For many people, the triode sounds familiar, because many people have heard his name, seen this symbol, and know that it can be used to amplify signals. It’s strange because in addition to the above, many people know nothing about it and have never directly used a triode. The reason why triode is so famous is that it has a very long history. In some early electronic devices, we can almost find the figure of triode. Early electronic magazines also published a large number of application circuits about triodes, but now there are few application scenarios for triodes. There are two main reasons: the first is the development of integrated circuits. For example, the circuits of the above triodes can be easily replaced by single-chip computers or other integrated circuits. It is more simple and convenient to use, and the effect is better. For many people, they have lost the motivation and scene to learn triode. The second and most important reason is the popularity of MOS transistor, which further compresses the living space of triode.

In this article, we will talk about the difference between MOS and triode, and what advantages it has over triode. First, let’s briefly talk about how the triode works. The triode has three electrodes, namely B, e and C, which are very simple to use. As long as the circuit is connected like this, when the IO port output of the single-chip microcomputer is at a high level, the triode opens the channel and the bulb opens; When the IO port output of the single-chip microcomputer is at low level, the triode is turned off and the bulb is turned off. Do you think this scene is very familiar? Yes, it is exactly the same as the MOS transistor we talked about before.

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What is the difference between MOS and triode?

The first and most important difference is that MOS transistor is a voltage controlled component, while triode is a current controlled component. So how do you understand these two sentences? Between B and e of the triode, it can be assumed that there is a diode, which creates a current path. When I apply a high level to the triode, a continuous current will be generated between B and E. only when this continuous current exists, the triode will be turned on. When I take away the high level, the current disappears instantly and the triode will close. Therefore, the necessary condition for maintaining the triode open is that there is a continuous current between B and E. therefore, the triode is called a current control element.

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For this MOS tube, there is no path between G and s, and there is only a parasitic capacitance between them. When the high level is applied to the G pole, there is no other current except the current that initially charges the parasitic capacitor. As long as the voltage difference between the G pole and the S pole is maintained, the MOS tube can be turned on. Even if I remove the high level of the IO port because of the parasitic capacitor, you will find that the bulb is still on. Until the charge on the capacitor slowly emits light, the MOS tube will not close, which is why MOS tubes are called voltage control devices.

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So you can see the advantages of MOS transistor here? Yes, it saves power. It does not need additional current to keep the MOS transistor open. You may think that the current consumed by the triode is too small to be large. But now there are hundreds of tubes or even hundreds of millions of tubes in an integrated circuit. If we use triodes, the power consumption will be amazing. That is why MOSFETs are basically used in today’s digital integrated circuits.

The second advantage of MOS transistor for triode is that the on impedance is small. After MOS transistor is on, D and s can be equivalent to a resistance, and this resistance can be easily less than 10 milliohms. At this time, even if the current passes through 10 amps, only 0.1 volt voltage drop will be generated, that is, 1W power consumption. After the triode is turned on, C and E are equivalent to a diode, which will produce a fixed voltage drop of about 0.4 v. At this time, if the current passing through is still 10 amps, the power consumption of the triode will reach 4W, which is about four times that of the MOS transistor. Therefore, most low-voltage power devices now use the MOS transistor for control.

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At this point, you will find that the triode seems to be defeated by MOS both in the internal of the integrated circuit and as a discrete component. So what are the advantages of triodes over MOS transistors? Generally speaking, triodes have two advantages. The first is that they are cheap. For some low-power control scenarios, such as LED lights or low-power DC motors. Their operating current may be less than 100 Ma, so using a triode is very cost-effective. Although the triode will consume a voltage drop of 0.4V, the current is very small. Therefore, the total power consumption of the triode can be ignored. This is the scene where I currently use the most triodes. I usually choose the s8050 triode. The price is only a few cents, which is very cost-effective. At the same time, the triode is also suitable for some level conversion circuits.

The second advantage of the triode is that it can withstand high voltage and large current. Due to the process problems of MOS transistors, if high voltage is required, the rdsn will be very large. At this time, the triode has an advantage. The triode can easily withstand high voltage, but this kind of scene is rarely encountered by ordinary people, so I won’t go into detail. To sum up, because of the advantages of small static current and low on impedance, we now use MOS transistors to replace triodes in most scenarios. We only use triodes in some scenarios with low power or insensitive to power consumption. After all, at present, triodes still have a great price advantage.

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In addition, this article did not explain the principle and structure of the triode in detail, and even I did not mention the amplification characteristics of the triode. That’s because I think the amplification characteristic of triode has been rarely used in reality. Even I have never used the amplification characteristic of triode in formal work. Of course, I’m not saying it’s unimportant. In fact, I still miss the radio made of two triodes and a pile of discrete components in high school. Although the whole process did not have much commercial value, it gained a lot of fun. It is these simple productions that make me interested in the world of hardware.

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