Transistor Emitter-Follower Buffer
For a voltage source, an emitter follower can serve as a buffer. The voltage divider on the left is so strongly affected by the value of the load resistor make it a poor voltage source. The same voltage divider on the right added with a transistor buffer will supply power to keep the voltage constant over its range of operation.
For an open circuit, the simple voltage divider will give a 4V output but quickly drops when larger loads (smaller load resistor) are connected. To keep the output voltage more nearly constant is the function of the buffer. To explore the effect of the buffer, you can change the value of the load resistor below. A 0.6 V diode drop was assumed for the transistor. To present essentially an open circuit, the input impedance to the transistor buffer is assumed to be so high.