We will cover the second of the three families of BJT amplifiers in this lecture by discussing theCommon_Base_Amplifier shown in Fig. 5.62a:
The small-signal equivalent circuit for this amplifier is shown in Fig. 5.62b (ignoring ro):
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
Since re is often small (on the order of 20 to 30bita), 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 bita 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
At the output,
The small-signal emitter current is
Substituting (3) and (4) into (2) gives the partial voltage gain to be
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
We can equivalently write this voltage gain as
with Av given in (5). By simple voltage division at the input to the small-signal equivalent circuit
Substituting this result and (5) into (7) yields the final
expression for the overall small-signal voltage gain
Since from (1) Rin= re then Gv simplifies to
If 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
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 current gain, Gi. By definition
Using current division at the output of the small-signal
equivalent circuit above
Because ii = -ie this expression gives
• Short circuit current gain, Ais. In the case of a short circuit
load (RL = 0), Gi in (13) reduces to the short circuit current
• Output resistance, Rout. Referring to the small-signal
equivalent circuit above and shorting out the input vsig = 0
which is the same as the CE amplifier (when ignoring ro).
Summary of the CB small-signal amplifier:
1. Low input resistance.
2. Gv can be very large, though critically dependent on Rsig.
3. Ais= á
4. Potentially large output resistance (dependent on RC).
One very important use of the CB amplifier is as a unity-gain current amplifier, which is also called a current buffer amplifier. This type of amplifier accepts an input signal current at a low impedance level and outputs nearly the same current amplitude, but at a high output impedance level. Even though this is a buffer amplifier, there is still power gain.