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- 1. SYLLABUS ELECTRONIC DEVICES AND CIRCUITS (19EC0402) UNIT – IV Transistor Biasing and Thermal Stabilization: Need for Transistor biasing, Operating point, Load line analysis, Biasing methods - Fixed bias, Collector to Base bias, Self-bias, stability factors, Bias compensation, Thermal Runaway, Thermal stability.
- 2. Need for Transistor biasing Transistor Biasing • The proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of signal is known as Transistor Biasing. • The circuit which provides transistor biasing is called as Biasing Circuit. Need for DC biasing • If a signal of very small voltage is given to the input of BJT, it cannot be amplified. Because, for a BJT, to amplify a signal, two conditions have to be met. • The input voltage should exceed cut-in voltage for the transistor to be ON. • The BJT should be in the active region, to be operated as an amplifier.
- 3. Need for Transistor biasing • If appropriate DC voltages and currents are given through BJT by external sources, so that BJT operates in active region and superimpose the AC signals to be amplified, then this problem can be avoided. • The given DC voltage and currents are so chosen that the transistor remains in active region for entire input AC cycle. • Hence DC biasing is needed.
- 4. Factors affecting the operating point • The main factor that affect the operating point is the temperature. • The operating point shifts due to change in temperature. • As temperature increases, the values of ICE, β, VBE gets affected. • ICBO gets doubled (for every 10o rise) • VBE decreases by 2.5mv (for every 1o rise) • So the main problem which affects the operating point is temperature. • Hence operating point should be made independent of the temperature so as to achieve stability. • To achieve this, biasing circuits are introduced.
- 5. Stabilization • The process of making the operating point independent of temperature changes or variations in transistor parameters is known as Stabilization. • Once the stabilization is achieved, the values of IC and VCE become independent of temperature variations or replacement of transistor. • A good biasing circuit helps in the stabilization of operating point. Need for Stabilization • Stabilization of the operating point has to be achieved due to the following reasons. • Temperature dependence of IC • Individual variations • Thermal runaway
- 6. Stabilization Temperature Dependence of IC As the expression for collector current IC is IC=βIB+ICEOIC=βIB+ICEO =βIB+(β+1)ICBO The collector leakage current ICBO is greatly influenced by temperature variations. To come out of this, the biasing conditions are set so that zero signal collector current IC = 1 mA. Therefore, the operating point needs to be stabilized i.e. it is necessary to keep IC constant.
- 7. Stability Factor • It is understood that IC should be kept constant in spite of variations of ICBO or ICO. • The extent to which a biasing circuit is successful in maintaining this is measured by Stability factor. • It denoted by S. • By definition, the rate of change of collector current IC with respect to the collector leakage current ICO at constant β and IB is called Stability factor.
- 8. Stability Factor • Hence we can understand that any change in collector leakage current changes the collector current to a great extent. • The stability factor should be as low as possible so that the collector current doesn’t get affected. • S=1 is the ideal value. • The general expression of stability factor for a CE configuration can be obtained as under.
- 9. Thermal Runaway • As the expression for collector current IC is • The flow of collector current and also the collector leakage current causes heat dissipation. • If the operating point is not stabilized, there occurs a cumulative effect which increases this heat dissipation. • The self-destruction of such an unstabilized transistor is known as Thermal run away. • In order to avoid thermal runaway and the destruction of transistor, it is necessary to stabilize the operating point, i.e., to keep IC constant.
- 10. Bias Compensation Diode Compensation for Instability • These are the circuits that implement compensation techniques using diodes to deal with biasing instability. The stabilization techniques refer to the use of resistive biasing circuits which permit IB to vary so as to keep IC relatively constant. • There are two types of diode compensation methods. They are – Diode compensation for instability due to VBE variation – Diode compensation for instability due to ICO variation – Let us understand these two compensation methods in detail.
- 11. Bias Compensation Diode Compensation for Instability due to VBE Variation • In a Silicon transistor, the changes in the value of VBE results in the changes in IC. • A diode can be employed in the emitter circuit in order to compensate the variations in VBE or ICO. • As the diode and transistor used are of same material, the voltage VD across the diode has same temperature coefficient as VBE of the transistor.
- 12. Bias Compensation Diode Compensation for Instability due to VBE Variation
- 13. Bias Compensation Diode Compensation for Instability due to ICO Variation
- 14. Thermal Stability • VBE and ICBO Variations – Many transistor circuits are required to operate over a wide temperature range. • So, another aspect of bias circuit stability is Bias Circuit Thermal Stability, or how stable IC and VCE remain when the circuit temperature changes. • Measures to deal with the effects of hFE variations have already been discussed. These apply whether the different hFE values are due to temperature changes or to hFE differences from one transistor to another.
- 16. Transistor load line analysis DC Load line • When the transistor is given the bias and no signal is applied at its input, the load line drawn at such condition, can be understood as DC condition. • Here there will be no amplification as the signal is absent. The circuit will be as shown below. • The value of collector emitter voltage at any given time will be VCE=VCC−ICRC • As VCC and RC are fixed values, the above one is a first degree equation and hence will be a straight line on the output characteristics.
- 17. Transistor load line analysis • This line is called as D.C. Load line. The figure below shows the • To obtain the load line, the two end points of the straight line are to be determined. • Let those two points be A and B.
- 18. Transistor load line analysis To obtain A • When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as VCE=VCC−ICRC 0=VCC−ICRC IC=VCCRC To obtain B • When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as VCE=VCC−ICRC =VCC
- 19. Biasing methods Transistors are one of the largely used semiconductor devices which are used for wide variety of applications including amplification and switching. • However to achieve these functions satisfactorily, transistor has to be supplied with certain amount of current and/or voltage. • The process of setting these conditions for a transistor circuit is referred to as Transistor Biasing. • This goal can be accomplished by variety of techniques which give rise to different kinds of biasing circuits.
- 20. Fixed Base Bias or Fixed Resistance Bias • A base resistor RB connected between the base and the VCC. Here the base-emitter junction of the transistor is forward biased by the voltage drop across RB which is the result of IB flowing through it. • From the figure, the mathematical expression for IB is obtained as
- 21. Fixed Base Bias or Fixed Resistance Bias
- 22. Collector to base Bias • The base resistor RB is connected across the collector and the base terminals of the transistor. • This means that the base voltage, VB and the collector voltage, VC are inter-dependent due to the fact that
- 23. Collector to base Bias
- 24. Voltage Divider Bias or self bias • This type of biasing network employs a voltage divider formed by the resistors R1 and R2 to bias the transistor. • This means that here the voltage developed across R2 will be the base voltage of the transistor which forward biases its base-emitter junction.
- 25. Voltage Divider Bias or self bias •In this kind of biasing, IC is resistant to the changes in both β as well as VBE which results in a stability factor of 1 (theoretically), the maximum possible thermal stability. • This is because, as IC increases due to a rise in temperature, IE also increases causing an increase in the emitter voltage VE which in turn reduces the base-emitter voltage, VBE. •This results in the decrease of base current IB which restores IC to its original value.
- 26. References • https:// www.tutorialspoint.com/amplifiers/bias_compensatio n.htm • http://ecetutorials.com/analog-electronics/transistor-b iasing-at-q-or-quiescent-pointstabilization-biasing-cir cuitsthermal-runaway / • https:// jntua.ac.in/gate-online-classes/registration/downloads /material/a159306483768.pdf • https:// electronicscoach.com/difference-between-jfet-and-mo sfet.html