To use a transistor as an amplifier,

In an $N-P-N$ transistor,when does the current flow?

In a common emitter amplifier,a change of $0.2 \text{ mA}$ in the base current causes a change of $5 \text{ mA}$ in the collector current. If the input resistance is $2 \text{ k}\Omega$ and the voltage gain is $75$,what is the load resistance used in the circuit?

In an $n-p-n$ transistor amplifier,the collector current is $9 \ mA$. If $90 \%$ of the electrons emitted reach the collector,then

For a transistor,the current ratio $\beta_{dc}$ is defined as the ratio of:

$A$ transistor is operating in the active mode. $v_1$ is the potential barrier across the base-emitter junction and $v_2$ is the potential barrier across the collector-base junction. $b_1$ is the width of the depletion layer of the base-emitter junction and $b_2$ is the width of the collector-base junction.