In the diagram shown, the resistance between points A and B is '$R_1$' when an ideal diode D is forward biased and is '$R_2$' when ideal diode D is reverse biased. The ratio $R_1 / R_2$ is

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