere characteristic curve of the voltage regulator tube. From this curve, we can see that the voltage at its end point remains basically unchanged when the reverse current changes greatly within a certain range. When RL becomes smaller, the current flowing through RL increases, but the current flowing through D decreases. When RL becomes larger, the current flowing through RL decreases, but the current flowing through D increases. Therefore, due to the existence of D, the current flowing through R is basically constant, and the voltage drop on R is basically unchanged, so the output voltage is also basically unchanged. When the load requires a larger output current, this circuit will not work. This is because the resistance of R must be reduced at this time. Since the reduction of R requires D to have a larger power consumption, but because the power consumption of the current general voltage regulator is relativelysmall, this circuit can only provide a current of tens of milliamperes to the load. The 30V tuning voltage of color TV is usually obtained by this circuit. 2. As shown in Figure 3, this circuit is an improved circuit for the shortcomings of the above-mentioned circuit. The difference from the first circuit is that R in the circuit is replaced with a transistor BG, the purpose is to expand the output current of the voltage regulator circuit. We know that the collector current IC of BG = β * Ib, β is the DC amplification factor of BG, and Ib is the base current of the transistor. For example, if a current of 500MA is to be provided to the load, BG's β = 100, then the circuit only needs to provide a current of 5MA to the base of BG. Therefore, this voltage stabilizing circuit is equivalent to expanding the first voltage stabilizing circuit by β times due to the addition of BG. In addition, since the base of BG is embedded at its nominal voltage stabilizing value by D, the output voltage of this voltage stabilizing circuit is V0=VD-0.7v, and 0.7V is the positive bias voltage drop of BG's B and E poles. In practical applications, we often provide different power supply voltages for different circuits, that is, the output voltage of the voltage stabilizing power supply is required to be adjustable, so the third form of series voltage stabilizing circuit appears. 3. Although the second voltage stabilizing circuit can provide a larger output current, its output voltage is restricted by the voltage stabilizing tube D. For this reason, people slightly modify the second circuit to make it a series voltage stabilizing power supply with continuously adjustable output voltage. The basic circuit is shown in Figure 4. From the circuit, we can see that this circuit has one more transistor and several resistors than the second circuit. R2 and D form the reference voltage of BG2, and R3, R4, and R5 form the output voltage sampling branch. The potential at point A is compared with the potential at point B (due to the existence of D, the potential at point B is constant). As a result of the comparison, the collector output of BG2 causes the potential at point C to change, thereby controlling the conduction degree of BG1 (at this time, BG1 acts as a variable resistor in the circuit) to stabilize the output voltage. R4 is a variable resistor. Adjusting it can change the potential at point A (that is, change the sampling value). Due to the change at point A, the potential at point C will also change, thereby changing the output voltage. This circuit has a flexible and variable output voltage, so it is widely used in various circuits. Thinking Draft About dbμV, dbm, dbw In cable TV technology, we often encounter the values of several signal parameters, which are logarithmic units - decibel (db). Decibel is used to facilitate expression, description and calculation (changing multiplication and division into addition and subtraction). Decibel is a unit that represents the ratio of two power levels, such as A=10lgP2/P1=20lgU2/U1=20lgI2/I1. There are three ways to use decibel in electromagnetic field strength measurement and testing: 1. Indicates the relative size of power (or voltage) between any two points in a signal transmission system. For example, for a CATV amplifier, when its input level is 70dbμV, its output level is 100dbμV, which means that the output of the amplifier is 30db different from the input, and this 30db is the gain of the amplifier. 2. When specifying a reference level, the absolute value of voltage or electric field strength can be expressed in decibels. This reference level is generally called 0db. For example, 1μV=0dbμV, 1mW=0dbm, 1mV=0dbmV. For example, there is a signal A whose level is 3dbμV. The conversion into voltage is: 3=20lgA/1μV, A=2μV, that is, the voltage of this 3dbμV signal is 2μV. 3. Use decibels to express the error of voltage or field strength, such as 30±3db. Usually db is a value that characterizes circuit loss and gain; dbmV and dbμV are relative level values that characterize signals. Since 1mV=1000μV, 0dbmV=60lg10=60dbμV. For example, if the signal level is 70dbμV, it is 70-60=10dbmV in dbmV; dbm and dbw are relative power values that characterize signals. Since 1W=1000mW, 0dbW=30lg10=30dbm. For example, if the optical power is 9dbm, the unit (watt) of power conversion is: 9=10lgx, x=7.9mW. Conversion between power and level (conversion between dBm and dbμV): In many cases, we only have one field strength meter, and its unit of measurement is usually dbμV. However, in some high-frequency power amplifiers, only the power value of the output signal is given. Therefore, the power value must be converted into a level value. For a signal source with a 50-ohm impedance, when its output power is 1mW (0dbm), its terminal voltage output should be U=50P-E2×1000000=223606.7978μV, which is expressed in decibels as: 20lg223606.7978=107dbμV. That is to say, the output level of a 50-ohm signal source with a 0dbm impedance is 107dbμV. For example 1: The output power of a 50 ohm high frequency power amplifier is 50 dBm. To calculate its output level, we have: 107+50=157 dBμV. Example 2: The minimum receiving power of a 50 ohm receiving device is -90dbm. To find its minimum receiving level, we have: 107-90=17dbμV. 50Ω system dbm, dbμV, watt conversion table Power (dBm) Level (dbμV) Power (W) 　 Power (dBm) Level (dbμV) Power (W) +53 160 200w 0 107 1.0mw +50 157 100w -1 106 .80mw +49 156 80w -3 104 .50mw +47 154 50w -7 100 .20mw +46 153 40w -10 97 .10mw +43 150 20w -20 87 .01mw +40 147 10w -27 80 　 +37 144 5w -30 77 .001mw +33 140 2w - 　 　 +30 137 1.0w - 　 　 +29 136 800mw - 　 　 +27 134 500mw - 　 　 +26 133 400mw - 　 　 +23 130 200mw - 　 　 +20 127 100mw - 　 　 +17 124 50mw - 　 　 +13 120 20mw - 　 　 +10 117 10mw - 　 　 +7 114 5mw 　 　 　 +3 110 2.0mw 　 　 　 Thought Draft