bias (redirected from biasing)

Bias, Greek sage

Bias (bī`əs), fl. 6th cent. B.C., Greek sage, b. Priene. He is at best semilegendary but was called one of the Seven Wise Men of Greece. Many epigrams were attributed to him by ancient writers.

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bias, in electricity

bias, a voltage, current, or other input applied to a device or system as a reference or to set its conditions of operation. A bias is usually steady but may vary with time, usually within a fixed and known range and with a fixed and known frequency. In electronics, the most common forms of bias are the voltage applied to the grid of an electron tube to set its operating conditions and the current applied to the base of a transistor to perform the same function. In tape recording, a bias current, usually alternating with a frequency in the ultrasonic range, is mixed with the signal to be recorded to minimize distortion produced by the tape itself.

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bias

A voltage applied to the gate (or base) of a transistor or vacuum tube, which causes the device to operate in its conductive state. When the control voltage (input voltage) is applied to the gate, it is added to the bias, causing the resultant voltage to be higher or lower, based on the design criteria.

Forward and Reverse Bias
Forward bias (same direction as control voltage) brings the device into or closer to the conductive state of the device. Reverse bias (opposite direction) holds the device in a non-conductive state until the control voltage is sufficient to bring it to the conductive state when added together. Bias is widely used in analog devices, such as an audio amplifier, to keep the output voltage constantly in the conductive region of the transistor or tube. It is also used in digital circuits to reach a certain threshold and open or close the switch faster.

Transistor Conduction Curve

This is the typical conduction curve of a bipolar and field effect transistor (FET). A forward bias pushes the voltage past the transition region and keeps the transistor operating in its conduction region.

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bias

1. Electronics the voltage applied to an electronic device or system to establish suitable working conditions

2. Bowls

a. a bulge or weight inside one side of a bowl

b. the curved course of such a bowl on the green

3. an inaudible high-frequency signal used to improve the quality of a tape recording

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Bias (electronics)

The establishment of an operating point on the transistor volt-ampere characteristics by means of direct voltages and currents.

Since the transistor is a three-terminal device, any one of the three terminals may be used as a common terminal to both input and output. In most transistor circuits the emitter is used as the common terminal, and this common emitter, or grounded emitter, is indicated in illus. a. If the transistor is to used as a linear device, such as an audio amplifier, it must be biased to operate in the active region. In this region the collector is biased in the reverse direction and the emitter in the forward direction. The area in the common-emitter transistor characteristics to the right of the ordinate V_CE = 0 and above I_C = 0 is the active region. Two more biasing regions are of special interest for those cases in which the transistor is intended to operate as a switch. These are the saturation and cutoff regions. The saturation region may be defined as the region where the collector current is independent of base current for given values of V_CC and R_L. Thus, the onset of saturation can be considered to take place at the knee of the common-emitter transistor curves. See Amplifier, Transistor

Translator circuits

In saturation, the transistor current I_C is nominally V_CC/R_L. Since R_L is small, it may be necessary to keep V_CC correspondingly small in order to stay within the limitations imposed by the transistor on maximum-current and collector-power dissipation. In the cutoff region it is required that the emitter current I_E be zero, and to accomplish this it is necessary to reverse-bias the emitter junction so that the collector current is approximately equal to the reverse saturation current I_CO. A reverse-biasing voltage of the order of 0.1 V across the emitter junction will ordinarily be adequate to cut off either a germanium or silicon transistor.

The particular method to be used in establishing an operating point on the transistor characteristics depends on whether the transistor is to operate in the active, saturation or cutoff regions; on the application under consideration; on the thermal stability of the circuit; and on other factors.

In a fixed-bias circuit, the operating point for the circuit of illus. a can be established by noting that the required current I_B is constant, independent of the quiescent collector current I_C, which is why this circuit is called the fixed-bias circuit. Transistor biasing circuits are frequently compared in terms of the value of the stability factor S = ∂I_C/∂I_CO, which is the rate of change of collector current with respect to reverse saturation current. The smaller the value of S, the less likely the circuit will exhibit thermal runaway. S, as defined here, cannot be smaller than unity. Other stability factors are defined in terms of dc current gain h_FE as ∂I_C/∂h_FE, and in terms of base-to-emitter voltage as ∂I_C/∂V_BE. However, bias circuits with small values of S will also perform satisfactorily for transistors that have large variations of h_FE and V_BE. For the fixed-bias circuit it can be shown that S = h_FE + 1, and if h_FE = 50, then S = 51. Such a large value of S makes thermal runaway a definite possibility with this circuit.

In collector-to-base bias, an improvement in stability is obtained if the resistor R_B in illus. a is returned to the collector junction rather than to the battery terminal. Such a connection is shown in illus. b. In this bias circuit, if I_C tends to increase (either because of a rise in temperature or because the transistor has been replaced by another), then V_CE decreases. Hence I_B also decreases and, as a consequence of this lowered bias current, the collector current is not allowed to increase as much as it would if fixed bias were used. The stability factor S is

(1) 

shown in Eq. (1). This value is smaller than h_FE + 1, which is the value obtained for the fixed-bias case.

If the load resistance R_L is very small, as in a transformer-coupled circuit, then the previous expression for S shows that there would be no improvement in the stabilization in the collector-to-base bias circuit over the fixed-bias circuit. A circuit that can be used even if there is zero dc resistance in series with the collector terminal is the self-biasing configuration of illus. c. The current in the resistance R_E in the emitter lead causes a voltage drop which is in the direction to reverse-bias the emitter junction. Since this junction must be forward-biased (for active region bias), the bleeder R₁-R₂ has been added to the circuit.

If I_C tends to increase, the current in R_E increases. As a consequence of the increase in voltage drop across R_E, the base current is decreased. Hence I_C will increase less than it would have had there been no self-biasing resistor R_E. The stabilization factor for the self-bias circuit is shown by Eq. (2), where R_B = R₁R₂/(R₁ + R₂).

(2) 

The smaller the value of R_B, the better the stabilization. Even if R_B approaches zero, the value of S cannot be reduced below unity.

In order to avoid the loss of signal gain because of the degeneration caused by R_E, this resistor is often bypassed by a very large capacitance, so that its reactance at the frequencies under consideration is very small.

The selection of an appropriate operating point (I_D, V_GS, V_DS) for a field-effect transistor (FET) amplifier stage is determined by considerations similar to those given to transistors, as discussed previously. These considerations are output-voltage swing, distortion, power dissipation, voltage gain, and drift of drain current. In most cases it is not possible to satisfy all desired specifications simultaneously.