Biasing of BJT IENGINEERS- CONSULTANTS LECTURE NOTES SERIES ELECTRONICS ENGINEERING 1 YEAR UPTU. Page 1
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1 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU iasing of JT As we know that JT can be operated in three regions: active, saturation and cutoff by applying proper voltage condition. n order to use JT in desired region a particular external voltage with correct polarity and magnitude should be applied at both the junctions. This application of voltage is called biasing. When we bias JT, we establish a certain amount of current and voltage condition for the JT. These conditions are called operating conditions or d.c. operating point or quiescent point. To bias JT in active region emitter junction of JT should be forward bias and collector junction should be reverse biased. For this at least two voltage sources will be required. For example if we bias npn JT in active region we should apply voltage V across emitter junction and voltage V between collector and emitter. f we use Si JT in active region V should be 0.7V and V should be greater than 1.4V for voltage across collector junction greater than voltage across emitter junction. (See Figure-1) Figure-1 Figure-2 ut this small voltage source (0.7V or 1.4V) will be very difficult to manage practically. So we can connect resistance to base and collector terminals while using large voltage source. So we can say that to bias JT in active region at least two voltage sources and two resistors are required. (See figure-2). 1
2 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU For proper operation of JT position of operating point should be stable. However position of operating point changes on changing parameters of JT such as β, O and V. As JT parameters are temperature dependent, the operating point also varies on changing temperature. We know that in a transistor, power is dissipated in the collector and hence it is made physically larger than the emitter and base region. As the power is dissipated, there is a chance for the collector base junction temperature to be raised. As the temperature at collector base junction increases, the reverse leakage current O increases. This is because O arises due to the flow of minority carriers which are thermally generated across reverse biased collector-base junction (reverse biased pn junction). As the temperature increases, thermal generation increases O increases. = α + O So, as O increases, increases. Power dissipated = 2 *. So, as collector current increases, power dissipated increases which in turn increases the collector base junction temperature. So the process is cumulative leading eventually to the destruction of the transistor. This is called thermal runaway. Thermal runaway can be prevented by using a heat sink or different biasing circuits. On changing temperature or any other parameters of JT position of operating point can be shifted from active to saturation or can be from active to cut-off. So to use JT as an amplifier operating point of JT should be stabilized in active region with respect to different parameters and temperature of JT. Shift in position of operating point can be understood by figure-a. 2
3 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU Figure-A shifting of operating point n this figure we can see that on changing, V or position of operating point, Q can be changed from active region (Q) to saturation region(towards point P) or it can be changed to cut-off region (towards point ). To stabilize operating with respect to change in JT parameters and temperature, time to time different circuits are used. n this discussion we will discuss different biasing circuits on the basis of their ability to stabilize operating point. To analyze biasing circuit on the basis of stability of operating point we can follow some steps which can make analysis easier. The steps are as follows. Stability with respect to temperature 1- Find current in term of using KVL in base terminal. 2- Differentiate with respect. For stability of operating point with respect to O we use current equation of JT in active region, = β + (β+1) O So variation in with respect to O will be / O. This is called stability factor with respect to O. This parameter also represents effect of temperature on. On differentiating equation-1 with respect to 1=β / + (β+1) O/ S= / O= (β+1)/ (1-β / )
4 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU 3-Find stability factor(s) with respect to temperature using equation-2. Stability with respect to β 1- Find in terms of β using KVL 2- Find using = β. 3-Analyse dependency of on β. Fixed bias circuit: The circuit shown in figure-3(a) is the fixed bias circuit. This is simplest circuit among all available. n D. analysis we consider only of D. source on biasing circuit. n presence of D. source only all capacitors in the circuit will be open circuit because reactance of capacitor for D. source, X= 1/2πf=1/2π0=. The D. equivalent circuit of fixed bias circuit is shown in figure-3(b). Figure-3(a) Figure-3(b) Stability with respect to temperature- Apply KVL in circuit connected to base terminal. efore applying KVL, simplify the biasing circuit, as shown in figure-4. Apply KVL in path G-A---G1. 4
5 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU V - - V=0 V V , S Figure-4 (Simplified circuit) Stability factor with respect to O, S=β+1 signify that variation in with respect to temperature or O is very large. Therefore we can say that stability of operating point with respect to temperature is very poor, because value of β is generally large. Stability with respect to β- From equation-3 we can find, V V quation-5 shows that is totally dependent on β and therefore we can say variation in β will vary and stability of operating point with respect to β is very poor. Advantages of fixed biasing circuit- 5
6 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU 1- This is very simple circuit to design. 2- We can change position of operating point only by changing value of resistance. Disadvantages of fixed biasing circuit- 1- Very poor stability of operating point with respect to temperature and O. 2- Very poor stability of operating point with respect to β. Fixed biasing with emitter resistance- The circuit shown in figure-5(a) is the fixed bias circuit with emitter resistance. n this biasing circuit a resistance is connected to emitter terminal in fixed biasing which can improve stability of operating point. n D. analysis we consider only of D. source on biasing circuit. n presence of D. source only all capacitors in the circuit will be open circuit because reactance of capacitor for D. source, X= 1/2πf=1/2π0=. The D. equivalent circuit of this biasing circuit is shown in figure-5(b). Figure-5(a) Figure-5(b) 6
7 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU Stability with respect to temperature- Apply KVL in circuit connected to base terminal. efore applying KVL, simplify the biasing circuit, as shown in figure-6. Apply KVL in path G-A---G1. V - - V - =0, we know =+ So, V - - V--=0, actually we have to find in term of So, V V, Figure-6 From equation-6 we can see that on increasing current will be decreased. As we know =β, due to decrement in, will be decreased. So we can say due to presence of in the circuit, increment in due to temperature will be compensated 7
8 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU by decrement in. Therefore we can say stability of operating point will be improved with respect to temperature which can be verified by value of stability factor. On differentiating equation-6 with respect to, So stability factor, 1 S f we compare value of stability factor with fixed biasing we can see, stability factor of 1 this biasing is reduced by a factor, 1, so stability of operating point of this biasing with respect to change in temperature and O is better than fixed biasing. f we use >> 1 S 1 Then stability factor, S is reduced to 1. Stability of operating 1 is much improved. So, to reduce value of stability factor to 1 we should use very large value of and very small value of. ut i) f we use large then there will be large power loss and large part of out put across will be feed backed to input. Due to feed backing of out put gain of amplifier using this circuit will be reduced. ii) f we use small then we will be forced to use very small voltage source, which is practically difficult to realize. Therefore we can say that by using this biasing circuit we can improve stability of operating point, but to reduce stability factor to 1 we have to compromise with large power loss and reduction in of amplifier. Stability with respect to β- To analyze stability with respect of β, first of all we should find out in term of β. For this apply KVL in G-A---G1. 8
9 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU V V 0 We know, So, ( ( V V 1) ) Then ( 1) ( ( V V 1) ) f ( 1), then ( V V From above equation we can see that is independent of β. ut for this large β, large and small is required. i) On using large β we will be restricted to use only JT with large β. ii) On using large there will be a large power loss and large part of output will be feed backed to input that will reduce gain of amplifier. iii) On using small we will be forced to use small voltage source, which is practically difficult to realize. From above discussion it is conform that stability of operating point with respect to β is much better than fixed biasing but we have to compromise with large power loss and reduction in gain and also small voltage source is required at emitter junction. So stability of operating point with respect to β can be improved up to some particular limit. Advantages- 1- Stability of operating point with respect to temperature and O is better than fixed biasing. 2- Stability of operating point with respect to temperature and β is better than fixed biasing. Disadvantages- 1- To improve stability we have to compromise with large power loss and reduction in gain. 2- To improve stability we are forced to use large β and small voltage source at base terminal. ) 9
10 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU 3-On extra resistor () is required in this biasing, so cost of this biasing circuit will be more. ollector to base biasing circuit- ircuit shown in figure-7(a) is called collector to base biasing circuit. n this biasing a feedback resistor is connected between collector and base terminal. This feedback resistance is used to improve stability of operating point with respect to change in temperature and β. n D. analysis we consider only of D. source on biasing circuit. n presence of D. source only all capacitors in the circuit will be open circuit because reactance of capacitor for D. source, X= 1/2πf=1/2π0=. The D. equivalent circuit of fixed bias circuit is shown in figure-7(b). Figure-7(a) ollector to base biasing Figure-7(b) D. equivalent circuit Stability with respect to temperature- For analysis of stability first of all we simplify biasing circuit as shown in figure-8. 10
11 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU Figure-8 To find out in terms of first of all apply KVL in loop G1-A----G in figure-8. V ( ) V 0 V V..7 From equation-7 we can see that on increasing current will be decreased. As we know =β, due to decrement in, will be decreased. So we can say due to presence of in feed back form in the circuit, increment in due to temperature will be compensated by decrement in. Therefore we can say stability of operating point will be improved with respect to temperature which can be verified by value of stability factor. On differentiating equation-7 with respect to, So stability factor, S f we compare value of stability factor with fixed biasing we can see, stability factor of 1 this biasing is reduced by a factor, 1, so stability of operating
12 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU point of this biasing with respect to change in temperature and O is better than fixed biasing. f we use >> 1 S 1 1 Then stability factor, S is reduced to 1. Stability of operating is much improved. So, to reduce value of stability factor to 1 we should use very large value of and very small value of. ut i) if we use large then there will be large power loss and large part of out put a will be feed backed to input through feed back resistance. Due to feed backing of out put gain of amplifier using this circuit will be reduced. ii) f we use small then we will be forced to use very small voltage source, which is practically difficult to realize. Therefore we can say that by using this biasing circuit we can improve stability of operating point, but to reduce stability factor to 1 we have to compromise with large power loss and reduction in of amplifier. Stability with respect to β- To analyze stability with respect of β, first of all we should find out in term of β. For this apply KVL in G1-A----G in figure-8. V ( ) V 0 We know, So, ( ( V V 1) ) Then ( 1) ( ( V V 1) ) f ( 1), then ( V V From above equation we can see that is independent of β. ut for this large β, large and small is required. i) On using large β we will be restricted to use only JT with large β. ) 12
13 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU ii) On using large there will be a large power loss and large part of output will be feed backed to input that will reduce gain of amplifier. iii) On using small we will be forced to use small voltage source, which is practically difficult to realize. From above discussion it is conform that stability of operating point with respect to β is much better than fixed biasing but we have to compromise with large power loss and reduction in gain and also small voltage source is required at emitter junction. So stability of operating point with respect to β can be improved only up to some particular limit. Advantages- 1- Stability of operating point with respect to temperature and O is better than fixed biasing. 2- Stability of operating point with respect to temperature and β is better than fixed biasing. Disadvantages- 1- To improve stability we have to compromise with large power loss and reduction in gain. 2- To improve stability we are forced to use large β and small voltage source at base terminal. Voltage divider biasing with emitter resistance with unbypassed resistance- 13
14 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU Figure-9(a) Voltage divider biasing Figure-9(b) D. equivalent circuit Figure-9 shows the voltage divider circuit. n this circuit biasing is provided by three resistors 1, 2 and. esistors 1 and 2 are used as voltage divider circuit to provide fixed voltage at base terminal. n D. analysis we consider only of D. source on biasing circuit. n presence of D. source only all capacitors in the circuit will be open circuit because reactance of capacitor for D. source, X= 1/2πf=1/2π0=. The D. equivalent circuit of fixed bias circuit is shown in figure- 9(b). efore to start analysis of circuit we should simplify biasing circuit, the simplified circuit will be as shown in figure-10. This circuit can be more simplified by replacing voltage divider circuit (using 1 and 2) by its Thevenin equivalent circuit. For equivalent circuit we use Thevenin theorem. 14
15 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU Figure-10 simplified voltage divider biasing circuit According to Thevenin theorem we find out voltage across points and G1, this voltage is called Thevenin equivalent voltage, VT. V T V The equivalent resistance across point and G1 is called TH TH figure-11) , TH is replaced by in the equivalent circuit. (See 15
16 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU Figure-12 Thevenin equivalent circuit of voltage divider part of biasing circuit Now the most simplified circuit will be as shown in figure-12. Figure-12 Simplified voltage divider circuit From figure-12 it is clear that it is similar to fixed biasing with emitter resistance as shown in figure-6. Then question arises that if it is similar to circuit in figure-6 then why we use voltage divider circuit. Actually, as we have discussed in fixed biasing with emitter resistance, to improve stability of operating point we have to use emitter resistor much larger than base resistance. To generate small we use voltage divider connection at base terminal in this circuit. f 2 used in this biasing is much smaller than 1 then value of TH which represents will be very small and for this we are not forced to use small V. This is benefit of using voltage divider connection at base terminal. Stability analysis of this biasing will be similar to fixed biasing with emitter resistance. The stability of this biasing be will better than all biasing circuits which we have discussed. ut still some problems are available in this biasing- 1- Due to presence of large, there will be large power loss. 2- Due to presence of some part of out put will be feed backed to input then gain of amplifier using this circuit will be reduced. 16
17 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU 3-Due to presence of 1 and 2 circuit will be complex and due to large 1 there will be large power loss. We can compromise with large power loss, complex circuit, but we can not compromise with low gain of an amplifier. The Gain of any amplifier concern with only a.c out put and a.c input. The gain of an amplifier reduces due to feed backing of out a.c out put to input. f we can protect a.c out put from feed backing across emitter resistance, then reduction in gain can be controlled. t can be possible if we provide a bypass for a.c. across emitter resistance. So to do this we use a bypass capacitor across emitter resistance, and biasing circuit is called voltage divider biasing with bypassed emitter resistance. Voltage divider biasing with bypassed emitter resistance- The circuit shown in figure-13(a) is voltage divider biasing with bypassed emitter resistance. n D. analysis we consider only of D. source on biasing circuit. n presence of D. source only all capacitors in the circuit will be open circuit because reactance of capacitor for D. source, X= 1/2πf=1/2π0=. The D. equivalent circuit of this bias circuit is shown in figure-13(b). Figure-13(a) Voltage divider circuit with bypassed (b) D. equivalent circuit. As we can see in figure-13(b) that D.. equivalent circuit of voltage divider biasing with bypassed and unbypassed emitter resistance is same. So from D. analysis point of view both biasing will represent same behavior and short comings. 17
18 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU n a.c. analysis we analyze circuit in presence of a.c. source only. n presence of a.c. source only all capacitor in the circuit will be short circuited and equivalent circuit will be called a.c equivalent circuit.(see figure-14) Figure-14(a) a.c equivalent circuit n a,c analysis effect of in this biasing circuit will be reduced to zero due to short circuit effect of bypass capacitor,.(see figure-14). Figure-14(b) a.c equivalent circuit with reduced to zero. Due to zero effect from resistance for a.c source, no part of a.c. out put will be feed backed to input. So, there will no reduction in gain of amplifier due to in the circuit. Therefore we can say that voltage divider biasing with bypassed emitter resistance is best among all which we have discussed. This biasing provides good stability of operating point with respect to variation in different JT parameters and temperature 18
19 NGNS- ONSULTANTS LTU NOTS SS LTONS NGNNG 1 YA UPTU also. ut for this we have to compromise with some power loss due to large and large 1 and also we have to compromise with complex circuit to design. So we have to also compromise with cost of circuit. 19
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