In a Common Emitter (CE) amplifier, there is a phase difference of $180^{\circ}$ (or $\pi$ radians) between the input and output signals. The output voltage magnitude is determined by multiplying the input voltage amplitude by the voltage gain ($A_v$). Given: Voltage gain, $A_v = 150$ Input signal, $V_i = 2\cos(15t + \frac{\pi}{3})$ Amplitude of input signal $= 2$ Amplitude of output signal $= 150 \times 2 = 300$ The output signal will have an added phase of $\pi$. Therefore, Output signal $V_o = 300\cos(15t + \frac{\pi}{3} + \pi) = 300\cos(15t + \frac{4\pi}{3})$.