Biasing

Biasing in electronics is the method of establishing predetermined voltages or currents at various points of an electronic circuit for the purpose of establishing proper operating conditions in electronic components. Many electronic devices whose function is signal processing time-varying (AC) signals also require a steady (DC) current or voltage to operate correctly. The AC signal applied to them is superposed on this DC bias current or voltage. Other types of devices, for example magnetic recording heads, require a time-varying (AC) signal as bias. The operating point of a device, also known as bias point, quiescent point, or Q-point, is the steady-state voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied.

Overview

In electrical engineering, the term bias has the following meanings:

1. A systematic deviation of a value from a reference value.
2. The amount by which the average of a set of values departs from a reference value.
3. Electrical, mechanical, magnetic, or other force (field) applied to a device to establish a reference level to operate the device.
4. In telegraph signaling systems, the development of a positive or negative DC voltage at a point on a line that should remain at a specified reference level, such as zero.

Note: A bias may be applied or produced by (i) the electrical characteristics of the line, (ii) the terminal equipment, and (iii) the signaling scheme.^[1]

Most often, bias simply refers to a fixed DC voltage applied to the same point in a circuit as an alternating current (AC) signal, frequently to select the desired operating response of a semiconductor or other electronic component (forward or reverse bias). For example, a bias voltage is applied to a transistor in an electronic amplifier to allow the transistor to operate in a particular region of its transconductance curve. For vacuum tubes, a (much higher) grid bias voltage is also often applied to the grid electrodes for precisely the same reason.

A hot bias can lower the tube life span, but a "cool" bias can induce crossover distortion.

Bias is also the term used for a high-frequency signal added to the audio signal recorded on magnetic tape. See tape bias.

Bias is used in direct broadcast satellites such as DirecTV and Dish Network, the integrated receiver/decoder (IRD) box actually powers the feedhorn or low-noise block converter (LNB) receiver mounted on the dish arm. This bias is changed from a lower voltage to a higher voltage to select the polarization of the LNB, so that it receives signals that are polarized either clockwise or counterclockwise, thereby allowing it to receive twice as many channels.

We still need to determine the optimal values for the DC biasing in order to choose resistors, etc. This bias point is called the quiescent or Q-point as it gives the values of the voltages when no input signal is applied. To determine the Q-point we need to look at the range of values for which the transistor is in the active region.

Importance in linear circuits

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).

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

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

1. Parameters of transistors depend on junction temperature. As junction temperature increases, leakage current due to minority charge carriers (I_CBO) increases. As I_CBO increases, I_CEO also increases, causing an increase in collector current I_C. This produces heat at the collector junction. This process repeats, and, finally, the Q-point may shift into the saturation region. Sometimes, the excess heat produced at the junction may even burn the transistor. This is known as thermal runaway.
2. When a transistor is replaced by another of the same type, the Q-point may shift, due to changes in parameters of the transistor, such as current gain () which varies slightly for each unique transistor.

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

Microphones

Electret microphone elements typically include a junction field-effect transistor as an impedance converter to drive other electronics within a few meters of the microphone. The operating current of this JFET is typically 0.1 to 0.5 mA and is often referred to as bias,^[2] which is different from the phantom power interface which supplies 48 volts to operate the backplate of a traditional condenser microphone. Electret microphone bias is sometimes supplied on a separate conductor.^[3]

See also

• Bipolar junction transistor
• Bipolar transistor biasing
• MOSFET
• Idling current
• Small signal model
• Tape bias

References

1. ↑  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
2. ↑ http://www.shure.com/ProAudio/Products/us_pro_ea_phantom
3. ↑ IEC Standard 61938

Sources

• Sedra, Adel; Smith, Kenneth (2004). Microelectronic Circuits. Oxford University Press. ISBN 0-19-514251-9. 
• P.K. Patil;M.M. Chitnis (2005). Basic Electricity and Semiconductor Devices. Phadke Prakashan. 
• Robert L. Boylestad;Louis Nashelsky (2005). Electronic Devices and Circuit Theory. Prentice-Hall Career & Technology. 

External links

• Bias - from Sci-Tech Encyclopedia