A bipolar junction transistor (BJT) can be in three modes:
Transistor acts like an open switch between collector and emitter (i.e.,
collector–emitter “resistance” is infinite).
Transistor acts like a dynamic resistor between collector and emitter that
adjusts its resistance in order to keep collector current at a set level (i.e.,
collector–emitter resistance is finite and positive).
Transistor acts like a closed switch between collector and emitter (i.e.,
collector–emitter “resistance” is very low).
In the active mode, the transistor adjusts the collector current to be a version of the base current amplified
by some constant > 0. If the base current falls to 0, the transistor enters cutoff mode and shuts off. When
the base current rises too far, the transistor loses its ability to decrease the collector–emitter resistance
to linearly increase the collector current. In this case, the transistor enters saturation mode. To keep the
transistor out of saturation mode, the collector and emitter should be separated by at least 0.2V.
A simple model for the operation of a transistor in active mode is shown in Figure. It requires knowing
the current gain in order to design the circuit. In both of these models,
iC = iB and iE = ( + 1)iB,
and the emitter is separated from the base by a diode. In order for this diode to conduct current, it must
be forward biased with 0.65V1.
Transistor acts like an open switch between collector and emitter (i.e.,
collector–emitter “resistance” is infinite).
Transistor acts like a dynamic resistor between collector and emitter that
adjusts its resistance in order to keep collector current at a set level (i.e.,
collector–emitter resistance is finite and positive).
Transistor acts like a closed switch between collector and emitter (i.e.,
collector–emitter “resistance” is very low).
In the active mode, the transistor adjusts the collector current to be a version of the base current amplified
by some constant > 0. If the base current falls to 0, the transistor enters cutoff mode and shuts off. When
the base current rises too far, the transistor loses its ability to decrease the collector–emitter resistance
to linearly increase the collector current. In this case, the transistor enters saturation mode. To keep the
transistor out of saturation mode, the collector and emitter should be separated by at least 0.2V.
A simple model for the operation of a transistor in active mode is shown in Figure. It requires knowing
the current gain in order to design the circuit. In both of these models,
iC = iB and iE = ( + 1)iB,
and the emitter is separated from the base by a diode. In order for this diode to conduct current, it must
be forward biased with 0.65V1.
