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EC6304-Electronics Circuits-I Part-B-Unit-I 1.Explain about fixed bias of BJT and derive the expression for stability factor . This form of biasing is also called base bias. In the example image on the right, the single power source (for example, a battery) is used for both collector and base of a transistor, although separate batteries can also be used. In the given circuit, Vcc = IBRB + Vbe Therefore, IB = (Vcc - Vbe)/RB For a given transistor, Vbe does not vary significantly during use. As Vcc is of fixed value, on selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit. Also for given circuit, Vcc = ICRC + Vce Therefore, Vce = Vcc - ICRC The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β on this page. Because IC = βIB we can obtain IC as well. In this manner, operating point given as (Vce,IC) can be set for given transistor. Merits: It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB). A very small number of components are required. Demerits: The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable. Changes in Vbe will change IB and thus cause IE to change. This in turn will alter the gain of the stage. When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift. For small-signal transistors (e.g., not power transistors) with relatively high values of β (i.e., between 100 and 200), this configuration will be prone to thermal runaway. In particular, the stability factor, which is a measure of the change in collector current with changes in reverse saturation current, is approximately β+1. To ensure absolute stability of the amplifier, a stability factor of less than 25 is preferred, and so small-signal transistors have large stability factors. 2.Explain about voltage divider bias of BJT and derive the expression for stability factor. Voltage divider bias is the most popular and used way to bias a transistor. It uses a few resistors to make sure that voltage is divided and distributed into the transistor at correct levels. One resistor, the emitter resistor, RE also helps provide stability against variations in β that may exist from transistor to transistor. The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor. In this circuit the base voltage is given by: voltage across provided . Also For the given circuit, Merits: Unlike above circuits, only one dc supply is necessary. Operating point is almost independent of β variation. Operating point stabilized against shift in temperature. Demerits: In this circuit, to keep IC independent of β the following condition must be met: which is approximately the case if where R1 || R2 denotes the equivalent resistance of R1 and R2 connected in parallel. As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low. o If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling. o If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current 3.Explain about the compensation technique to stabilize Q point using diode and thermistor. Compensation techniques refer to the use of temperature sensitive devices such as diodes, transistors, thermistors etc. which provide compensating voltages and currents to maintain the operating point constant. In comparison, stabilization technique use only resistive biasing circuits. Due to bias circuit, feedback is there and it reduce drastically the amplification of the signal. If this loss in signal gain is intolerable in particular application, it is often possible to use compensating techniques to reduce the drift of the operating point. Diode compensation for is shown. : A circuit utilizing self bias stabilizing technique and diode compensation The diode is kept biased in the forward direction by the source same material and type of transistor, the voltage coefficient as the base to emitter voltage and resistance . If the diode is of across the diode will have the same temperature . If KVL is applied around the base circuit then, Since tracks compensation of w.r.t. temperature, then will be insensitive to variations in . In practice, the is not like this exact, but it is sufficiently effective to take care of great part of transistor drift due to variations in Diode Compensation for : The changes of with temperature contribute significantly to changes in collector current of Si transistors. On the other hand, for Ge transistors changes in with temperature play the more important role in collector current stability. The compensation circuit is shown. It offers stabilization against variations of If diode and the transistor are of same type and material, The reverse saturation current of the diode will increase with temperature at the same rate as the transistor collector saturation current . Now Since the diode is reverse biased by an amount through D is . The base current . for Ge devices, it follows that the current If > > 1 and if D and of transistor track each other over the desired temperature range, then remains essentially constant. 4.Write the factors that affect the stability of Q point and explain how self bias circuit acts as a constant current circuit? It is desirable that once selected, the operating (or Q) point should remain stable i.e. the operating point should not shift its position owing to change in temperature etc. Unfortunately it is not possible in practice unless special efforts are made to achieve it. The maintenance of the operating point stable is called the stabilization. The stabilization of operating point is essential because of a) Temperature dependence of IC (b) Individual variations and (c) Thermal runaway. With the increase in temperature, the collector leakage current ICO, the current gain ß tend to increase and VBE required to produce a given collector current IC tends to decrease. Thus increase in temperature tends to cause increase in IC. The value of ß and VBE are not exactly the same for any two transistors even of the same type. So when a transistor is replaced by another one (even of the same type) the operating point (zero signals IC and VCE) is shifted. The collector current IC, being equal to ßIB + (1+ ß ) ICO, increases with the increase in temperature. This leads to increased power dissipation with further increase in temperature. Being a cumulative process, it can lead to thermal runaway resulting in burn out of the transistor. However, if by some modification, IC is made to fall with increase in temperature automatically, then decrease in the term ß can be made to neutralize the increase in the term (1 +ß ) ICO, thereby keeping IC almost constant. This will achieve thermal stability resulting in bias stability. The biasing network associated with the transistor should fulfill the requirements of (i) ensuring proper zero signal collector current, (ii) ensuring VCE not falling below 0.5 V for Ge transistors and 1 V for Si transistors at any instant and (iii) ensuring stabilization of operating point (zero signal IC and VCE) 5. Potential divider biasing from Given = 15 V is obtained for an npn transistor. Determine the operating point when (a) = 125 and (b) is doubled and comment on stability of operating with change in . Ans. 6. Determine the resistor stability factors? Ans. For fixed bias; For collector to base bias; for a fixed bias and collector to base bias and compare the two The stability factor is lesser using collector to base bias than fixed bias. Hence collector to base bias is more stable. 7. 3. A transistor with = 50, Vbe = 0.7V, Vcc = 22.5V & Rc = 5.6K is used in a biasing circuit. It is designed to establish the quiescent point at Vce = 12V, Ic = 1.5mA, S = 3. Find the values of RE, R1 & R2. 8. Explain the biasing technique of enhancement type MOSFET