Biasing (electronics)

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 are used in amplification or rectification.

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). More detail follows.

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:

# Parameters of transistor depend on temperature. As it increases, leakage current due to minority charge carriers (ICBO) increases. As ICBO increases, ICEO also increases, causing increase in collector current IC. This produces heat at the collector junction. This process repeat, and finally Q-point may shift into saturation region. Sometimes the excess heat produced at the junction may even burn the transistor. This is known as thermal runaway.
# When a transistor is replaced by another of the same type, the Q-point may shift, due to change in parameters of transistor such as "current gain" (eta ) which changes from unit to unit.

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





See also

* Small signal model
* Bipolar Junction Transistor
* Discrete Bipolar Transistor Biasing

External links

* [ Bias] - from Sci-Tech Encyclopedia

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