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Bias, Greek sage
bias, in electricity
- any situation in which the accuracy RELIABILITY, VALIDITY, etc. of sociological data or findings are held to be distorted by the limitations of a research method employed, or by a researcher's or a theorist's presuppositions (e.g. political or moral beliefs). See also OBJECTIVITY, VALUE FREEDOM AND VALUE NEUTRALITY.
- in a more narrowly technical sense, in statistical analysis, a difference between a hypothetical ‘true value’ of a variable in a population and that obtained in a particular sample of respondents. See also BIASED SAMPLE.
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 VCE = 0 and above IC = 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 VCC and RL. Thus, the onset of saturation can be considered to take place at the knee of the common-emitter transistor curves. See Amplifier, Transistor
In saturation, the transistor current IC is nominally VCC/RL. Since RL is small, it may be necessary to keep VCC 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 IE 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 ICO. 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 IB is constant, independent of the quiescent collector current IC, 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 = ∂IC/∂ICO, 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 hFE as ∂IC/∂hFE, and in terms of base-to-emitter voltage as ∂IC/∂VBE. However, bias circuits with small values of S will also perform satisfactorily for transistors that have large variations of hFE and VBE. For the fixed-bias circuit it can be shown that S = hFE + 1, and if hFE = 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 RB 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 IC tends to increase (either because of a rise in temperature or because the transistor has been replaced by another), then VCE decreases. Hence IB 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
If the load resistance RL 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 RE 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 IC tends to increase, the current in RE increases. As a consequence of the increase in voltage drop across RE, the base current is decreased. Hence IC will increase less than it would have had there been no self-biasing resistor RE. The stabilization factor for the self-bias circuit is shown by Eq. (2), where RB = R1R2/(R1 + R2).
In order to avoid the loss of signal gain because of the degeneration caused by RE, 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 (ID, VGS, VDS) 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.
bias(1) A weight given to a neuron in a neural network. See neuron.
(2) 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 sum of the two.
Forward and Reverse Bias
Forward bias voltage brings the transistor or tube into or closer to its conductive state. For example, if the gate requires positive voltage to conduct, forward biasing adds positive voltage.
In contrast, reverse bias holds the device in a non-conductive state until the sum of the control voltage and bias is sufficient to bring it to the conductive state. For example, if the gate requires positive voltage to conduct, reverse biasing adds negative voltage.
Bias is widely used in analog devices, such as an audio amplifier, to keep the input voltage within 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.|