From Kirchhoff's voltage law, the voltage across the base resistor is This resistor introduces negative feedback that stabilizes the Q-point. ![]() The fixed bias circuit is modified by attaching an external resistor to the emitter. Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted. The negative feedback also increases the input impedance of the amplifier as seen from the base, which can be advantageous. Because an increase in collector current with temperature is opposed, the operating point is kept stable. A lower base-resistor voltage drop reduces the base current Ib, which results in less collector current Ic . ![]() However, a larger Ic causes the voltage drop across resistor Rc to increase, which in turn reduces the voltage Vbr across the base resistor Rb. If Vbe is held constant and temperature increases, then the collector current Ic increases. So any thermal runaway will induce a voltage drop across the Rc resistor that will throttle the transistor's base current.įrom Kirchhoff's voltage law, the voltage Vrb across the base resistor Rb is In this form of biasing, the base resistor Rb is connected to the collector instead of connecting it to the DC source Vcc. This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. However, one application of fixed bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a DC signal derived from the AC output of a later stage. Instead, it is often used in circuits where transistor is used as a switch. Usage:ĭue to the above inherent drawbacks, fixed bias is rarely used in linear circuits (i.e., those circuits which use the transistor as a current source). In this manner, operating point given as (Vce,IC) can be set for given transistor. The common-emitter current gain of a transistor is an important parameter in circuit design,and is specified on the data sheet for a particular transistor. Therefore, this type is called fixed bias type of circuit. As Vcc is of fixed value, on selection of RB, the base current IB is fixed. The single power source (for example, a battery) is used for both collector and base of a transistor, although separate batteries can also be used.įor a given transistor, Vbe does not vary significantly during use. Voltage divider bias or potential divider ![]() The following are five common biasing circuits used with class-A bipolar transistor amplifiers:
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