Introduction and Application of Electron Tube

The basic electron tube generally has 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 heating type, which is the cathode that emits electrons by direct current passing through the cathode to heat the cathode; the other is the indirect heating type cathode, which is generally a hollow metal tube with a spiral filament inside the tube. The filament voltage is added to heat the filament, which heats the cathode and emits electrons. Most of the electron tubes used in daily life are of this type (as shown in the figure). The electrons emitted from the cathode pass through the gaps between the grid metal wires and reach 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 that of 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.

为了提高电子管的放大系数,在三极管的阳极和控制栅极之间另外加入一个栅极称之为帘栅极,而构成四极管,由于帘栅极具有比阴极高很多的正电压,因此也是一个能力很强的加速电极,它使得电子以更高的速度迅速到达阳极,这样控制栅极的控制作用变得更为显著.因此比三极管具有更大的放大系数.但是由于帘栅极对电子的加速作用,高速运动的电子打到阳极,这些高速电子的动能很大,将从阳极上打出所谓二次电子,这些二次电子有些将被帘栅吸收形成帘栅电流,使帘栅电流上升这会导致帘栅电压的下降,从而导致阳极电流的下降,为此四极管的放大系数受到一定而限制.

In order to solve the above contradiction, a pair of collector-emitter electrodes connected to the cathode are added on both sides outside the tetrode screen grid. Since the collector-emitter potential is the same as the cathode, it has a repulsive effect on electrons, so that after passing through the screen grid, the electrons move in a certain direction under the action of the collector-emitter electrode and form a flat beam. The electron density of this flat electron beam is very large, thus forming a low-voltage area. The secondary electrons shot out 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 electron tube is called a beam tetrode. The beam tetrode not only has a higher amplification factor than the triode, but also has a larger anode area, allowing a larger current to pass. Therefore, it is often used in current power amplifiers as a power amplifier.

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 circuits are mostly used in various high and low frequency amplifier circuits. Because of its wide application, we will discuss the negative feedback circuit here. Negative feedback has four effects on amplifier performance:

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 the distortion of the amplifier.
4. Negative feedback can improve the signal-to-noise ratio of the amplifier.
5. Negative feedback has an impact on 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 operating 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 to the input end through this branch. Since the signal at the output end 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 size of the negative feedback amount, 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 saturated 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.

The basic principle of impedance matching

In the right figure, R is the load resistance, r is the internal resistance of the power source 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:
From the above formula, it can be seen that when R=r, the value of (Rr) in the denominator of the formula is the minimum 0, and the power obtained by the load is the maximum at this time. Therefore, when the load resistance is equal to the internal resistance of the power source, the load will obtain the maximum power. This is the basic principle of impedance matching in electronic circuits.

Color Temperature

There is a little-known parameter of color TV sets - 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. However, due to different luminous substances, the spectral components are also very different. How to distinguish the differences between various light sources due to different spectral components? For this reason, physics uses a radiation source called a black body as a standard. 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 at what temperature its radiation is equivalent to the radiation characteristics of the black body (that is, their spectral components are the same), and the temperature (absolute temperature) of the black body at this time is called the color temperature of a certain 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; the color temperature is lower when the red component is high.
In daily life, the film used for photography has high and low color temperatures. Daylight
type Film is high color temperature film, while tungsten film is low color temperature film. If you use tungsten film to take pictures under sunlight or flash, the color of the scene 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 will be distorted.

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, the 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, the 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)

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 magazines. 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 amplifiers with large voltage gains. For this reason, we often design the load resistance of the transistor collector to be as large as possible. However, if this resistance is too large, the transistor will easily enter a 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 a small DC voltage drop, as shown in Figure 6.

The voltage regulator circuit composed of the voltage regulator 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 the 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 great voltage gain. In fact, this circuit is used in many integrated circuits.
Series voltage regulator power supply
The 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, and its working principle is to use the characteristics of the voltage regulator tube working in reverse breakdown to achieve. Figure 2 is the volt-ampere characteristic curve of the voltage regulator tube. From this curve, we can see that when the reverse current changes greatly within a certain range, the voltage at its end point remains basically unchanged. 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. The reduction of R requires D to have a larger power consumption. However, since the power consumption of general voltage regulator tubes is relatively small, this circuit can only provide tens of milliamperes of current to the load. The 30V tuning voltage of color TVs is usually obtained using 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 by a transistor BG, the purpose is to expand the output current of the voltage stabilizing circuit. We know that the collector current IC of BG = β * Ib, β is the DC amplification factor of BG, Ib is the base current of the transistor, for example, now to provide a current of 500MA to the load, BG β = 100, then the circuit only needs to provide a current of 5MA to the base of BG. Therefore, this voltage stabilizing circuit is actually 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, 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, for which 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 adds a triode and several resistors compared with 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 of point A is compared with the potential of point B (due to the existence of D, the potential of point B is constant). The result of the comparison is that the collector output of BG2 causes the potential of point C to change, thereby controlling the conduction degree of BG1 (at this time, BG1 plays the role of a variable resistor in the circuit) to stabilize the output voltage. R4 is a variable resistor. Adjusting it can change the potential of point A (that is, change the sampling value). Due to the change of point A, the potential of point C will also change, so that the output voltage will also change. This circuit has a flexible and variable output voltage, so it is widely used in various circuits.