When used as switches the transistors are used in the saturation mode where additional base-emitter current will produce no additional collector emitter current. The values hfe represents DC gain - a small base-emitter current creates a larger emitter-collector current. This too forward biases the base-emitter junction. Push Sw2 this allows current from GND through the emitter that spits to form Ib and Ic for the NPN transistor. This combines with the collector current to produce the emitter current back to the 12-volt supply. Push switch Sw1 current flows from GND through R1 and forward biases the base (B) with respect to the emitter. Sink and source are important to know when connecting programmable logic controllers (PLCs) used to control machinery in industry. With the NPN transistor on the right the switch is in the ground side of the load and is said to sink the current. Operating as a switch to turn the motor ON-OFF the PNP transistor is located in the +Vcc side of the load and will source the current. Note the position of the transistors in relation to GND, the +12-volts, and the load in this case DC motors. Note the arrows dictating current flow - with the PNP the collector current (Ic) is from collector (C) to emitter (E) while the NPN Ic is emitter to collector. Note current flow in these illustrations is from negative to positive. Plate 3 illustrates the typical electrical connections for both PNP and NPN bipolar transistors on a negative ground micro-controller system.