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BJT common Base

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Whites, EE 320

Lecture 20

Page 1 of 5

Lecture 20: Common Base Amplifier.
We will cover the second of the three families of BJT amplifiers
in this lecture by discussing the common base amplifier shown
in Fig. 5.62a:

(Fig. 5.62a)
The small-signal equivalent circuit for this amplifier is shown in
Fig. 5.62b (ignoring ro):

(Fig. 5.62b)
© 2009 Keith W. Whites

Whites, EE 320

Lecture 20

Page 2 of 5

As before, let’s determine the small-signal AC characteristics of
this amplifier by solving or Rin, Gv, Gi, Ais, and Rout.
• Input resistance, Rin. From direct inspection of the smallsignal equivalent circuit, we see that
(5.137),(1)
Rin = re
Since re is often small (on the order of 20 to 30 Ω), then Rin of
the CB amplifier is very small. Generally this is not desirable,
though in the case of certain high frequency amplifiers input
impedances near 50 Ω is very useful (to reduce so-called
“mismatch reflections” at the input).
• Small-signal voltage gain, Gv. We’ll first calculate the partial
voltage gain
v
(2)
Av ≡ o
vi
At the output,
vo = −α ie ( RC || RL )
(3)
The small-signal emitter current is
−v
(4)
ie = −ii = i
re
Substituting (3) and (4) into (2) gives the partial voltage gain
to be

Av =

α
re

( RC || RL ) = g m ( RC || RL )

(5.138),(5)

Whites, EE 320

Lecture 20

Page 3 of 5

This is the same gain as for the CE amplifier (without ro),
except the gain here for the CB amplifier is positive.
The overall (from the input to the output) small-signal voltage
gain Gv is defined as
v
Gv ≡ o
(6)
vsig
We can equivalently write this voltage gain as
v v
vi
Gv = i ⋅ o =
Av
N
vsig vi ( 2) vsig

(7)

with Av given in (5).
By simple voltage division at the input to the small-signal
equivalent circuit
Rin
(8)
vi =
vsig
Rin + Rsig
Substituting this result and (5) into (7) yields the final
expression for the overall small-signal voltage gain
α ( RC || RL ) Rin
(9)
Gv =
re
Rin + Rsig
Since from (1) Rin = re then Gv simplifies to
α ( RC || RL )
Gv =
re + Rsig

(5.142),(10)

If α ≈ 1, we can interpret this small-signal overall voltage
expression in (10) as the ratio of the total resistance in the
collector lead to the total resistance in the emitter lead. This

Whites, EE 320

Lecture 20

Page 4 of 5

gain can be fairly large, though if Rsig is nearly the same size
as the total emitter resistance the gain will be small. In other
words, if this amplifier is connected to a high output
impedance stage, it will be difficult to realize high gain.
• Overall small-signal curr...
Whites, EE 320 Lecture 20 Page 1 of 5
© 2009 Keith W. Whites
Lecture 20: Common Base Amplifier.
We will cover the second of the three families of BJT amplifiers
in this lecture
by discussing the common base amplifier shown
in Fig. 5.62a:
(Fig. 5.62a)
The small-signal equivalent circuit for this amplifier is shown in
Fig. 5.62b (ignoring r
o
):
(Fig. 5.62b)
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