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Analysis of the role of field effect tubes

Source: Company Dynamics Editor: PingShang Click: Release time: 2021-11-27 09:13:47

What is the role of field effect transistors? Pingshang Technology will analyze it for you from a professional perspective.


1. It can be applied to zoom in. Because the input impedance of the field effect tube amplifier is very high, the coupling capacitor can have a smaller capacity, and it is not necessary to use an electrolytic capacitor. 2. The FET has a very high input impedance and is very suitable for impedance transformation. It is often used for impedance conversion in the input stage of a multi-stage amplifier. 3. It can be used as a variable resistor. 4. It can be conveniently used as a constant current source. 5. It can be used as an electronic switch.


Classification of FET 1. The junction field effect tube is named after it has two PN junctions; 2. Insulated gate type field effect tube is also called metal-oxide-semiconductor field effect tube, which is named because the gate is completely insulated from other electrodes. 3. According to the different conduction mode, MOSFET is divided into enhancement type and depletion type. The so-called enhanced type refers to: when VGS=0, the tube is in an off state, and after adding the correct VGS, the majority of carriers are attracted to the gate, thereby "enhancing" the carriers in this area and forming a conductive channel . The depletion type means that the channel is formed when VGS=0, and when the correct VGS is added, the majority of carriers can flow out of the channel, thus "depleting" the carriers and turning the tube off.


Triodes and FETs are very common electronic components used in amplifying and switching circuits. The triode was first invented, and it quickly replaced the electron tube with its excellent performance, but later in the application, the triode exposed some congenital shortcomings-structural problems The resulting defects, in this situation, it is urgent to manufacture a transistor that can overcome the defects of the triode, so the application of field effect transistors is born.


The major feature of FET is its extremely high input impedance, which is unmatched by triodes. However, its emergence does not completely replace triodes like transistors eliminate electron tubes, nor is it. In some respects, they are inferior to triodes, so we cannot say in general terms. Good or bad, because it is developed on the basis of a triode, it has similarities with a triode in many aspects, and the two are perfect for a wide range of applications. Through the comprehensive understanding of triode and FET, in order to better use them.

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1. Electrode difference: The triode has three electrodes: base b, emitter e, and collector c. The field effect tube also has three electrodes G, source S, and drain D. They have a corresponding relationship. The role of the electrode Similar, that is, both the base and the gate are control electrodes, the emitter corresponds to the source, and the collector corresponds to the drain, which are all controlled electrodes;

2. Control type: The triode is a current-controlled device, that is, the change of the collector current is controlled by the change of the base current; the field effect transistor is a voltage-controlled device, that is, the source and drain current is controlled by the change of the gate voltage Size; the working principles of the two are different. The transistor controls the collector current through the base current, while the field effect transistor controls the current change by changing the width of the conductive channel through the gate voltage;

3. Impedance difference: the input impedance of the triode is low, between a few hundred ohms and several thousand ohms, the base current is large, and the output resistance is high, which has a large impact on the previous circuit, and it can hardly work when the impedance is not matched; The input impedance of the effect tube is extremely high, reaching more than megohms, and the MOS tube is higher, and there is almost no current in the grid, which has little effect on the previous circuit, and the output resistance is also higher like the triode;

4. Carrier difference: The triode has two kinds of carriers to participate in conduction, namely, minority carriers and many carriers, which are bipolar devices; the field effect transistor has only one carrier to participate in conduction, which is a unipolar device;

5. Differences in stability: The triode is also susceptible to temperature and thermal stability due to the minority carrier is also involved in the conduction. Therefore, its noise is high and the manufacturing is complicated; the field effect transistor is thermally stable due to its multi-carrier conduction. Better, so the noise is small; the manufacturing process is simple, easy to integrate, low power consumption, small size, and wide safe working area; large-scale and very large-scale integrated circuits are mostly made of field effect transistors;

6. Classification difference: transistors are divided by structure There are two types of PNP and NPN; and there are more types of field effect transistors. According to the conductive channel, they are divided into n-type and p-type. According to the principle structure, they are divided into junction field effect transistors JFET and insulated gate field effect transistor MOSFET. MOS transistors are divided into enhanced Two types: type and depletion type;

7. The difference of characteristic curve: the characteristic curve of triode is divided into cut-off zone, amplification zone, saturation zone, and breakdown zone; FET is divided into cut-off zone, amplification zone, variable resistance zone, and breakdown zone. There is a corresponding relationship between the two; There are input and output characteristic curves on the curve; from the circuit analysis and calculation, the field effect tube is simpler than the triode; the transfer characteristic of the triode (IC-Vbe) changes according to the exponential law, and the transfer characteristic of the field effect tube changes according to the square law, so The non-linear distortion of the field effect tube is larger than that of the triode;

8. Amplification ability: The important parameter that characterizes the amplifier ability of the transistor is the current amplification factor β, and the FET uses the transconductance to represent gm. Its value is small, the amplification ability is poor, and the voltage amplification factor is smaller than that of the transistor circuit;

9. The difference in flexibility: the emitter and collector of the triode cannot be interchanged, otherwise the β is extremely low and cannot work normally. For some specific conditions (the substrate is not connected to the source), the source and The drain is interchangeable. The gate voltage of the depletion-type field effect transistor can be positive or negative, with high flexibility; 10. Differences in connection methods: There are three connection methods for triodes: common emitter, common base, and common collection. Field effect transistors also have three connections: common source, common gate, and common drain connection; the two correspond; the phase relationship is the same;

11. Bias difference: the triode needs proper bias current to work normally, and the FET needs proper bias voltage when it works normally;

12. Discrimination difference: there are only two types of triode, and the judgment is relatively simple, field effect There are many types of tubes, and the discrimination is more complicated. The triode only needs to prevent the temperature from being too high during the welding process, and there is no problem of static electricity; the MOS tube must discharge the grid and source, otherwise the induced static electricity will easily break down the tube. The gate and source must be short-circuited during storage and transportation;

13. Capacitance difference: the capacitance between the transistors is larger, and the capacitance between the MOS transistors is lower; The coupling capacitance of the triode is large, the input resistance of the field effect tube is extremely high, and the coupling capacitance is small;

14. Other differences: both triodes and field effect transistors can be used as variable resistors and switching devices; the power consumption of triodes is higher, but it is cheaper; when only allowing less current (high output impedance) from the signal source, The field effect tube is selected to achieve the purpose of impedance matching, which is suitable for the front circuit with low noise and high input resistance; when the signal voltage is low and current is allowed, a triode is used. As a switching tube, the field effect tube has higher efficiency, and is mostly used in high-current, high-speed switching power supplies; field effect tubes should be used in situations where the ambient temperature changes greatly.