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

According to modern manufacturing technology, three doping regions are manufactured 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 its ability to amplify signals. It is the core component of the amplifier circuit, able to control the conversion of energy and amplify any tiny changes in the input without distortion.

The following are some issues we need to pay attention to when using transistors in circuit design:

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

This circuit is common in various analog circuit textbooks in China and is 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 the ground. At this time, the voltage gain of the common emitter is:

b. When the input signal frequency is relatively low, XC will be much greater than zero, which is equivalent to an open circuit. At this time, the voltage gain of the common emitter 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 capacitors of a certain value inside the transistor. When the input signal frequency reaches a certain level, the amplification effect of the transistor will be greatly reduced. What's worse, it will also cause additional phase difference.

Due to the existence of Cbe, 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 this feedback signal has a phase difference of 180° with 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 understand 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 collector and emitter, collector and base, and 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 try to reduce CL as much as possible, so that fH can be higher. First, 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 using a transistor as a switch, pay attention to its reliability:

Like a diode, the emitter junction of a transistor also has a turn-on voltage of about 0.7V. When the transistor is used as a switch, the input signal may cause the transistor to conduct when it is at a low level (0.7V

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 to have a negative potential, so that the transistor can be reliably cut off, and the collector will output the high level we want.

(5) It is necessary to accept the fact that the switching speed of transistors 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:

One of the biggest advantages of the emitter follower is that it has a high input impedance, so its load carrying capacity is also enhanced. However, you still need to understand its principle in the process of application, otherwise it may cause unexpected "sources of problems". The following is an introduction to its principle. For this circuit, there is the following equation:

From this we can see 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 "a glimpse of the whole picture", because there are too many precautions in the use of triodes, which cannot be covered in a blog post. Moreover, it is not easy to master the triode as a device. However, if we consciously continue to experience and summarize in practice, triodes will also be familiar to us.