Electronic Devices and Circuits
Engineering Sciences 154

Basic BJT Amplifier Configurations

There are plenty of texts around on basic electronics, so this is a very brief look at the three basic ways in which a bipolar junction transistor (BJT) can be used. In each case, one terminal is common to both the input and output signal. All the circuits shown here are without bias circuits and power supplies for clarity.

Common Emitter Configuration


Here the emitter terminal is common to both the input and output signal. The arrangement is the same for a PNP transistor. Used in this way the transistor has the advantages of a medium input impedance, medium output impedance, high voltage gain and high current gain.

 

Common Base Configuration


Here the base is the common terminal. Used frequently for RF applications, this stage has the following properties. Low input impedance, high output impedance, unity (or less) current gain and high voltage gain.

Common Collector Configuration


This last configuration is also more commonly  known as the emitter follower. This is because the input signal applied at the base is "followed" quite closely at the emitter with a voltage gain close to unity. The properties are a high input impedance, a very low output impedance, a unity (or less) voltage gain and a high current gain. This circuit is also used extensively as a "buffer" converting impedances or for feeding or driving long cables or low impedance loads.


 
 
Transistor Configuration Comparison Chart
(see Sedra & Smith and "Detailed Analysis" below)
AMPLIFIER TYPE
  COMMON BASE 
  COMMON EMITTER 
  COMMON EMITTER
(Emitter Resistor) 
  COMMON COLLECTOR
(Emitter Follower)
    INPUT/OUTPUT PHASE RELATIONSHIP
180°
180°
VOLTAGE GAIN
HIGH

MEDIUM

MEDIUM

LOW

CURRENT GAIN
LOW

a

MEDIUM

MEDIUM

b

HIGH

POWER GAIN
LOW
HIGH
HIGH
MEDIUM
INPUT RESISTANCE
LOW

MEDIUM

MEDIUM

HIGH

OUTPUT RESISTANCE
HIGH

MEDIUM

MEDIUM

LOW

 

Detailed Analysis

Common or Grounded Emitter Amplifier (actual circuit configuration)
CE Amplifier Small-Signal Equivalent Circuit
To analyze this configuration, we first set down the complete nodal equations:
Using the relationship , the nodal equations can be rewrite in a more homogeneous form:

Eliminating vo from the last two nodal equations we find that

and if we substitute this expression into the first nodal equation we find that
 

Finally, substituting these two expressions into the second nodal equation we find the following expression for the voltage gain:


  • When   this expression reduces to


  • When   but   it reduces to


 
Common or Grounded Collector Amplifier (actual circuit configuration)
CE ("Emitter-Follower") Amplifier Small-Signal Equivalent Circuit
Again to analyze this configuration, we first set down the complete nodal equations:
Again using the relationship , the nodal equations can be rewrite in a more homogeneous form:


Substituting the second nodal equation into the first we find the following expression for the voltage gain:

 
A "trickly" calculation is required to obtain the output impedance.  To do so we first shut off the input voltage and then apply test voltage source,  vx , to the output terminal.  Under these circumstances, the current into the output terminal is given by:

Therefore, the relatively low output impedance is given by:

while the relatively high input impedance is given by:


This page was prepared and is maintained by R. Victor Jones
Comments to: jones@deas.harvard.edu.

Last updated November 7,  2001