Transistor amplification principle

I. Current amplification principle of Transistor
Transistor (hereinafter referred to as Transistor) has two kinds of materials: GE tube and silicon tube. However, each type has two structure modes, the two most commonly used are silicon, the special transistor, and the special transistor, which have different power polarity, the working principle is the same. The following describes only the current amplification principle of the Special silicon tube.

Figure 1: Structure of the transistor (PNP)

Figure 1 shows the structure of an-inch diode. It is composed of two n-type semiconductors with a P-type semiconductor. As shown in the figure, the PN junction formed between the launch zone and the base zone is called the launch knot, the PN junction formed between the collector and the base zone is called the collector junction, and the three leads are called the emission pole E, the base pole B, and the Collector respectively. When the point B potential is higher than the point E potential, the emission junction is in the positive bias state, while the point C potential is in the reverse bias state when the point B potential is higher than the point B potential, the collector power EC is higher than the base power EBO.
When manufacturing a transistor, consciously make the majority of carrier concentrations in the emission zone greater than the base zone, while the base zone is very thin, and strictly control the impurity content, so that once the power is connected, due to the correct launch knot, the majority of carriers (electrons) in the launch zone and the majority of carriers (points of control) in the base zone can easily intercept and spread across the transmission structure to the inverse, however, because the concentration base of the former is greater than that of the latter, the current sent through is basically an electronic stream, which is called the emission pole current IE. Due to the thin base area and the reverse bias of the collector junction, most of the electrons injected into the base area enter the collector area to form the collector current IC, with only a few (1-10%) remaining) the electrons in the base zone are composite by holes in the base zone, and the holes in the composite base zone are re-supplemented by the base pole power EB, thus forming the base pole current Ibo according to the current continuity principle: ie = IB + IC this means that when a small IB is added to the base pole, a large IC can be obtained on the collector, which is called current amplification, IC and IB maintain a certain proportional relationship, that is, in the Beta 1 = IC/IB formula: Beta, which is called a DC magnification, the ratio of the variation of the collector current △ic to the variation of the base current △ib is: β = △ic/△ib type β, called the AC current magnification, due to the low frequency, the values of β1 and β are not much different, so sometimes for convenience, we do not strictly distinguish them. The Beta value is about dozens to one hundred. Transistor is a type of current amplifier. However, in actual use, the current amplification function of the transistor is often used to convert the resistor to the voltage amplification function.

Ii. Switching Characteristics of Transistor

1. Static features

The transistor consists of two PNDS: the collector junction and the transmitter junction. Based on the bias polarity of the two PN knots, the transistor has three operating States: Cut-off, zoom-in, and saturation. Figure 3.5 (a) and (B) respectively provide a simple circuit consisting of a transistor with a common emitter transistor of type and its output characteristic curve.

The circuit has the following features:

1). cutoff status: UB <0, two pn are reversed, IB ≈ 0, IC ≈ 0, uce ≈ UCC. The transistor presents a high impedance, similar to switching off.
2). Zoom in status: UB> 0, positive offset of the emission junction, reverse offset of the collector junction, and Ic = beta IB.
3 ). saturation status: UB> 0, both pnknots are positive, IB ≥ IBS (base critical saturation current) ≈ UCC/β RC, at this time Ic = ICS (collector saturation current) ≈ UCC/RC. The transistor shows low impedance, similar to switching on.

In a digital logic circuit, a transistor is used as a switching component to work in both saturation and cutoff states, which is equivalent to a non-contact switch controlled by the baseline signal, it corresponds to the "closed" and "disconnected" of the contact switch"
.
Figure 3.6 (a) and (B) provide the equivalent circuit of the circuit shown in Figure 3.5 in the transistor cutoff and saturation states.

Figure 3.5 crystal transistor circuit and Output Characteristic Curve

 

 
 

2. Dynamic Features

The dynamic characteristics of transistor are the characteristics of transistor in the saturation and cutoff state conversion process.

The switching process of the transistor is the same as that of the diode. The formation and disappearance of the charge also exists in the tube. Therefore, switching between the saturation and the cutoff states also takes some time to complete.
If an ideal rectangular wave voltage is input at the input end of the circuit shown in Figure 3.5 (A), then the waveform of the IC and UCE should be shown in 3.7 (a) ideally. However, as shown in waveform 3.7 (B) of the IC and UCE in the actual conversion process, there is a gradual change in either the turn-off or the turn-on process.

Figure 3.6 circuit equivalent of transistor cutoff and saturation
The characteristics of the crystal transistor in both the cutoff and saturation states are called the static switch characteristics of the transistor.

 

1. activation time

Activation time: the time required for the transistor from the cutoff state to the saturation state is the activation time.
When the transistor is in the cutoff state, the emission is reversed, and the space charge zone is relatively wide. When the UI of the input signal changes from-U1 to + U2, because the space charge area of the launch terminal is still kept at the end of the time, the electronics in the launch zone cannot immediately pass through the launch terminal to reach the base zone. At this time, the electrons in the launch area enter the space charge area, narrowing the space charge area, and then the launch area starts to emit electrons to the base area, and the transistor starts to turn on. The time required for this process is called the delay time TD.
After the delay (TD), the emission area continuously injects electrons into the base area. The electrons accumulate in the base area and spread to the collector area to form the collector current IC. With the increase of the base zone electronic concentration, the IC is increasing. The time required for the IC to rise to the maximum of 90% is called the time tr.

Activation time ton = TD + Tr

The length of activation time depends on the transistor structure and circuit operating conditions.

2. Close time

Closing time: the time required for the transistor to go from saturation to cutoff is called closing time.
After entering the saturation state, the collector's ability to collect electrons is weakened. Excess electrons accumulate in the base area, which is called excessive storage charge. At the same time, the collector area is close to the boundary and a certain number of holes are accumulated, the collector end is in a positive offset.
When the input voltage UI changes from + U2 to-u1, the storage charge cannot disappear immediately. Instead, it generates a drift motion under the reverse voltage to form a reverse base flow, prompting excessive storage charge to be discharged. Before the storage charge completely disappears, the collector current remains ICS unchanged until all the storage charge is dispersed, the transistor begins to exit the saturation state, and the IC begins to decline. The time required for this process is stored in ts.
After all the excess charge stored in the base area disappears, the electrons in the base area become fewer and fewer under the action of reverse voltage, and the collector current IC keeps decreasing, and gradually approaches zero. The time required to reduce the collector current from 0.9ics to 0.1ics is called the descent time tF.

Toff = TS + TF

Similarly, the length of shutdown time depends on the structure and application of the transistor.
The activation time ton and toff indicate the switching speed of the transistor from the cutoff time to the saturation time and from the saturation to the cutoff time. They are the main factors affecting the circuit working speed.