Several issues that need to be paid attention to when using triodes

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Several issues that need to be paid attention to when using triodes [Copy link]

As the name implies, a triode has three electrodes. A diode is made up of a PN structure, while a triode is made up of two PN structures. The shared electrode becomes the base of the triode (indicated by the letter b). The other two electrodes become the collector (indicated by the letter c) and the emitter (indicated by the letter e). Due to different combinations, one is an NPN type triode and the other is a PNP type triode. It is a semiconductor device that controls current. Its function is to amplify weak signals into electrical signals with larger amplitudes, and it is also used as a contactless switch.

The triode is just a general term for amplifier devices with three pins in the Chinese meaning, which we often call a triode. According to modern manufacturing technology, three doping regions are created on the same silicon wafer according to different doping methods, and two PN junctions are formed, thus forming a transistor. The biggest advantage of the transistor is that it can amplify signals. It is the core component of the amplifier circuit, which can control the conversion of energy and amplify any small changes in the input without distortion.

The following are some issues we need to pay attention to when using transistors in circuit design. Let’s ——“look at the pictures and talk”:

(1) Pay attention to the effect of bypass capacitor on voltage gain:

This circuit is commonplace in various analog circuit textbooks in China, and it can be considered a classic. Due to the existence of this bypass capacitor, different situations will occur in different frequency environments: a. When the input signal frequency is high enough, XC will be close to zero, that is, the emitter is short-circuited to ground. At this time, the voltage gain of the common emitter is: 383933 b. When the input signal frequency is high enough, XC will be close to zero, that is, the emitter is short-circuited to ground. 51)]When the rate is relatively low,XC will be much greater than zero, which is equivalent to an open circuit. At this time, the common emitter voltage gain is:

From this we can see that when designing circuits using transistors, it is necessary to consider the impact of bypass capacitors on voltage gain. (2) Pay attention to the influence of the junction capacitance inside the transistor: Due to the semiconductor manufacturing process, there will inevitably be junction capacitance of a certain value inside the transistor. When the input signal frequency reaches a certain level, they will greatly reduce the amplification effect of the transistor. What's worse, it will also cause additional phase difference.

Due to the existence ofCbe, the internal resistance RS of the input signal source and XCbe form a little-known voltage divider, which can also be regarded as an LPF. When the frequency of the input signal is too high, the potential of the base of the transistor will decrease, and the voltage gain will decrease accordingly.

Due to the existence of Cbc, when the frequency of the input signal is too high, part of Vout will be fed back to the base through Cbc. Because there is a 180° phase difference between the feedback signal and the input signal, this will also reduce the potential of the base, and the voltage gain will also decrease.

(3) It is necessary to clearly grasp the cutoff frequency of the transistor:

This circuit diagram is an equivalent diagram, in which CL is the equivalent capacitance of the junction capacitance between the collector to the emitter, the collector to the base, and the load capacitance.

When the frequency of the input signal reaches , the gain of the transistor begins to decrease rapidly. In order to solve this problem well, we have to spend time to reduce CL as much as possible, so that fH can be higher. First of all, we can deliberately choose a transistor with a smaller inter-electrode capacitance value when designing the circuit, which is usually called an RF transistor; we can also reduce the value of RL, but this will come at a price: the voltage gain will decrease.

(4) When the transistor is used as a switch, you need to pay attention to its reliability:

Like a diode, the emitter junction of the transistor will also have a turn-on voltage of about 0.7V. When the transistor is used as a switch, the input signal may be at a low level (0.7V

The following figure is a solution to this problem:

Here, since a negative power supply VEE is artificially connected to the base, even if the low level of the input signal is slightly greater than zero, the base of the transistor can be made negative, so that the transistor is reliably cut off, and the collector will output the high level we want.

(5) We need to accept a fact: the switching speed of the transistor is generally not satisfactory.

As mentioned above, the existence of the internal junction capacitance of the device greatly limits the switching speed of the transistor, but we can still think of some ways to effectively improve its shortcomings. The following figure provides a practical method:

It can be seen from the figure that when the rise time of the input signal is very small (the signal frequency is very high), that is, dV/dt is very large, then ZC is very small, and as a result, Ib is very large, so that the transistor can be quickly saturated or cut off, which naturally increases the switching speed of the transistor. (6) You should understand the principle of emitter follower: 383943 One of the biggest advantages of emitter follower is that its input impedance is very high, so its load capacity is also enhanced. However, you still need to understand its principle during use, otherwise it may cause unexpected "source of problem". The following is an introduction to its principle. For this circuit, there is the following equation:

From this, it can be seen that the load impedance connected to the emitter looks like a very large impedance value at the base, and the load is easily driven by the signal source.

This blog post mainly uses the common emitter circuit as an example to illustrate the problem. The above-mentioned issues can only be regarded as "seeing the leopard in the tube", because there are too many precautions for the use of triodes, which cannot be covered in one blog post. Moreover, it is not easy to master the triode device well. However, if we consciously experience and summarize it in practice, triodes will also be familiar to us.

maychang

The circuit in the text and the picture does not correspond, and the circuit is wrong. Suggest modification.

qwqwqw2088

The following pictures are repeated with the text.

bioger

The article is good, but the text and pictures sometimes don't match.

szjlcw