Three working states of the transistor - NPN type - saturation state

Cut-off state:
  When "≤0", the collector current is very
small, the transistor is equivalent to cut-off, and the power supply
voltage mound is almost entirely applied to both ends of the tube.

Amplification state:
  "Gradually increasing from O, the collector current
J also increases according to a certain proportion. A small
change in I causes a large change. The transistor
plays an amplifying role.

Saturation state:
  When the three-input tube is saturated, the voltage drop across the tube is very
small, and almost all of the power supply voltage E is applied to the collector
load. The larger the I port at both ends of the collector load resistor, the more
sensitive the control.