The difference between bipolar junction transistor (BJT) and field-effect transistor (FET)
There are many differences between bipolar junction transistors (BJTs) and field-effect transistors (FETs). Here are a few simple examples:
1. Transistor current control, MOS transistor belongs to voltage control, BJT amplifies current, FET converts gate voltage into drain current. The first parameter of BJT is the current amplification factor β value, and the first parameter of FET is the breakdown gm;
2. Driving capability: MOS transistors are commonly used power switch transistors, as well as high current local switch circuits;
3. Cost issue: Transistors are cheap, MOSFETs are expensive;
4. BJT has poor linearity, while FET has good linearity;
5. BJT noise is relatively high, while FET noise is relatively low;
6. BJT polarities are only available in NPN and PNP types, while FET polarities include N-channel, P-channel, depletion, and enhancement types, making FET selection and use more complex;
7. Power consumption issue: BJT has a small input resistance and consumes a large amount of current, while FET has a large input resistance and consumes almost no current;
In fact, the transistor is relatively cheap and easy to use, commonly used in digital circuit switch control; MOS transistors are used in high-frequency and high-speed circuits, high current applications, and places where the base or drain control current is sensitive.
Overall, the replacement of BJT with FET is a major trend in both discrete component circuits and integrated circuits.
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