Need for biasing
Bias establishes the DC operating point for proper linear operation of an amplifier. If an amplifier is not biased with correct DC voltages on the input and output, it can go into saturation or cutoff when an input signal is applied. Figure 1.1 shows the effects of proper and improper DC biasing of an inverting amplifier. In part (a), the output signal is an amplified replica of the input signal except that it is inverted, which means that it is 1800 out of phase with the input. The output signal swings equally above and below the dc bias level of the output, VDC(out).
Improper biasing can cause distortion in the output signal, as illustrated in parts (b) and (c). Part (b) illustrates limiting of the positive portion of the output voltage as a result of a Q-point (dc operating point) being too close to cutoff.
Part (c) shows limiting of the negative portion of the output voltage as a result of a de operating point being too close to saturation.