Transistor bias circuits 1 Objectives Discuss the concept of dc - PowerPoint PPT Presentation

Chapter 5 Transistor bias circuits 1

Objectives ● Discuss the concept of dc biasing of a transistor for linear operation ● Analyze voltage-divider bias, base bias, emitter bias and collector-feedback bias circuits. and collector-feedback bias circuits. 2

Biasing Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal. 3

Operating Point The DC input establishes an operating or quiescent point called the Q- point . 4

The Three States of Operation • Active or Linear Region Operation • Base–Emitter junction is forward biased Base– Collector junction is reverse biased • Cutoff Region Operation • Base–Emitter junction is reverse biased Base–Emitter junction is reverse biased • Saturation Region Operation • Base–Emitter junction is forward biased Base– Collector junction is forward biased 5

DC Biasing Circuits • Fixed-bias circuit • Emitter-stabilized bias circuit • Collector-emitter loop • Voltage divider bias circuit • Voltage divider bias circuit • DC bias with voltage feedback 6

Fixed Bias 7

The Base - Emitter Loop From Kirchhoff’s voltage law: +V CC – I B R B – V BE = 0 Solving for base current : Solving for base current : − V BE V CC I B = = = = R B 8

Collector-Emitter Loop Collector current: = = = = β β β β I B I C From Kirchhoff’s voltage law: V CE = = = = V CC − − − − I C R C 9

Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value. VCC = = = = I Csat Csat R R C VCE ≅ ≅ ≅ ≅ 0 V 10

Load Line Analysis The end points of the load line are: I Csat I C = V CC / R C V CE = 0 V V CEcutoff V CEcutoff V CE = V CC I C = 0 mA The Q -point is the operating point: • where the value of R B sets the value of I B • that sets the values of V CE and I C 11

Circuit Values Affect the Q-Point 12

Circuit Values Affect the Q-Point 13

Circuit Values Affect the Q-Point 14

Emitter-Stabilized Bias Circuit Adding a resistor (R E ) to the emitter circuit stabilizes the bias circuit. 15

Base-Emitter Loop From Kirchhoff’s voltage law: + V CC - I E R E - V BE - I E R E = = = = 0 Since I E = ( β β + 1)I B : β β V CC - I B R B - ( β V CC - I B R B - ( β β + β + + 1)I B R E = + 1)I B R E = = 0 = 0 β β β β + + + + = = = = Solving forI B : V CC - V BE I B = = R = = + + ( β + + β β β + + + 1)R + B E 16

Collector-Emitter Loop From Kirchhoff’s voltage law: IERE + V + I − V = 0 R CE C C CC Since I E ≅ ≅ I C : ≅ ≅ V CE = V CE = = = = V CC – I C (R C + = V CC – I C (R C + = = + R E ) + + + R E ) + + Also: V E = = = = I E R E V C = = V CE + + V E = = V CC - I C R C = = + + = = V B = = V CC – I R R B = = V BE + + V E = = = = + + 17

Improved Biased Stability Stability refers to a circuit condition in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor Beta ( β β β β ) values. Adding RE to the emitter improves the stability of a transistor. 18

Saturation Level The endpoints can be determined from the load line. V CEcutoff : I Csat : V CE = = = = V CC V CE = = = 0 V = I C = = = 0mA = VCC IC = = = = RC + + R + + 19

Voltage Divider Bias This is a very stable bias circuit. The currents and The currents and voltages are nearly independent of any variations in β β . β β 20

Approximate Analysis Where I B << I 1 and I 1 ≅ ≅ ≅ ≅ I 2 : R 2 V CC = = = = V B + + R + + R 1 2 Where β β R E > 10R 2 : β β = V E = = = I E R E V E = = V B − = = − − − V BE From Kirchhoff’s voltage law: V CE = V CC − I C R C − I E R E I E ≅ I C V CE = V CC − I C (R C + R E ) 21

Voltage Divider Bias Analysis Transistor Saturation Level V CC I = = = = I Cmax = = = = Csat + + R R + + C E Load LineAnalysis Load LineAnalysis Cutoff: Saturation: VCC IC = = = = V CE = = = = V CC RC + + + + RE I C = = 0mA = = V = = 0V = = CE 22

DC Bias with Voltage Feedback Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. collector to base. In this bias circuit the Q- point is only slightly dependent on the transistor beta, β β β β . 23

Base-Emitter Loop From Kirchhoff’s voltage law: V CC – I ′ ′ ′ ′ C R C – I B R B – V BE – I E R E = = = = 0 Where I B << I C : = IC + IB ≅ IC I' C Knowing I C = β Knowing I = β β I B and I E ≅ β β I and I ≅ β β β ≅ I , the loop ≅ ≅ ≅ I C , the loop ≅ ≅ equation becomes: V CC – β β β β I B R C − − I B R B − − − − − V BE − − − − − β β β β I B R E = = = = 0 Solving for I B : V CC − − V BE − − I B = = = = R B + + β β (R C + + R E ) + + β β + + 24

Collector-Emitter Loop Applying Kirchoff’s voltage law: I E + V CE + I ’ C R C – V CC =0 Since I ′′ ′′ ′′ ′′ C ≅ ≅ I C and I C = β ≅ ≅ β β β I B : I C (R C + RE ) + V CE – V CC =0 Solving forV CE : V CE = V CC – I C (R C + R E ) 25

Base-Emitter BiasAnalysis Transistor Saturation Level V CC I = I Cmax = = = = = = = Csat + + R + + R C E Load LineAnalysis Cutoff: Saturation: V = V CC V CE CC = I C R + R I C = 0mA C E V CE = 0 V 26

PNP Transistors The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference isthat the currents are flowing in the opposite direction . 27

Analysis of Voltage Bias for PNP Transistor � � � Base voltage R � � 1 = V V � � B EE + β β β β R R R � � 1 2 DC E = + V V V � Emitter voltage E B BE = = + + + + + + V V V V I I R R I I R R V V � � By Ohm’s Law, EE B B B E E BE � And, − − V V V = EE B BE = V I R I C C C E R B + R = − − β V V I R I R E DC EC CC C C E E 28

Example 1 � Evaluate IC and VEC for pnp transistor circuit in Figure below. Given VEE = +15V, R1 = 63k Ω , R2 = 27k Ω , RC = 1.8k Ω , RE = 2.6k Ω , β DC =120. 29

Example 2 � Figure below shown the schematic with a negative supply voltage, determine IC and VCE for a pnp transistor circuit with given values: R1 = 25k Ω , R2 = 60k Ω , RC = 6k Ω , RE = 9k Ω , VCC = -12V, and β DC = 90 30

Example 3 � Construct a complete circuit required to replace the transistor in Figure below with a pnp transistor. Given VCC = 10V, R1 = 78k Ω , R2 = 100k Ω , RC = 18k Ω , RE = 8k Ω . 31