current is fixed for some performance reason, any increase in load resistance automatically leads to a lower voltage for V_out. which in turn lowers the voltage drop V_CB between collector and base, limiting the signal swing at the amplifier output (if the output swing is larger than V_CB, the transistor is driven out of active mode during part of the signal cycle).

In contrast, using the active load of Figure 2, the AC impedance of the ideal current source is infinite regardless of the voltage drop V_CC