Question

The current gain of a transistor in common emitter mode is 35. The change in collector current is 140mV at constant collector to emitter voltage. The change in the base current will be.
A. 10mA
B. 2mA
C. 0.1mA
D. 4mA

Answer 1

Hint: Current gain of a transistor in common emitter amplifier mode is given by,
\[\beta = {\left( {\dfrac{{\vartriangle {I_C}}}{{\vartriangle {I_B}}}} \right)_{{V_{CE}}}}\]

Complete step by step solution:
Current gain of a transistor in common emitter amplifier mode is given by,
\[\beta = {\left( {\dfrac{{\vartriangle {I_C}}}{{\vartriangle {I_B}}}} \right)_{{V_{CE}}}}\]

We can also write it as,
\[\vartriangle {I_B} = \dfrac{{\vartriangle {I_C}}}{\beta }\]

Now putting the values,
\[
   = \dfrac{{140 \times {{10}^{ - 3}}}}{{35}} \\
   = 4 \times {10^{ - 3}}A \\
   = 4mA \\
 \]

Therefore, the correct option is D.

Additional information:
The single stage common emitter amplifier circuit shown above uses what is commonly called “Voltage Divider Biasing”. This type of biasing arrangement uses two resistors as a potential divider network across the supply with their centre point supplying the required Base bias voltage to the transistor. Voltage divider biasing is commonly used in the design of bipolar transistor amplifier circuits.

This method of biasing the transistor greatly reduces the effects of varying Beta, (\[\beta \]) by holding the Base bias at a constant steady voltage level allowing for best stability. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, R1, R2 and the power supply voltage Vcc as shown with the current flowing through both resistors.

Note: Students need to be able to know this way of finding current gain. This method of biasing the transistor greatly reduces the effects of varying Beta, (\[\beta \]) by holding the Base bias at a constant steady voltage level allowing for best stability. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, R1, R2 and the power supply voltage Vcc as shown with the current flowing through both resistors.