This mode of operation allows the output voltage to increase and decrease around the amplifiers Q-point without distortion as the input signal swings through one complete cycle. When a bipolar transistor is biased so that the Q-point is near the middle of its operating range, that is approximately halfway between cut-off and saturation, it is said to be operating as a Class-A amplifier. This central operating point is called the “Quiescent Operating Point”, or Q-point for short. The correct biasing point for a bipolar transistor, either NPN or PNP, generally lies somewhere between the two extremes of operation with respect to it being either “fully-ON” or “fully-OFF” along its DC load line. Therefore, if the transistor is to operate correctly as a linear amplifier, it must be properly biased around its operating point as improper transistor biasing will result in a distorted output.Įstablishing the correct operating point requires the selection of bias resistors and load resistors to provide the appropriate input current and collector voltage conditions. The additional gate resistor R G1 from gate to supply voltage facilitates in larger adjustment of the DS bias point and permits use of larger valued R S.The steady state operation of a bipolar transistor depends a great deal on its base current, collector voltage, and collector current values. The voltage V 2 across R G2 provides the necessary bias. The resistor R G1 and R G2 form a potential divider across drain supply V DD. Let the given JFET be replaced by another JFET having the double conductance then drain current will also try to be double but since any increase in voltage drop across R S, therefore, gate-source voltage, V GS becomes more negative and thus increase in drain current is reduced.Ī slightly modified form of DC potential-divider bias is provided by the circuit shown in figure below. Self-biasing of a JEFT stabilizes its quiescent operating point against any change in its parameters, like transconductance. Thus, DC conditions of JFET amplifier are fully specified. The operating point (that is zero signal ID and V DS) can easily be determined from equation and equation given below: So, voltage drop across resistance R S provides the biasing voltage VGg and no external source is required for biasing and this is the reason that it is called self-biasing. With a drain current I D the voltage at the S is Since no gate current flows through the reverse-biased gate-source, the gate current I G=0 and, therefore, V G = i GR G =0. Since the FET has such a high input impedance that no gate current flows and the DC voltage of the gate set by a voltage divider or a fixed battery voltage is not affected or loaded by the FET. For a JFET drain current is limited by the saturation current I DS. Various FET biasing circuits in printed circuit board (PCB) design, fabrication and assembly are discussed below.ĭC bias of a FET device needs setting of gate-source voltage V GS to give desired drain current I D. With few exceptions, MOSFET bias circuits are similar to those used for JFETs. To obtain reasonable limits on quiescent drain currents ID and drain-source voltage VDS, source resistor and potential divider bias techniques must be used. However, the wide differences in maximum and minimum transfer characteristics make ID levels unpredictable with simple fixed-gate bias voltage. Unlike BJTs, thermal runaway does not occur with FETs. The AC signal applied to them is super-positioned on this DC bias current or voltage.įor proper working of a transistor, it is essential to apply external voltages of correct polarity across its emitter-base and collector-base junctions. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing time-varying (AC) signals, also require a steady (DC) current or voltage at their terminals to operate correctly. In electronics, Biasing is the setting of initial operating conditions (current and voltage) of an active device in an amplifier.
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