Analogue Electronics II Lec 5 PDF

Analogue Electronics II Lec 5 Lecturer Arwa Alaaeldin Mursi Elamin 2023 Lecture Contents Transistor Biasing Circuits:  Transistor Biasing Circuits: an Introduction Biasing refers to the establishment of suitable dc values of different currents and voltages of a...

Analogue Electronics II Lec 5 Lecturer Arwa Alaaeldin Mursi Elamin 2023 Lecture Contents Transistor Biasing Circuits:  Transistor Biasing Circuits: an Introduction Biasing refers to the establishment of suitable dc values of different currents and voltages of a given transistor. Through proper biasing, a desired DC operating point or quiescent point; Q-Point of the transistor amplifier, in the active region (linear region) of the characteristics is obtained. The goal of amplification, in most cases, is to increase the amplitude of an ac signal without distortion or clipping the wave form. 3 Transistor Biasing Circuits: an Introduction The selection of a proper DC operating point or quiescent point, generally depends on the following factors: (a) The amplitude of the ac signal to be handled by the amplifier and distortion level in signal. Applying large ac voltages to the base would result in driving the collector current into saturation or cutoff regions resulting in a distorted or clipped wave form. (b) The load to which the amplifier is to work for a corresponding supply voltage. 4 The DC Operating Point: Biasing and Stability  The goal of amplification, in most cases, is to increase the amplitude of an ac signal without distortion or clipping the wave form. 5  Transistor Output Characteristics: IC IC IB = 40A IB = 30A IB = 20A IB = 10A VCE Early voltage Cutoff region  At a fixed IB, IC is not dependent on VCE Nasim Zafar 6 Transistor Output Characteristics: Load Line – Biasing and Stability: The requirement is to set the Q-point such that that it does not go into the saturation or cutoff regions when an a ac signal is applied. 7 The DC Operating Point: Biasing and Stability Slope of the Load Line: VCC = VCE + VRC VCE = VCC -- VRC VCE = VCC -- IC RC 1 VCC I c ( )VCE  Rc RC 8 The DC Operating Point: Biasing and Stability Load Line drawn on output characteristic curves. Determines quiescent point, Q Q is between saturation and cutoff Best Q for a linear amplifier: Midway between saturation and cutoff. 9  Optimum Q-point with amplifier operation. IC IC(sat) IB = 50 A IB I C βI B IB = 40 A IC(sat)/2 Q-Point IB = 30 A IB = 20 A IB = 10 A IB = 0 A VCE VCC/2 VCC VCE VCC  I C RC 10  The DC Operating Point: Biasing and Stability For this particular transistor we see that 30 mA of collector current is best for maximum amplification, giving equal amount above and below the Q-point. A generic dc load line. IC VCC  VCE VCC IC  I C (sat)  RC RC VCE (off ) VCC VCE 12 The DC Operating Point: Biasing and Stability Q-Point and Current Gain βdc βdc not a constant βdc Dependent on: Operating Point Q Temperature 13 The DC Operating Point: Biasing and Stability  The DC operating point of a transistor amplifier shifts mainly due to changes in the temperature, since the transistor parameters: — β, ICO and VBE —are functions of temperature.  100 < βdc < 300  = dc current gain = hFE  We will discuss some of the methods used for biasing the transistor circuits. 14 Transistor Biasing Circuits: Biasing - Circuit Configurations:  1. Fixed-Biased Transistor Circuits.  2. Fixed-Biased with Emitter Resistance Circuits.  3. Voltage-Divider-Biased Transistor Circuits. 15 Transistor Biasing Circuits: 1. Fixed-Biased Transistor Circuits. - Highly dependent on βdc 2. Fixed-Bias with Emitter Resistance Circuits. Add emitter resistor Greatly reduces effects of change of β Equations highly dependent on βdc 16 17 DC Voltages and Currents in a BJT:  Active region - Amplifier: BJT acts as a signal amplifier. 1. B-E Junction Forward BiasedC C IC IC VBE ≈ 0.7 V for Si B B IB 2. B-C Junction Reverse Biased IB IE IE E E 3. KCL: I E = I C + IB 18 1. Fixed-Biased Transistor Circuits: – Single Power Supply 19 1. Transistor Fixed-Bias Circuits: Base–Emitter Loop: Collector–Emitter Loop: VCE = VCC -- IC RL (a) Fixed-Bias Circuit. (b) Equivalent Circuit. 20 1. Transistor Fixed-Bias Circuits: Current-Voltage Equations for Fixed-Bias circuits: VCC  VBE IB  RB I C  I B VCE VCC  I C RC 21 Example 3. V  0.7V 8V  0.7V I B  CC  +8 V RB 360kΩ 20.28μA RC I C hFE I B 100 20.28μA  I 2 k 2.028mA RB C 360 k VCE VCC  I C RC IB 8V  2.028mA 2kΩ  hFE = 100 3.94V +0.7 V IE The circuit is midpoint biased. VBE 22 Example 4. Construct the dc load line for the circuit shown in the above figure, and plot the Q-point from the values obtained in Example 3. Determine whether the circuit is midpoint biased. IC (mA) V 8V I C (sat )  CC  4mA 4 RC 2kΩ 3 2 Q VCE off  VCC 8V 1 VCE (V) 2 4 6 8 10 23 Example 5. (Q-point shift.) The transistor in the figure below has values of hFE = 100 when T = 25 °C and hFE = 150 when T = 100 °C. Determine the Q-point values of IC and VCE at both of these temperatures. +8 V Temp(°C) IB (A) IC (mA) VCE (V) RC 25 20.28 2.028 3.94 I 2 k RB C 100 20.28 3.04 1.92 360 k IB hFE = 100 (T = 25 C) hFE = 150 (T = 100 C) +0.7 V IE VBE 24  2. Fixed-Bias with Emitter Resistance:  1. Base-Emitter Loop: KCL: IE = IC + IB The emitter current can be written as: From the above two equation we get: Fixed-Bias Circuit with Emitter Resistance 25  2. Fixed-Bias with Emitter Resistance.  2. Collector-Emitter Loop with the base current known, IC can be easily calculated by the relation IC = β IB. Fixed-Bias Circuit with Emitter Resistance 26  3. Voltage-Divider-Bias Circuits: Voltage-Divider Bias Circuits: Sometimes referred to as Universal-Bias Circuit: Most stable Need IB > 1), hFE = 50 VE 1.37V I CQ   1.25mA R2 RE 1.1kΩ I2 RE 4.7 k IE 1.1 k VCEQ VCC  I CQ RC  RE  10V  1.25mA 4.1kΩ  4.87V 36 Example 2. Verify that I2 > 10 IB. +10 V V 2.07V I2  B  440.4μA IC RC R2 4.7kΩ R1 3 k I1 18 k IE 1.25mA IB IB   hFE  1 50+1 hFE = 50 24.51μA R2 I2 RE 4.7 k 1.1 k  I 2  10 I B IE 37 Example 3. A voltage-divider bias circuit has the following values: R1 = 1.5 k, R2 = 680 , RC = 260 , RE = 240  and VCC = 10 V. Assuming the transistor is a 2N3904, determine the value of IB for the circuit. R2 680Ω VB VCC 10V  3.12V R1  R2 2180Ω VE VB  0.7V 3.12V  0.7V 2.42V VE 2.42V I CQ  I E   10mA RE 240Ω hFE ( ave )  hFE (min) hFE (max)  100 300 173 IE 10mA IB   57.5μA hFE (ave)  1 174 38 Base input resistance VCC R2 // RIN (base) VB  VCC R1  R2 // RIN (base) R2 // hFE RE  I1 R1  VCC R1  R2 // hFE RE  IB VB REQ  VCC R1  REQ REQ R2 // hFE RE  I2 R2 IB RIN(base) 39 Example 4 REQ R2 // hFE RE  VCC=20V 10kΩ// 50 1.1kΩ  8.46kΩ REQ VB VCC RC R1  REQ IC R1 6.2k 8.46kΩ I1 68k 20V  2.21V 68kΩ  8.46kΩ VE VB  0.7V I CQ  I E   hFE = 50 RE RE IE 2.21V  0.7V R2  1.37mA I2 RE 1.1kΩ 10k 1.1k VCEQ VCC  I CQ  RC  RE  20V  1.37mA 7.3kΩ  9.99V 40  Voltage-divider bias characteristics +VCC Circuit recognition: The voltage divider in the base circuit. IC RC Advantages: The circuit Q- I1 R1 point values are stable against changes in hFE. IB Output Disadvantages: Requires more components than most Input other biasing circuits. I2 R2 Applications: Used primarily IE RE to bias linear amplifier. 41  Voltage-divider bias characteristics +VCC VCC Load line I  equations: C (sat ) RC  RE VCE (off ) VCC IC RC I1 R1 Q-point equations (assume that hFERE > 10R2): IB R2 Output VB VCC R1  R2 Input VE VB  0.7V I2 R2 VE IE RE I CQ  I E  RE VCEQ VCC  I CQ  RC  RE  42 Other Transistor Biasing Circuits Emitter-bias circuits Feedback-bias circuits Collector-feedback bias Emitter-feedback bias 43 Emitter Biased Transistor Circuits:  This type of circuit is independent of  making it as stable as the voltage-divider type, The drawback is that it requires two power supplies.  Two key equations for analysis of this type of bias circuit are given below. With these two currents known we can apply Ohm’s law and Kirchhoff's law to solve for the voltages. IB ≈ IE/ IC ≈ IE ≈( -VEE-VBE)/(RE + RB/DC) 44 Example 5 Determine the values of ICQ and VCEQ for the amplifier shown in Fig.7.27. +12 V 12V  0.7V IB  RB  (hFE  1) RE IC RC 11.3V 750  37.47μA 100Ω+2011.5kΩ IB I CQ hFE I B 200 37.47μA Q1 Output hFE = 200 7.49mA Input RB VCEQ VCC  I C  RC  RE   ( VEE ) 100 RE IE 1.5k 24V  7.49mA 750Ω  1.5kΩ  7.14V -12 V 45  Load Line for Emitter-Bias Circuit IC VCC  ( VEE ) VCC  VEE I C (sat )   RC  RE RC  RE IC(sat) VCE ( off ) VCC   VEE  VCC  VEE VCE(off) VCE 46  Emitter-bias +V characteristics CC Circuit recognition: A split IC RC (dual-polairty) power supply and the base resistor is connected to ground. IB Advantage: The circuit Q-point Output Q1 values are stable against changes in hFE. Input RB Disadvantage: Requires the use of RE dual-polarity power supply. IE Applications: Used primarily to bias linear amplifiers. -VEE 47  Emitter-bias characteristics +VCC Load line equations: VCC  VEE I C (sat )  IC RC RC  RE VCE (off ) VCC  VEE IB Output Q1 Q-point equations: Input  VBE  VEE RB I CQ hFE  RE RB  hFE  1 RE IE VCEQ VCC  I CQ  RC  RE   VEE -VEE 48 Summary: βdc Dependent on: Operating Point Q Temperature For stability of the Q-point: Make R2   RE 1 10 49