A power BJT must dissipate \(30 \mathrm{~W}\) of power. The maximum allowed junction temperature is \(T_{j, \max }=150^{\circ} \mathrm{C}\), the ambient temperature is \(25^{\circ} \mathrm{C}\), and the device-to-case thermal resistance is \(\theta_{\operatorname{dev}-\mathrm{case}}=2.8^{\circ} \mathrm{C} / \mathrm{W}\).

(a) Find the maximum permissible thermal resistance between the case and ambient.

(b) Using the results of part (a), determine the junction temperature if the power dissipated in the transistor is \(20 \mathrm{~W}\).