Question

The current gain in a transistor is 40 in a common emitter mode. The emitter current is8.2 mA, then base current is:
a)0.02 mA
b)0.2 mA
c)2 mA
d)0.4 mA

Answer 1

Two important relations in a Bipolar Transistor:\[I_E=I_B+I_C\] and \[I_C=\beta I_B\] the current in the emitter will be then: \[I_E=(1+\beta)I_B\]

With this you can find the solution

Answer 2

What do you mean by "current gain"?

Answer 3

\[\beta = I_C/I_B\] is the current gain

Answer 4

Thanks!

Answer 5

And the solution is...?

Answer 6

LOL.. It must be 0.2 mA

Answer 7

Excellent...Keep in mind these relations I have told you. This is the only thing you need to analyze a simple circuit with transistors

Answer 8

What does the term " common emitter mode" mean?

Answer 9

In a nutshell, just thing of your circuit as a black box having an input, an output and a terminal that is common to input and output (normally connected to ground). In common emitter input is the base, output is the collector and common terminal is emitter

Answer 10

A transistor can function in all thre modes, common emitter, common base and common collector, depending of which is the input, the output and the common terminal. The features of the amplifier varies depending of the mode you use the transistor

Answer 11

So, would it make any difference if there was "common base" or "common collector" instead of "common emitter" in the question?

Answer 12

That is correct. The relations always exist for a transistor in the active region

Answer 13

what difference would it make?

Answer 14

It is when you use the transistor as amplifier, depending on the mode you will have different input inpedances, output impedances and gains

Answer 15

I guess, I need to study more. Thanks for your help!

Answer 16

welcome. It is not rocket science anyway, just a question of practice and seeing them perform in lab

Answer 17

@ujjwal, good stuff here: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html