<P> In magnetic tape recording, the term bias is also used for a high - frequency signal added to the audio signal applied to the recording head, to improve the quality of the recording on the tape . This is called tape bias . </P> <P> Linear circuits involving transistors typically require specific DC voltages and currents for correct operation, which can be achieved using a biasing circuit . As an example of the need for careful biasing, consider a transistor amplifier . In linear amplifiers, a small input signal gives larger output signal without any change in shape (low distortion): the input signal causes the output signal to vary up and down about the Q - point in a manner strictly proportional to the input . However, because a transistor is nonlinear, the transistor amplifier only approximates linear operation . For low distortion, the transistor must be biased so the output signal swing does not drive the transistor into a region of extremely nonlinear operation . For a bipolar transistor amplifier, this requirement means that the transistor must stay in the active mode, and avoid cut - off or saturation . The same requirement applies to a MOSFET amplifier, although the terminology differs a little: the MOSFET must stay in the active mode (or saturation mode), and avoid cut - off or ohmic operation (or triode mode). </P> <P> For bipolar junction transistors the bias point is chosen to keep the transistor operating in the active mode, using a variety of circuit techniques, establishing the Q - point DC voltage and current . A small signal is then applied on top of the Q - point bias voltage, thereby either modulating or switching the current, depending on the purpose of the circuit . </P> <P> The quiescent point of operation is typically near the middle of the DC load line, so as to obtain the maximum available peak - to - peak signal amplitude without distortion due to clipping . The process of obtaining an appropriate DC collector current at a certain DC collector voltage by setting up the operating point is called biasing . </P>

What is the need of biasing in bjt