Biasing

Overview

In electronic 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.

In electronics, bias usually refers to a fixed DC voltage or current applied to a terminal of an electronic component such as a diode, transistor or vacuum tube in an circuit in which alternating current (AC) signals are also present, in order to establish proper operating conditions for the device. 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 grid bias voltage is often applied to the grid electrodes for the same reason.

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.

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 horizontally or vertically, 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)(collector base current with emitter open) increases. As I_CBO increases, I_CEO(collector emitter current with base open) 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 ( β ) 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.

Vacuum tubes

Grid bias is a DC voltage applied to electron tubes (or valves in British English) with three electrodes or more, such as triodes. The control grid (usually the first grid) of these devices is used to control the electron flow from the heated cathode to the positively charged anode. Bias point in small-signal applications is set to minimize distortion and achieve sufficiently low power draw. In high-power applications, biasing is typically set for maximum available output power or voltage, with a secondary target of either low distortion or high efficiency.

In a typical voltage amplifier, including power stages of most audio power amplifiers, DC bias voltage is negative relative to cathode potential. Instant grid voltage (sum of DC bias and AC input signal) should never rise above cathode potential to prevent grid-to-cathode currents that overload preceding amplifier stages and may cause severe even-order distortion. High transconductance tubes develop significant grid currents even with small negative bias; in these cases, maximum instant voltage ceiling is lowered to -1.0..-0.5 Volt.

High efficiency Class B+ push-pull amplifiers operate at higher bias points (near zero or even positive values). These designs take care of grid currents through the use of cathode followers or interstage transformers easing current load on the driver stages, and deep negative feedback to minimize distortion.

High power transmitter tubes (oscillators and modulators) are frequently positively biased to maximize radio frequency output. Distortion is minimized by using band-pass filter loads tuned to the desired radio frequency.

Bias voltage is obtained through:

An external voltage source (fixed bias) - a battery or a dedicated DC power supply. When the cathode potential is raised above ground (as in cascode circuits), bias voltage is obtained by tapping into main (positive) plate power supply.

Automatic bias or self bias - using a cathode resistor to raise cathode potential above grid (tied to ground) and stabilize plate current;

Grid leak bias - diverting DC grid current through a high value grid resistor, as used in the grid-leak detector.

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, 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.