Video Transcript
For transistors connected in a
common base configuration, which of the following is true? (A) The current gain vanishes. (B) The current gain is greater
than one. (C) The current gain is less than
one but does not vanish. And (D) there is not enough
information to determine the answer.
Looking at these answers, it
appears that this question is asking us about the current gain of transistors
connected in common base configuration.
Let’s draw a diagram of a
transistor in the common base configuration. That would look something like
this. For an npn transistor in such a
configuration, when we think about conventional current, we know that the direction
of conventional current is into the forward-biased n-type doped semiconductor on the
left, which means that it is the collector, which we denote using the letter C in
this transistor. The centermost p-type semiconductor
must then be the base, indicated with letter B. And the direction of conventional
current is out from the rightmost n-type semiconductor, meaning it must be the
emitter, which we label with letter E.
We now know the direction of
current through all parts of the circuit. The reason this configuration is
called common base, or CB, is because the base, which in this case is the p-type
semiconductor, is connected as a common terminal for both input and output. We have to be careful when we think
about these words. Input does not mean where the
electric charge enters the transistor, the collector, nor does output refer to where
the electric charge exits, the emitter. Instead, when we talk about input
and output, we’re referring to how we connect this transistor to a circuit.
When connecting this transistor to
a circuit for a common base configuration, we’ll attach the emitter as an input,
meaning an input signal will be on that side. And the collector will be attached
as an output, meaning a signal will be output on that side. The base, meanwhile, is attached as
both an input and output or grounded.
All of this means that the input
current of the transistor will be 𝐼 sub E, the emitter current, and the output
current will be 𝐼 sub C, the collector current. The reason we’ve talked about input
and output here is because current gain is calculated as a proportion between the
input and output currents. Specifically, it is the proportion
between the output current over the input current. And since we already know our input
and output currents as being 𝐼 sub E and 𝐼 sub C, we can then substitute them into
this equation.
Now that we have this, we just need
to determine our relative values between 𝐼 sub C and 𝐼 sub E, since we’ve not been
given specific values. Let’s recall how the currents in
all three parts of a transistor are related. The emitter current is equal to the
sum of the collector current and the base current. Since we know that the current that
leads into the base, 𝐼 sub B, is a nonzero value, this must mean that the collector
current is smaller than the emitter current. Thinking about this for the current
gain equation, if the collector current is smaller than the emitter current, then it
means that the current gain would have to be a number that is lower than one. And in fact, this specific
proportion for the current gain in CB circuits has its own character, an 𝛼
symbol. This proportion is constant for a
given transistor.
So, knowing all of this, it seems
that answer choice (C), the current gain is less than one but does not vanish, is
correct. When the answer options refer to
the current vanishing, what they mean is that the current gain is so small that we
can ignore it or is zero. As long as the transistor is
correctly set, then we shouldn’t expect the current gain to vanish, as the collector
current will be smaller than the emitter current but won’t hit zero. And since the emitter current can’t
be larger than the collector current, the current gain cannot be above one. This means the correct answer to
“which of the following statements is true?” is answer choice (C).
