Chapter-6 DC biasing-1.ppt
- 2. Outlines DC load line Operating region Selection of operating point Operating point Transistor biasing Stabilization Stability factor Types of biasing ckt 1. Fixed Biasing Circuits (Base Resistor Method) 2. Emitter Bias 3. Biasing with collector-feedback resistor 4. Voltage Divider Biasing Si Vs. Ge 2
- 3. DC load line Load line: locus of operating point on o/p characteristics of transistor A line on which operating point moves according to input ac signal 3
- 4. Q-point: It is point on load line which represents DC value of VCE and IC in absence of signal. Best position is mid-point on load line, VCE= ½ VCC 4
- 5. Operating regions Cut-off region Saturation region 5
- 6. Selection of operating point 6 Near cut-off Near saturation Active region
- 7. Operating Point or Quiescent Point For transistor amplifiers the resulting dc current and voltage establish an operating point on the characteristics that define the region that will be employed for amplification of the applied signal. Since the operating point is a fixed point on the characteristics, it is also called the quiescent point (abbreviated Q-point). By definition, quiescent means quiet, still, inactive. Figure shows a general output device characteristic with four operating points indicated. 7
- 9. Contd... The biasing circuit can be designed to set the device operation at any of these points or others within the active region. The maximum ratings are indicated on the characteristics of Fig. by a horizontal line for the maximum collector current ICmax and a vertical line at the maximum collector-to-emitter voltage VCEmax. The maximum power constraint is defined by the curve PCmax in the same figure. At the lower end of the scales are the cutoff region, defined by IB = 0 , and the saturation region, defined by VCE=VCEsat. 9
- 10. Need Of Operating Point Temperature causes the device parameters such as the transistor current gain (ac) and the transistor leakage current (ICEO) to change. Higher temperatures result in increased leakage currents in the device, thereby changing the operating condition set by the biasing network. The result is that the network design must also provide a degree of temperature stability so that temperature changes result in minimum changes in the operating point. This maintenance of the operating point can be specified by a stability factor, S. 10
- 11. Example 1. For CE configuration Vcc = 10 v, Rc = 8k. Draw DC load line. Find Q-point for zero signal if base current is 15μA, β=40 Ic = 1.25mA, Zero signal Ic=β IB = 0.6mA, VCE = 5.2v 2. In transistor ckt Rc= 5k, quiescent current is 1.2mA. Determine Q-point when Vcc=12 v. how will point change when Rc is changed from 5k to 7.5k Operating pt 1: (6 v, 1.2mA) Operating pt 2: (3 v, 1.2mA) 11
- 12. Transistor biasing Faithful amplification: “The process of raising strength of weak signal without change in its general shape is known as faithful amplification” Proper zero signal collector current Min proper base-emitter voltage at any instant Min proper collector-emitter voltage at any instant Key point for Faithful Amplification 1 2 3 12
- 13. BJT to be biased in its linear or active operating region the following must be true: 1. Proper zero signal collector current: Zero sig Ic ≥ max Ic due to signal alone 13
- 15. 2. Proper min VBE: The base–emitter junction must be forward-biased (p-region voltage more positive), with a resulting forward-bias voltage of about 0.3v (Ge) and 0.7 V (Si). 15
- 16. 3. Proper min VCE: The base–collector junction must be reverse-biased (n-region more positive), with the reverse-bias voltage being any value within the maximum limits of the device. β falls if VCE is not proper unfaithful amplification VCE = 0.5 v(Ge) and 1v (Si) 16
- 17. Example An NPN – Si transistor has Vcc = 6v, Rc=2.5k Find (1) max Ic that can be allowed during application of signal for faithful amplification (2) min zero sig collector current required Ic max = 2mA Ic min 0 signal = 1mA 17
- 18. 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 basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction properly reverse biased during the application of signal This can be achieved with a bias battery or associating a circuit with a transistor The circuit which provides transistor biasing is known as biasing circuit Biasing is very essential for the proper operation of transistor in any circuit. 18
- 19. Inherent Variations of Transistor Parameters Parameters such as β, VBE are not the same for every transistor even of the same type The major reason for these variations is manufacturing techniques have not too much advanced. For instance, it has not been possible to control the base width and it may vary, although slightly, from one transistor to the other even of the same type. Such small variations result in large change in transistor parameters such as β, VBE The inherent variations of transistor parameters may change the operating point, resulting in unfaithful amplification 19
- 20. Stabilization The collector current in a transistor changes rapidly when, 1. The temperature changes, 2. The transistor is replaced by another of the same type. When the temperature changes or the transistor is replaced, the operating point (i.e. zero signal IC and VCE) also changes The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilisation Need for stabilisation: 1. Temperature dependence of IC 2. Individual variations 3. Thermal runaway 20
- 21. 1. Temperature dependence of IC IC = β IB + ICEO = β IB + (β + 1) ICBO Temp ↑ •ICBO Influenced by Temp ICBO ↑ •Rise in 10˚ will double ICBO IC ↑ •Apply proper Bias to set zero sig Ic Q – point change •Make Ic constant inspite of changes in Temp 21
- 22. 2. Individual variations The value of β and VBE are not exactly the same for any two transistors even of the same type VBE itself decreases when temperature increases When a transistor is replaced by another of the same type, these variations change the operating point 22
- 23. 3. Thermal runaway Temp ↑ ICBO ↑ IC↑ β ICBO IC = β IB + ICEO = β IB + (β + 1) ICBO If no stabilization applied • The self-destruction of an unstabilised transistor is known as Thermal runaway • To avoid thermal runaway - IC is kept constant • This is done by causing IB to decrease automatically with temperature increase by circuit modification • Then decrease in β, IB will compensate for the increase in (β + 1) ICBO, keeping IC nearly constant. cumulative 23
- 24. Essentials of a Transistor Biasing Circuit Transistor biasing is required for faithful amplification. The biasing network associated with the transistor should meet the following requirements : 1. It should ensure proper zero signal collector current. 2. It should ensure that VCE does not fall below 0.5 V for Ge transistors and 1 V for silicon transistors at any instant 3. It should ensure the stabilisation of operating point 24
- 25. Stability Factor The rate of change of collector current IC w.r.t. the collector leakage current ICO at constant β and IB is called stability factor i.e. To achieve greater thermal stability, it is desirable to have as low stability factor as possible. The ideal value of S is 1 Experience shows that values of S exceeding 25 result in unsatisfactory performance 25
- 26. General expression of S-factor 26
- 27. Types of Biasing Circuits It is desirable that transistor ckt should have a single source of supply—the one in the output circuit (i.e. VCC). The following are commonly used methods of obtaining transistor biasing from one source of supply (i.e. VCC ) : 1. Fixed Biasing Circuits (Base Resistor Method) 2. Emitter Bias 3. Biasing with collector-feedback resistor 4. Voltage Divider Biasing In all the methods, the same basic principle is employed i.e. required value of IB(and hence IC) is obtained from VCC in the zero signal conditions. The value of RC is selected so that proper min VCE maintained 27
- 28. 1. Base resistor method The required value of zero signal IB (and hence IC = βIB) can be made to flow by selecting the proper value of base resistor RB VCC is a fixed known quantity and IB is chosen at some suitable value. Hence, RB can always be found directly, and for this reason, this method is called fixed-bias method 28
- 29. Stability factor In fixed-bias method of biasing, IB is independent of IC so that dIB/dIC = 0. Putting the value of dIB / dIC = 0 in the above expression, we have, Stability factor, S = β + 1 IC changes (β + 1) times as much as any change in ICO. For instance, if β = 100, then S = 101 which means that IC increases 101 times faster than ICO Due to the large value of S in a fixed bias, it has poor thermal stability 29
- 30. Advantages : 1. Circuit is very simple as only one resistance RB is required 2. Biasing conditions can easily be set and the calculations are simple 3. There is no loading of the source by the biasing circuit since no resistor is employed across base-emitter junction Disadvantages : 1. This method provides poor stabilisation. It is because there is no means to stop a self increase in collector current due to temperature rise and individual variations. For example, if β increases due to transistor replacement, then IC also increases by the same factor as IB is constant. 2. The stability factor is very high. Therefore, there are strong chances of thermal runaway Due to these disadvantages, this method of biasing is rarely employed 30
- 31. Example Fig shows biasing with base resistor method. (i) Determine IC and VCE. Neglect small base- emitter voltage. Given that β = 50 (ii) If RB in this circuit is changed to 50 kΩ, find the new operating point 1. IB = 20 μA IC = 1mA VCE = 7 v 2. IB = 40 μA IC = 2mA VCE = 5 v 31
- 32. Example Design base resistor bias circuit for a CE amplifier such that operating point is VCE = 8V and IC = 2 mA. You are supplied with a fixed 15V d.c. supply and a silicon transistor with β = 100. Take base-emitter voltage VBE = 0.6V. Calculate also the value of load resistance that would be employed. Rc = 3.5 k RB = 720k 32
- 33. 2. Emitter Bias circuit This ckt uses two separate d.c. voltage sources ; one positive (+ VCC) and the other negative (– VEE). Normally, the two supply voltages will be equal. For example, if VCC = + 20V (d.c.), then VEE = – 20V(d.c.) There is a resistor RE in the emitter circuit 33
- 34. Circuit Analysis of Emitter Bias IC: – IB RB – VBE – IE RE + VEE = 0 ∴ VEE = IB RB + VBE + IERE 34
- 35. Circuit Analysis of Emitter Bias VCE: Applying KVL to the collector side: VCC – IC RC – VCE – IC RE + VEE = 0 or VCE = VCC + VEE – IC (RC + RE) 35
- 36. Stability of Emitter bias 36
- 37. Example For the emitter bias circuit shown in Fig. find IE, IC,VC and VCE for β = 85 and VBE = 0.7V. IE = IC = 1.73mA VC = 11.9v VCE = 14.6v 37
- 38. 3. Biasing with collector-feedback resistor Stability factor, S < (β + 1) 38
- 39. Advantages 1. It is a simple method as it requires only RB 2. This circuit provides some stabilisation of the operating point as discussed below : VCE = VBE + VCB Temp ↑ ICO ↑ IC↑ VCE ↓ VCB ↓ IB↓ IC↓ VCC = IC RC + VCE 39 VCB = IB RB
- 40. Disadvantages 1. The circuit does not provide good stabilisation because 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 other effects. 2. This circuit provides a negative feedback which reduces the gain of the amplifier as explained hereafter. During the positive half-cycle of the signal, the collector current increases. The increased collector current would result in greater voltage drop across RC. This will reduce the base current and hence collector current 40
- 41. Example Fig. shows a silicon transistor biased by collector feedback resistor method. Determine the operating point. Given that β = 100. IB = 0.096 mA IC = 9.6 mA VCE = VCC − IC RC = 10.4 V ∴ Operating point is 10.4 V, 9.6 mA. 41
- 42. Example It is desired to set the operating point at 2V, 1mA by biasing a silicon transistor with collector feedback resistor RB. If β = 100, find the value of RB.. 42
- 43. Example Find the Q-point values (IC and VCE) for the collector feedback bias circuit shown in Fig 43
- 44. Stability of Q-point 44 β ↑ VBE ↓ TEMP Temp ↑ VBE ↓ IB ↑ IC ↑ ICRC ↑ VC ↓ IBRB↓ IB ↓ IC ↓ β ↑ IC= βIB ↑
- 45. Voltage Divider Bias Method Most widely used method Two resistances R1 and R2 are connected across the supply voltage VCC The emitter resistance RE provides stabilisation The voltage drop across R2 forward biases the EB junction. This causes the base current and hence collector current flow in zero signal conditions 45
- 48. Stabilization Temp ↑ IC ↑ ICRE ↑ VBE ↓ IB↓ IC ↓ V2 = VBE + IC*RE 48
- 49. Stability factor S S factor is given by, Proof If the ratio R0/RE is very small, then R0/RE can be neglected as compared to 1 and the stability factor becomes : 49
- 50. Example A transistor uses potential divider method of biasing.R1=50kΩ, R2=10 kΩ and RE = 1kΩ. If VCC = 12 V, find : (i) the value of IC ; given VBE = 0.1V (ii) the value of IC ; given VBE = 0.3V. Comment on the result. 50 Comment: From the above example, it is clear that although VBE varies by 300%, the value of IC changes only by nearly 10%. This explains that in this method, IC is almost independent of transistor parameter variations
- 51. Example The circuit shown in Fig. uses silicon transistor having β = 100. Find the operating point and stability factor. 51
- 52. 52
- 53. Si Vs. Ge Parameter Si Ge ICBO 0.01 μA to 1μA 2 to 15 μA variation of ICBO with temperature ICBO doubles with each 12°C rise ICBO doubles with each 8 to 10°C rise working temperature 150°C 70°C PIV rating of diode 1000V 400V 53
- 55. 1. Diode compensation for variation in VBE 55
- 56. Diode compensation for variation in VBE 56
- 57. 2. Diode compensation for variation in ICO 57
- 58. 3. Thermistor compensation 58 Negative temperature co-efficient
- 59. 4. Sensistor compensation 59 Positive temperature co-efficient