Video Transcript
For transistors connected in a
common emitter configuration, which of the following is true? (A) The current gain vanishes. (B) The current gain is larger than
one. (C) The current gain is less than
one but does not vanish. (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 the common emitter configuration. To help us with this question then,
let’s draw a diagram of a transistor in the common emitter configuration. That would look something like
this, for an npn transistor in such a configuration.
When we think about the direction
of conventional current in this circuit, we know that the direction of conventional
current is into the forward biased n-type doped semiconductor on the left. This means the n-type semiconductor
region on the left 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 the 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 the letter E.
Let’s now recall what we mean when
we talk about the gain of a transistor. For a common emitter circuit, this
proportion expresses the current gain of a transistor in the common emitter
configuration. Here, 𝛽 is the current gain, 𝐼 C
is the current into the collector, and 𝐼 B is the current into the base. We want to relate the collector
current and the base current because their magnitudes relative to each other will
determine the current gain.
If 𝐼 C is larger than 𝐼 B, then
it means that the current gain will be larger than one. If 𝐼 C is smaller, then the
current gain would have to be smaller than one. So which one is smaller, the base
current or the collector current? Well, regardless of the
configuration or construction of a transistor, the current into the base is much
smaller than either the collector or emitter current. This is because the base is lightly
doped, meaning it has a low electrical conductivity compared to the emitter and
collector, resulting in a smaller value of current.
This means that answer choice (B),
the current gain is larger than one, is correct. As long as the transistor is
correctly set, then we shouldn’t expect the current gain to vanish, as neither the
base current nor the collector current is zero. And since the base current isn’t
larger than the collector current, the current gain cannot be below one.
Since we have enough information to
work out the answer, we can eliminate answer option (D) too. Hence, we choose option (B) as our
final answer. The current gain is larger than
one.
