Problem $4$: $(6$points$)$

In order to maintain the emitting light intensity of a laser$-$diode $($LD$),$ a circuit shown below which consists of BJT and photo$-$diode $($PD$)$ is used to control the laser$-$diode.

$($a$)$ Explain why the circuit can do the job. $3-$points$)$

$($b$)$ Find the value of R such that the emitting light intensity can be maintained at $10$ mW$.(3-$points$)$

Device specification:

$1.$ LD: $10$ mW light$-$power @ $50$ mA with $2$ V voltage drop.

$2.$ PD: operate @ $0\mathrm{.}15$ mA

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$0\mathrm{.}13$

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