Question
Sun February 06, 2011 By: Sruthisreenivasan

can you please explain the graphs for input and output characteristic obtained by the common emitter configuration for npn transistor

Expert Reply
Tue February 15, 2011
Dear student,
 
 

 

Input Characteristics

  • Input characteristics are found by considering the base loop.



    A graph of the base current IB versus VBE, which is the voltage between the base and the emitter, looks like that of an ordinary diode. The current is zero until VBE reaches 0.7 volts, where it then increases very suddenly.



    The base voltage forward biases the emitter. The equation to find the voltage across the resistor RB is VBB -- VBE, where VBB is the base voltage. The current IB is found using VBB -- VBE / RB.
  • Output Characteristics

  • Output characteristics are found by considering the collector loop.



    A graph of the collector current IC versus the collector-emitter voltage VCE shows much the same shape for different transistors, though the numbers will be different. When VCE is zero, so is IC. As VCE increases, IC will remain zero and then suddenly shoot up when the voltage reaches a certain value, much the same way as IB. Unlike IB, IC will reach a plateau and then remain basically constant as VCE increases. The graph illustrates that IC = Bdc * IB, or that a small increase in IB leads to a large increase in IC.



    IB will be constant until the breakdown region of the transistor is reached. This region is where the transistor will become damaged when the voltage is too large, and is dependent on the transistor type. IB will rapidly increase when the breakdown voltage is reached.



    The collector voltage reverse-biases the collector. The collector-emitter voltage is equal to the collector voltage minus the voltage across the collector resistor. It is VCE = VC -- IC * RC.
  • Input characteristics are found by considering the base loop.



    A graph of the base current IB versus VBE, which is the voltage between the base and the emitter, looks like that of an ordinary diode. The current is zero until VBE reaches 0.7 volts, where it then increases very suddenly.



    The base voltage forward biases the emitter. The equation to find the voltage across the resistor RB is VBB -- VBE, where VBB is the base voltage. The current IB is found using VBB -- VBE / RB.
  • Output Characteristics

  • Output characteristics are found by considering the collector loop.



    A graph of the collector current IC versus the collector-emitter voltage VCE shows much the same shape for different transistors, though the numbers will be different. When VCE is zero, so is IC. As VCE increases, IC will remain zero and then suddenly shoot up when the voltage reaches a certain value, much the same way as IB. Unlike IB, IC will reach a plateau and then remain basically constant as VCE increases. The graph illustrates that IC = Bdc * IB, or that a small increase in IB leads to a large increase in IC.



    IB will be constant until the breakdown region of the transistor is reached. This region is where the transistor will become damaged when the voltage is too large, and is dependent on the transistor type. IB will rapidly increase when the breakdown voltage is reached.



    The collector voltage reverse-biases the collector. The collector-emitter voltage is equal to the collector voltage minus the voltage across the collector resistor. It is VCE = VC -- IC * RC.
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