Biasing (electronics)

Biasing in electronics is the method of establishing predetermined voltages and/or currents at various points of a circuit to set an appropriate operating point.

The operating point of a device, also known as bias point or quiescent point (or simply Q-point), is the DC voltage and/or current which, when applied to a device, causes it to operate in a certain desired fashion. The term is normally used in connection with devices such as transistors and diodes which ar

Importance in linear circuits

Linear circuits involving transistors typically require specific DC voltages and currents to operate correctly, 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).

Bipolar junction transistors

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.

The quiescent point of operation is typically near the middle of DC load line. The process of obtaining certain DC collector current at a certain DC collector voltage by setting up operating point is called biasing.

After establishing the operating point, when input signal is applied, the output signal should not move the transistor either to saturation or to cut-off. However, this unwanted shift might occur due to various reasons outlined below:

To avoid a shift of Q-point, bias-stabilization is necessary. Various biasing circuits can be used for this purpose.

Types of bias circuit

The following discussion treats five common biasing circuits used with bipolar transistors:

1. Fixed bias
2. Collector-to-base bias
3. Fixed bias with emitter resistor
4. Voltage divider bias

Types of bias circuit

The following discussion treats five common biasing circuits used with bipolar transistors:

1. Fixed bias
2. Collector-to-base bias
3. Fixed bias with emitter resistor
4. Voltage divider bias
5. Emitter bias