Question

Explain the working of CB configuration of a BJT and draw its input, output characteristics.

Answer 1

Common Base Configuration 

 In common base configuration, emitter is the input terminal, collector is the output terminal and base terminal is connected as a common terminal for both input and output. That means the emitter terminal and common base terminal are known as input terminals whereas the collector terminal and common base terminal are known as output terminals.  In common base configuration, the base terminal is grounded so the common base configuration is also known as grounded base configuration. Sometimes common base configuration is referred to as common base amplifier, CB amplifier, or CB configuration.  The input signal is applied between the emitter and base terminals while the corresponding output signal is taken across the collector and base terminals. Thus the base terminal of a transistor is common for both input and output terminals and hence it is named as common base configuration.

Input characteristics 

 The input characteristics describe the relationship between input current (IE) and the input voltage (VBE).  First, draw a vertical line and horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The input current or emitter current (IE) is taken along the y-axis (vertical line) and the input voltage (VBE)is taken along the x-axis (horizontal line).  To determine the input characteristics, the output voltage VCB (collector-base voltage) is kept constant at zero volts and the input voltage VBE is increased from zero volts to different voltage levels. For each voltage level of the input voltage (VBE), the input current (IE) is recorded on a paper or in any other form.  A curve is then drawn between input current IE and input voltage VBE at constant output voltage VCB (0 volts).  Next, the output voltage (VCB) is increased from zero volts to a certain voltage level (8 volts) and kept constant at 8 volts. While increasing the output voltage (VCB), the input voltage (VBE) is kept constant at zero volts. After we kept the output voltage (VCB) constant at 8 volts, the input voltage VBE is increased from zero volts to 

different voltage levels. For each voltage level of the input voltage (VBE), the input current (IE) is recorded on a paper or in any other form.

Output characteristics 

 The output characteristics describe the relationship between output current (IC) and the output voltage (VCB).  First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The output current or collector current (IC) is taken along the y-axis (vertical line) and the output voltage (VCB)is taken along the x-axis (horizontal line).  To determine the output characteristics, the input current or emitter current IE is kept constant at zero mA and the output voltage VCB is increased from zero volts to different voltage levels. For each voltage level of the output voltage VCB, the output current (IC) is recorded.  When the emitter current or input current IE is equal to 0 mA, the transistor operates in the cut-off region. 

 Next, the input current (IE) is increased from 0 mA to 1 mA by adjusting the input voltage VBE and the input current IE is kept constant at 1 mA. While increasing the input current IE, the output voltage VCB is kept constant.  After we kept the input current (IE) constant at 1 mA, the output voltage (VCB) is increased from zero volts to different voltage levels. For each voltage level of the output voltage (VCB), the output current (IC) is recorded