Common Collector (CC) Mode Configuration of BJT- Notes for Electronics Engineering 1st Year

Bipolar Junction Transistor (BJT):

A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. In an N-P-N transistor, a thin and lightly doped P-type base is sandwiched between a heavily doped N-type emitter and another N-type collector; while in a P-N-P transistor, a thin and lightly doped N-type base is sandwiched between a heavily doped P-type emitter and another P-type collector. In the following, we will only consider NPN BJTs.

Bipolar Junction Transistors Characteristics:

The three parts of a BJT are collector, emitter and base. Before knowing about the bipolar junction transistor characteristics, we have to know about the modes of operation for this type of transistors. The modes are

1. Common Base (CB) mode
2. Common Emitter (CE) mode
3. Common Collector (CC) mode

Common Collector (CC) Mode Configuration:

In this configuration, we use collector terminal as common for both input and output signals. This configuration is also known as emitter follower configuration because the emitter voltage follows the base voltage.

In this configuration the input signal is applied between the base-collector region and the output is taken from the emitter-collector region. Here the input parameters are VBC and IB and the output parameters are VEC and IE. The common collector configuration has high input impedance and low output impedance. The input and output signals are in phase. Here also the emitter current is equal to the sum of collector current and the base current. Now let us calculate the current gain for this configuration.

Now let us calculate the current gain for this configuration.

Current gain,

A_i = output current/Input current

A_i = I_E/I_B

A_i = (I_C + I_B)/I_B

A_i = (I_C/I_B) + 1

A_i = β + 1

The common collector transistor circuit is shown above. This common collector configuration is a non-inverting amplifier circuit. The voltage gain for this circuit is less than unity but it has large current gain because the load resistor in this circuit receives both the collector and base currents.

Input Characteristics:

The input characteristics of a common collector configuration are quite different from the common base and common emitter configurations because the input voltage V_BC is largely determined by V_EC level. Here,

V_EC = V_EB + V_BC

V_EB = V_EC – V_BC

The input characteristics of a common-collector configuration are obtained between inputs current I_B and the input voltage V_CB at constant output voltage V_EC. Keep the output voltage V_EC constant at different levels and vary the input voltage V_BC for different points and record the I_B values for each point. Now using these values we need to draw a graph between the parameters of V_BC and I_B at constant V_EC.

Output Characteristics:

The operation of the common collector circuit is same as that of common emitter circuit. The output characteristics of a common collector circuit are obtained between the output voltage V_EC and output current I_E at constant input current I_B. In the operation of common collector circuit if the base current is zero then the emitter current also becomes zero. As a result no current flows through the transistor

If the base current increases then the transistor operates in active region and finally reaches to saturation region. To plot the graph first we keep the I_B at constant value and we will vary the V_EC value for various points, now we need to record the value of I_E for each point. Repeat the same process for different I_B values.

Now using these values we need to plot the graph between the parameters of I_E and V_CE at constant values of I_B. The below figure show the output characteristics of common collector.

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