This section introduces the characteristics of the transistor in a clear and easy-to-understand manner, using a popular water flow model to enhance memory of the principles of the transistor. This method of learning by analogy is called analogical learning and is used by foreign scientists to simplify the learning, understanding, and memory of complex concepts such as gravitational distortion of space-time in the theory of relativity.
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| Figure 1-26 NPN and PNP BJT |
The transistor we commonly refer to is the bipolar junction transistor (BJT), which has two PN junctions. The circuit symbols for NPN and PNP transistors are shown in Figure 1-26. They have three pins: the base (B), the collector (C), and the emitter (E). To determine whether a transistor is NPN or PNP, remember that the arrow in the symbol always points from P to N. In the left symbol, the arrow starts from P and ends at N, so P is in the middle of the transistor, making it an NPN transistor. In the right symbol, the arrow starts from P and ends at N, so N is in the middle of the transistor, making it a PNP transistor.
This section focuses on the characteristics
of the NPN transistor. Figure 1-27 shows the output characteristic curve of an
NPN transistor, with VCE as the horizontal axis and IC as the vertical axis.
These curves describe the relationship between the base current (IB), the
collector current (IC), and the voltage between the collector and emitter
(VCE). The transistor has three operating regions: saturation, amplification,
and cutoff.
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| Figure 1-27 Output characteristic curve of an NPN |
1.Characteristics of the saturation region:
The current flowing through the transistor (IC) depends on both IB and VCE.
When VCE is constant, a change in IB has little effect on IC. However, when IB
is fixed, a slight change in VCE can cause a dramatic change in IC. In other
words, the transistor is saturated, meaning that it is full. We can use the
analogy of pouring water into a cup to remember this process, as shown in
Figure 1-28. IB is the flow of water from the tap, IC is the water level, and
VCE is the height of the cup. When the cup is full (i.e., the transistor is
saturated), the water level (IC) is no longer controlled by IB, but by the
height of the cup (VCE). To further increase IC, the height of the cup (VCE)
must be increased. This analogy makes it easier to understand and remember the
concept of saturation.
2.Characteristics of the amplification region:
As IB increases, IC also increases. IC is mainly controlled by IB and
is not strongly affected by VCE. As shown in Figure 1-27, when VCE increases in
the amplification region, IC remains relatively constant, while increasing IB
causes IC to increase. This process can also be easily understood using the
water cup model. In the cup representing the amplification region, the height
of the water (IC) is only controlled by the flow of water from the tap (IB).
This is why we call the transistor a current-controlled device.
3.Characteristics of the cutoff region:
When
IB is zero or close to zero, IC is also zero, regardless of the value of VCE.
Using the water cup model, this can be understood as follows: when the flow of
water from the tap (IB) is very small, close to zero, the water level (IC) in
the cup is also close to zero, regardless of the height of the cup (VCE).
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| Figure 1-28 Equivalent Model of Current-Controlled Transistor |
When the current IC is not large,
transistors are often used as amplifiers or switches. When large currents are
required, MOS transistors are often used as switches. The above is an introduction
to the relevant characteristics of transistors. Understanding and remembering
can be much easier when combined with the equivalent model of
current-controlled transistors.
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