This will result in unfaithful amplification. If base emitter voltage V BE falls below these values during any part of the signal, that part will be amplified two lesser extent due to small collector current. The value of potential barrier is 0.7 volt for silicon and once the potential barrier overcome the base current and hence collector current increases sharply. Proper minimum base-emitter voltage (V BE) The value of zero signal collector current must be at least equal to the maximum collector current due to the signal alone.Ģ. If we introduce a battery source in the base circuit and no signal is applied a DC current I C will flow in the collector circuit due to the battery, this is known as zero signal collector current I C. In other words, the fulfilment of these conditions will ensure that transistor operates between the active region of the output characteristics i.e. On the other hand, condition (3) ensure that base-collector junction shall remain properly reverse biased at all times. The condition (1) and (2) ensure that base-emitter junction shall remain properly forward biased during all parts of the signal. Minimum proper collector emitter voltage at any instant.Minimum proper base emitter voltage at any instant.To achieve faithful amplification the following basic condition most be satisfy : The theory of a transistor suggests that it will function properly if its input circuit (that is base-emitter junction) is forward biased and the output circuit (that is collector-base junction) is reverse biased at all times. The basic purpose of biasing is to keep the base emitter junction forward biased and collector base junction is reverse biased, during the application of signal.ĭuring amplification the magnitude of signal should increase and there should be no change in signal shape, this increase in magnitude of the signal without any change in shape is known as faithful amplification. This is the smallest possible value of S and leads to the maximum possible thermal stability.Transistor biasing is required for proper operation of transistors in any circuit. As R B can be found directly, this method is called as fixed bias method. We know that V CC is a fixed known quantity and I B is chosen at some suitable value. Since V BE is generally quite small as compared to V CC, the former can be neglected with little error. Therefore,Ĭonsidering the closed circuit from V CC, base, emitter and ground, while applying the Kirchhoff’s voltage law, we get, Let I C be the required zero signal collector current. The figure below shows how a base resistor method of biasing circuit looks like. The required value of zero signal base current and hence the collector current (as I C = βI B) can be made to flow by selecting the proper value of base resistor RB. The base emitter junction is forward biased, as base is positive with respect to emitter. The required zero signal base current is provided by V CC which flows through R B. In this method, a resistor R B of high resistance is connected in base, as the name implies. Biasing with Collector feedback resistorĪll of these methods have the same basic principle of obtaining the required value of I B and I C from V CC in the zero signal conditions. The commonly used methods of transistor biasing are It is economical to minimize the DC source to one supply instead of two which also makes the circuit simple. The biasing in transistor circuits is done by using two DC sources V BB and V CC. Emitter Follower & Darlington Amplifier.Transformer Coupled Class A Power Amplifier.
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