Biasing is needed in a transistor amplifier circuit to set baseline, or steady, voltages. In general, biasing is a method in electronics of establishing predetermined voltages at points throughout a circuit.
From a baseline, any changes in voltages in a circuit are recorded as differences from the proper operating bias. Circuit elements like resistors affect the bias of a transistor amplifier. In an experiment described on the University of Saint Andrews website, a constructed transistor amplifier is recorded at an initial bias. Then, applied input voltage makes its way through the circuit and results in output at a given voltage based on selected resistors.