TRANSISTOR THEORY
PN junction
is comparable to a low-resistance circuit element because it passes a high
current for a given voltage. In turn, a reverse-biased PN junction is
comparable to a high-resistance circuit element. By using the Ohm's law
formula for power (P = I2R) and assuming current is held
constant, you can conclude that the power developed across a high resistance
is greater than that developed across a low resistance. Thus, if a crystal
were to contain two PN junctions (one forward-biased and the other
reverse-biased), a low-power signal could be injected into the
forward-biased junction and produce a high-power signal at the
reverse-biased junction. In this manner, a power gain would be obtained
across the crystal. This concept, which is merely an extension of the
material covered in chapter 1, is the basic theory behind how the transistor
amplifies. With this information fresh in your mind, let's proceed directly
to the NPN transistor.
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