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Transistor Biasing


The basic function of transistor is amplification. The process of raising the strength of a weak signal without any change in its general shape is referred to as faithful amplification. For faithful amplification it is essential that 
                        
                   1.       Emitter-base junction is forward biased.
                   2.       Collector –base junction is reverse biased;
                   3.       Proper zero signal collector current.

For achieving faithful amplification, fulfillment of the following basic conditions is essential :

1.       Proper Zero Signal Collector Current :- Zero signal collector current should be atleast  equal to the maximum collector current due to signal alone i.e  zero signal >or equal to maximum collector current.
2.       Minimum Proper base-emitter Voltage  at Any Instant :- The base-emitter voltage Vbe  should not fall below  0.3V for germanium transistors and 0.7 V for silicon transistors at any instant. If the base emitter voltage  Vbe  falls below these values during any part of signal, that part will be amplified to smaller extent due to smaller collector current and therefore, faithful amplification will not be available.
3.       Minimum Proper Collector-Emitter Voltage At Any Instant  :-  The collector-emitter voltage Vce  should not fall below Knee voltage(0.5V for germanium and 1.0V for silicon Transistors).
With Collector-emitter  voltage Vce less than Knee voltage, the collector-base junction is not properly reverse biased causing increase in base current Ib and decrease in collector current Ic and so decrease in the value of B(beta). Thus when  Vce  falls below Knee voltage Vknee during any part of the signal, that part will be amplified to lesser extent due to reduced value of B(beta)  and result in unfaithful amplification. For collector emitter voltage exceeding knee voltage, the collector-base junction is properly reverse biased and the value of B(beta)  remains constant, resulting in unfaithful amplification.

The conditions 1 and 2 ensure that emitter-base junction remains properly forward biased during all parts of the signal and condition 3 ensures that  collector-base junction remains properly reverse-biased at all times. By fulfilling these conditions the transistor is made to operate over range of its characteristic curves which are linear-parallel and equi-spaced for equal increments of the parameter.
                           The proper flow of zero signal collector current and the maintenance of proper collector-emitter voltage during the passage of signal is called the transistor biasing. If the transistor is not biased properly, it would work inefficiently and produce distortion in the output signal. A transistor is biased either with the help of battery or associating a circuit with the transistor. The latter method is more efficient and is frequently used. The circuit used for transistor biasing is called the biasing circuit. It may be noted that transistor biasing is very essential for the proper operation of transistor in any circuit.

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