In a common emitter transistor,the current gain is $80$. What is the change in collector current,when the change in base current is $250 \ \mu A$?

In a transistor,the base is very lightly doped as compared to the emitter because by doing so:

When an $NPN$ transistor is used as an amplifier,which of the following occurs?

When $n-p-n$ transistor is used as an amplifier :

An $n-p-n$ transistor is connected in common-emitter configuration as shown in the figure. If the collector current is $5 \text{ mA}$,$V_{BE} = 0.6 \text{ V}$,$V_{CE} = 3 \text{ V}$,and the common-emitter current amplification factor $\beta = 50$,then the values of $R_1$ and $R_2$ are respectively.

Consider an amplifier circuit wherein a transistor is used in common emitter mode. The change in collector current and base current respectively are $4 mA$ and $20 \mu A$ when a signal of $40 mV$ is added to the base-emitter voltage. If the load resistance is $10 k\Omega$, then the power gain in the circuit is: