BJT, Early Effect, PA DC-DC

Early Effect: Collector voltage (Vcc) has some effect on collector current (Ic)– Ic increases slightly as Vcc increases for the same amplication (within the active region, since the gradient is linear, amplication gan is linear no matter where the Q point is, resulting in the same level of output for the same input level). Due to this effect, current in Ic can be saved by varying Vcc depending on the signal size (base current or base voltage) to be amplified and this is why using PA DCDC can save some current. When input power to PA is small, we can reduce the Vcc level accordingly, putting the operation point (Q point) in the middle of the PA active region, and when the PA input power is large, we can increase the Vcc level to move the Q point well within the PA's active region. Simply put, the fact that the gradients of the slopes in the Vce-Ic graph are greater than 0 degree means there's some room for current saving through variation of Vcc (supply current to the PA).