CE EMITTER-BIAS and EMIITER FOLLOWER CONFIGURATION

CE EMITTER-BIAS CONFIGURATION
The networks examined in this section include an emitter resistor that may or may
not be bypassed in the ac domain. We will first consider the unbypassed situation and
then modify the resulting equations for the bypassed configuration.
Unbypassed
The most fundamental of unbypassed configurations appears in Fig. 8.10. The re
equivalent model is substituted in Fig. 8.11, but note the absence of the resistance ro.
The effect of ro is to make the analysis a great deal more complicated, and considering
the fact that in most situations its effect can be ignored, it will not be included in
the current analysis. However, the effect of ro will be discussed later in this section.
Applying Kirchhoff’s voltage law to the input side of Fig. 8.11 will result in







EMITTER-FOLLOWER CONFIGURATION
When the output is taken from the emitter terminal of the transistor as shown in Fig.
8.17, the network is referred to as an emitter-follower. The output voltage is always
slightly less than the input signal due to the drop from base to emitter, but the ap-

























proximation Av 1 is usually a good one. Unlike the collector voltage, the emitter
voltage is in phase with the signal Vi. That is, both Vo and Vi will attain their positive
and negative peak values at the same time. The fact that Vo “follows” the magnitude
of Vi with an in-phase relationship accounts for the terminology emitterfollower.
The most common emitter-follower configuration appears in Fig. 8.17. In fact, because
the collector is grounded for ac analysis, it is actually a common-collector configuration.
Other variations of Fig. 8.17 that draw the output off the emitter with Vo
Vi will appear later in this section.
The emitter-follower configuration is frequently used for impedance-matching purposes.
It presents a high impedance at the input and a low impedance at the output,
which is the direct opposite of the standard fixed-bias configuration. The resulting effect
is much the same as that obtained with a transformer, where a load is matched
to the source impedance for maximum power transfer through the system.
Substituting the re equivalent circuit into the network of Fig. 8.17 will result in
the network of Fig. 8.18. The effect of ro will be examined later in the section.