A bipolar transistor is a three-terminal device (Figure 1), in which a small current applied to the base controls a much larger current flowing between the collector and emitter. It is available in two flavors (npn and pnp), with properties that meet the following rules for npn transistors (for pnp simply reverse all polarities):
where β , the current gain (sometimes called hFE), is typically about 100. Both IB and IC flow to the emitter. Note: the collector current is not due to forward conduction of the base–collector diode; that diode is reverse-biased. Just think of it as “transistor action.”
Rule 4 gives the transistor its usefulness: a small current flowing into the base controls a much larger current flowing into the collector. An important warning: the current gain β is not a “good” transistor parameter; for instance, its value can vary
from 50 to 250 for different specimens of a given transistor type. It also depends on the collector current, collector-to-emitter voltage, and temperature. A circuit that depends on a particular value for beta is a bad circuit.
Note particularly the effect of rule 2. This means you can’t go sticking an arbitrary voltage across the base–emitter terminals, because an enormous current will flow
if the base is more positive than the emitter by more than about 0.6 to 0.8V (forward diode drop). This rule also implies that an operating transistor has VB ≈ VE +0.6V (VB = VE +VBE). Again, polarities are normally given for npn transistors; reverse them for pnp. Let us emphasize again that you should not try to think of the collector current as diode conduction.
It isn’t, because the collector–base diode normally has voltages applied across it in the reverse direction. Furthermore, collector current varies very little with collector voltage (it behaves like a not-too-great current source), unlike forward diode conduction, in which the current rises very rapidly with applied voltage.
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