CB Characteristics Of A Transistor 3
Experiment No: 9
Date:
COMMON BASE CHARACTERISTICS OF A TRANSISTOR
Aim: To plot the input and output characteristics of an NPN transistor in common base configuration and calculate various parameters.
Components and Equipments Required: Transistor, voltmeters, ammeters, rheostats, dc sources, breadboard and CRO.
Theory: In CB configuration, the base of a transistor is common to input and output circuits. Input voltage is applied between emitter and base and output is taken across collector and base. CB configuration is also called grounded base configuration. In this setup, an increase in emitter current causes an increase in collector current. Collector current is given by the expression IC = I E − I B. Since IB is in the order of microamperes, collector current is almost same as the emitter current in spite of the variations in the collector base junction. Another important expression for a CB transistor configuration is
IC = αIE + ICBO
, where
ICBO
is the current flowing through the collector circuit when the collector base junction is reverse biased and emitter base junction is open circuited. Transistor offers a low input resistance and very high output impedance when it is in CB configuration. It provides almost unity current gain and high voltage gain.
Dynamic input resistance: Input characteristics are plotted between the emitter current
I E and emitter to base voltage VEB for a constant value of collector to base voltage VCB . The reciprocal of the slope of the curves gives the value of dynamic input resistance ri and is given by the expression
ri = ∆VEB / ∆I E
keeping VCB constant.
Dynamic output resistance: Output characteristics are plotted between the collector current
IC and
collector to base voltage
VCB
for a constant value of emitter current IE . The dynamic output resistance can be obtained from these curves and is given by the expression ro = ∆VCB / ∆IC keeping
IE constant.
Common base current gain: It is given by the
Date:
COMMON BASE CHARACTERISTICS OF A TRANSISTOR
Aim: To plot the input and output characteristics of an NPN transistor in common base configuration and calculate various parameters.
Components and Equipments Required: Transistor, voltmeters, ammeters, rheostats, dc sources, breadboard and CRO.
Theory: In CB configuration, the base of a transistor is common to input and output circuits. Input voltage is applied between emitter and base and output is taken across collector and base. CB configuration is also called grounded base configuration. In this setup, an increase in emitter current causes an increase in collector current. Collector current is given by the expression IC = I E − I B. Since IB is in the order of microamperes, collector current is almost same as the emitter current in spite of the variations in the collector base junction. Another important expression for a CB transistor configuration is
IC = αIE + ICBO
, where
ICBO
is the current flowing through the collector circuit when the collector base junction is reverse biased and emitter base junction is open circuited. Transistor offers a low input resistance and very high output impedance when it is in CB configuration. It provides almost unity current gain and high voltage gain.
Dynamic input resistance: Input characteristics are plotted between the emitter current
I E and emitter to base voltage VEB for a constant value of collector to base voltage VCB . The reciprocal of the slope of the curves gives the value of dynamic input resistance ri and is given by the expression
ri = ∆VEB / ∆I E
keeping VCB constant.
Dynamic output resistance: Output characteristics are plotted between the collector current
IC and
collector to base voltage
VCB
for a constant value of emitter current IE . The dynamic output resistance can be obtained from these curves and is given by the expression ro = ∆VCB / ∆IC keeping
IE constant.
Common base current gain: It is given by the