common base configuration
Problem:
The base of a pnp bipolar transistor is grounded. A battery is connected between the
emitter and the base. Another battery is connected between the base and the collector.
This is known as the common base configuration.
(a) Draw the circuit indicating the polarities of the batteries that would put the
transistor in the forward active mode. Explain why you have chosen these polarities.
(b) Plot the minority charge density in the emitter, base, and collector indicating
whether electrons or holes are the minority charge carriers.
(c) Why is the emitter more heavily doped than the collector?
(d) How do the carriers that are emitted into the base reach the collector?
Solution:
a)
E
B
C
p+
n
p
+
-
Veb
Vbc
-
+
If the transistor is put in forward active mode the emitter base pn-junction is forward
biased and the collector base pn-junction reverse biased. With this configuration
minority carriers get injected into the base and because of the electric field of the
reverse biased collector-base pn-junction they get swept into the collector.
The next pictures show a simulation in pspice with a sweep of the collector base voltage
with four different emitter base voltages.
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Circuit:
Simulation:
b)
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c)
If the emitter is more heavily doped than the base, the amount of injected electrons into
the emitter is very small compared to the injected holes into the base. So the forward
bias current is mostly made up of injection of holes into the base. As a reason of this
almost the whole current gets swept to the collector and the collector current can be
controlled with a small base current.
The collector is more lightly doped than the base to avoid punchthrough. This means
that the whole base would be depleted and there won’t be the desired gain anymore.
The lightly doped collector ensures that most of the BC-depletion width is in the
collector.
d)
Because the base width is shorter than the diffusion length the injected holes don’t
recombine with electrons. They get swept to the collector by the electric field of the
reverse biased base-collector pn-junction.
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