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UNIT 3 TRANSISTOR BIASING

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(1) To study and analyze basic biasing circuits like Base bias, Collector to Base bias, and Voltage divider bias. (2) To compare these basic bias circuits. (3) To design these basic Bias circuits and analyze the voltage & current levels, and (4) Qualitative study of thermal stability of bias circuits.

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The process of energizing the transistor to amplify input a.c. signal such that the fluctuations in ac signal should not drive the transistor to either cut-off or saturation region is called biasing. Hence, biasing is nothing but setting up a Q-point of the transistor near the middle of the dc load line.

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1. To keep the Emitter-Base junction forward biased and Collector-Base junction reverse biased during the entire cycle of input signal. 2. To stabilize the Q-point against the changes in temperature, variations in transistor parameters, aging of the components etc. The first condition will ensure the linear operation of transistor. The second condition will protect the transistor from thermal runaway.

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(1) Base bias (also called fixed bias). (2) Base bias with collector feedback (also called collector feedback bias). (3) Voltage divider bias (also called self bias).

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Base bias is used to keep the Emitter-Base junction forward biased and Collector-Base junction reverse biased during the entire cycle of input signal.

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--- Eqn. (1) --- Eqn. (2) --- Eqn. (3)

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(i) This biasing circuit is very simple as only one resistance RB is required. (ii) Biasing conditions can easily be set and the calculations are simple. (iii) There is no loading of the source by the biasing circuit since no external resistor is employed at emitter terminal.

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(i) The values of collector current (I C ) and collector-to-emitter voltage (V CE ) are dependent on current gain. But we know that is strongly dependent upon the temperature. (ii) The stability factor is very high. Therefore, there are large chances in I C which leads thermal runaway.

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The basic circuit used in the base bias (or fixed bias) is that of common-emitter configuration. We know that in common emitter configuration, the stability factor is equal to (1+ ). Therefore, for base-bias circuit the stability factor is given by :

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In order to decrease the stability factor further (ideally S = 1), collector to base bias is used. In collector to base bias, the collector voltage provides necessary bias voltage to Base-Emitter junction. This circuit tries to decrease thermal runaway problem.

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Putting (I C + I B )= I C

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3.3.3 Stability of Collector to Base Bias : Differentiating the above expression with respect to I C, The stability factor (S) of a collector feedback bias is smaller than (1 + ).

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(i) It is a simple method as it requires only two resistors R B & R C. (ii) This circuit provides some stabilization of the operating point as discussed below: If there is an increase in collector leakage current due to increase in temperature, the total collector current tends to increase. As a result, V CE tends to decreases due to greater drop across R C. This will decreases V CB & hence I B. Such decrease in I B decreases I C to original value compensating previous increase.

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(i) The circuit does not provide good stabilization and the stability factor is fairly high, though it is lesser than that of fixed bias. Therefore, the operating point does change, although to lesser extent, due to temperature variations and aging effects. (ii) This circuit provides a negative feedback which reduces the gain of the amplifier.

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This circuit provides a d.c. bias which is independent of the transistor current gain ( ) Circuit Diagram :

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(i) Base Voltage (V B ): (ii) Emitter current (I E ): (iii) Collector current (I C ): Thus in this circuit, collector current I C is independent on.

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Thus the voltage divider bias provides stability factor close to unity.

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1. Voltage divider bias circuit can successfully provide a d.c. Bias which is independent of the transistor current gain (β). 2. This bias circuit has the smallest possible value of stability factor S and leads to the maximum possible thermal stability. Due to design considerations, R T /R E has a value that cannot be neglected as compared to 1. In actual practice, the circuit may have stability factor around 10.

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1. Replace the NPN transistor by a PNP transistor. 2. Reverse all voltages and hence the direction of currents will be reversed.

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3.7.1 Operating Point Stability In any transistor amplifier which is not properly biased, the collector current in a transistor changes rapidly when: (i) the temperature changes, (ii) the transistor is replaced by another of the same type. This is due to the inherent variations of transistor parameters.

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Need for stabilization: Stabilization of the operating point is necessary due to the following reasons : (i) Temperature dependence of I C (ii) Individual component parameter variations (iii) Thermal runaway.

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1. Why Si transistor is commonly used in electronic circuits? (i) Smaller collector leakage current I CO : (ii) Smaller variation of I CO with temperature: (iii) Greater range of working temperature:

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