(a) Draw a circuit diagram of an n-p-n transistor with its emitter-base junction forward biased and base-collector junction reverse biased. Briefly describe its working. Explain how a transistor in its active state exhibits a low resistance at its emitter-base junction and high resistance at its base-collector junction.
(b) Derive the expression for the voltage gain of a transistor amplifier in CE configuration in terms of the load resistance RL, current gain a and input resistance. Explain why input and output voltages are in opposite phase
(a) A circuit diagram of the n-p-n transistor;-
When the emitter-base junction is forward biased, the negative (-ve) pole of the battery is connected with n-diode and positive (+ve) pole with p-diode, electrons are majority carriers, repelled towards base due to -ve potential on which combines with holes present in the base.
As here the base is thin, most of the electron will go the collector
In the active mode of the transistor, emitter-base junction is forward biased and collector-base junction is reversed biased. When transistor operates in this mode, collector current increases linearly with base current. So in the active state it exhibits low resistance at emitter-base and high resistance at its base-collector.
(b) Applying Kirchoff's loop rule to input loop
____(i)
Where,
Output loop
______(ii)
Voltage gain:
Where,
The negative sign in the voltage gain shows that the input voltage and output voltage has a phase difference of 180 degrees.