Electronic Devices and Circuits Engineering Sciences 154

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	0°	180°	180°	0°
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

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 v_o 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,  v_x , 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: