ElectronicDevices and CircuitsEngineeringSciences 154

Basic BJT AmplifierConfigurations

There are plenty of texts around on basic electronics,so this is a very brief look at the three basic ways in which a bipolarjunction transistor (BJT) can be used. In each case, one terminal is commonto both the input and output signal. All the circuits shown here are withoutbias circuits and power supplies for clarity.

Common Emitter Configuration

Here the emitter terminal is common to both the inputand output signal. The arrangement is the same for a PNP transistor. Usedin 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 frequentlyfor RF applications, this stage has the following properties. Low inputimpedance, high output impedance, unity (or less) current gain and highvoltage gain.

Common Collector Configuration

This last configuration is also more commonly  knownas the emitter follower. This is because the input signal applied at thebase is "followed" quite closely at the emitter with a voltage gain closeto unity. The properties are a high input impedance, a very low outputimpedance, a unity (or less) voltage gain and a high current gain. Thiscircuit is also used extensively as a "buffer" converting impedances orfor feeding or driving long cables or low impedance loads.

 
 

TransistorConfiguration Comparison Chart
(see Sedra & Smith and "Detailed Analysis"below)

AMPLIFIER TYPE	COMMON BASE	COMMON EMITTER	COMMON EMITTER (Emitter Resistor)	COMMON COLLECTOR (Emitter Follower)
INPUT/OUTPUTPHASE RELATIONSHIP	0°	180°	180°	0°
VOLTAGEGAIN	HIGH	MEDIUM	MEDIUM	LOW
CURRENTGAIN	LOW a	MEDIUM	MEDIUM b	HIGH
POWERGAIN	LOW	HIGH	HIGH	MEDIUM
INPUTRESISTANCE	LOW	MEDIUM	MEDIUM	HIGH
OUTPUTRESISTANCE	HIGH	MEDIUM	MEDIUM	LOW

 

Detailed Analysis

Common or Grounded Emitter Amplifier(actual circuit configuration)

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 v_ofromthe last two nodal equations we find that

and if we substitute this expressioninto the first nodal equation we find that
 

Finally, substitutingthese two expressions into the second nodal equation we find the followingexpression for the voltage gain:

• When 
  
   this expression reduces to

• When 
  
   but 
  
   it reduces to

 

Common or Grounded Collector Amplifier(actual circuit configuration)

Again to analyze thisconfiguration, 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 nodalequation into the first we find the following expression for the voltagegain:

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

Therefore, the relatively lowoutput impedance is given by:

while the relatively highinput impedance is given by: