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Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can ...
Typology: Schemes and Mind Maps
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A transistors steady state of operation depends a great deal on its base current, collector voltage, and collector current and therefore, if a transistor is to operate as a linear amplifier, it must be properly biased to have a suitable operating point. Establishing the correct operating point requires the proper selection of bias resistors and load resistors to provide the appropriate input current and collector voltage conditions.
BJT Biasing methods
Fixed Bias The transistors base current, I B remains constant for given values of Vcc, and therefore the transistors operating point must also remain fixed.Hence referred as fixed biasing
Collector to base bias This self biasing collector feedback configuration is another beta dependent biasing method that requires only two resistors to provide the necessary DC bias for the transistor.
Collector to base bias The collector to base feedback configuration ensures that the transistor is always biased in the active region regardless of the value of Beta (β) as the DC base bias voltage is derived from the collector voltage, V C providing good stability.
L
L
C
B
Dual Feedback Transistor Biasing An additional resistor to the base bias network of the previous configuration improves stability even more with respect to variations in Beta, ( β ) by increasing the current flowing through the base biasing resistors.
Dual Feedback Transistor Biasing The current flowing through R B 1 is generally set at a value equal to about 10 % of collector current, I C
. Obviously it must also be greater than the base current required for the minimum value of Beta, β. One of the advantages of this type of self biasing configuration is that the resistors provide both automatic biasing and Rƒ feedback at the same time
Transistor Biasing with Emitter Feedback The current flowing from the emitter, I E (which is a combination of I C
B ) causes a voltage drop to appear across R E in such a direction, that it reverse biases the base- emitter junction. So if the emitter current increases, voltage drop I.R E also increases. Since the polarity of this voltage reverse biases the base-emitter junction, I B automatically decrease.
Voltage Divider Bias This voltage divider biasing configuration is the most widely used transistor biasing method, as the emitter diode of the transistor is forward biased by the voltage dropped across resistor R B 2
