What are CE CB CC transistor configurations?

There are three types of transistor configurations CB (common base), CE (common emitter), and CC (common collector). In the below section we will learn in detail about the CE CB CC transistor configurations along with their input and output characteristics.

Common Base (CB)

The notation and symbols of pnp and npn transistors are given below:

• Here the base is common to both the input and output sides of the configuration.
• The flow of holes will govern the direction of current.
• Hence, Ic = Ib + Ie
• Where Ic, Ib, Ie are the collector, base and emitter currents respectively.
• The graphical symbol of the PNP common base configuration is 

• The arrow in the above symbol shows the direction of emitter current in the device.
• Now, to study the behavior of the device we require two characteristics:

Input Characteristic Curve of CB

• It is the relation between the input current I_E to the input voltage V_BE for various levels of output voltage V_CB.
• It is also known as driving point characteristics.

Output Characteristic Curve of CB

• It is also the relation between the output current I_C to the output voltage V_CB for various levels of input current I_E.
• Also known as the collector set of characteristics.
• Therefore, it has three basic regions:

Active Region

• Here, the base-emitter junction is forward biased and collector-base junction is reverse biased.
• Since the input current I_E increases above zero, output current I_C increases to a magnitude equal to I_E as determined by the basic transistor current relationship.
• So, the first approximation determined by the curve is

                                       I_C ≈ I_E

Cut-off Region

• It is defined as the region where the collector current I_C is equal to 0A.
• Here, the base-emitter junction and the collector-base junction both are in reverse bias.

Saturation Region

• It is the region that lies towards the left of V_CB = 0V.
• Since the base-emitter junction and the collector-base junction both are in forward bias.

Common Emitter Configuration (CE)

• The notation and symbols of npn and pnp transistors are given below:

• In the above figure all the currents are shown in their actual conventional directions.
• The current relation developed earlier is still applicable,
•                I_E = I_B + I_C
• Where I_E , I_B , I_C are the collector, base and emitter currents respectively.
• The graphical symbol of the PNP common emitter configuration is 

• Now, to study the behavior of the device we require two characteristics:

Input Characteristic Curve

• It is the graph between the input current I_B to the input voltage V_BE for a range of values of output voltage V_CE.
• Note that the magnitude I_B of is in micro amperes and that of I_C is in milli amperes.

Output Characteristic Curve

• It is the graph between the output current I_C to the output voltage V_CE for a range of values of input current I_B.
• The three basic regions are:

Active Region

• Here, the base-emitter junction is forward biased and collector base junction is reverse biased.
• These are the same conditions that existed in the active region of the common base configuration.
• This can also be employed for voltage, current or power amplification.

Cut-off Region

• Here I_C is not equal to zero when I_B is zero.
• For linear amplification purposes, it is defined as I_C = I_CEO.
• The region below I_B = 0µA is to be avoided for undistorted output signals.
• When the transistor is used as a switch, the condition should be ideally I_C = 0mA for a chosen V_CE voltage.

Saturation Region

• It is the region that lies towards the left of V_CE = 0V.

Common Collector Configuration (CC)

The notation and symbols of npn and pnp transistors are given below:

• In the above figure all the currents are shown in their actual conventional directions.
• It is used for impedance matching purposes as it has high input impedance and low output impedance.
• It can also be designed using common emitter characteristics.
• Then output characteristics of a common collector is the same as that of common emitter configuration for all practical purposes.
• Then output characteristics are a plot between  I_E versus V_CE for all values of I_B.
• The input current of the common collector is also the same as that of common emitter configuration.
• Here the region of operation will ensure that maximum ratings are not being exceeded and also output ratings have minimum distortion.

• The characteristics specifying the minimum V_CE that can also be applied without entering the non-linear region is the saturation region.
• Then maximum power dissipation is given by,

P = V_CE. I_C