lectronic ircuits Transistor Bias ircuits Manar Mohaisen Office: F208 mail: manar.subhi@kut.ac.kr Department of
Review of the Precedent Lecture Bipolar Junction Transistor (BJT) BJT haracteristics and Parameters BJT as an Amplifier and as a Switch Phototransistor and its Applications Transistor ategories and Packaging
lass Objectives xplain the D Operating Point xplain the oltage-divider Bias Other Bias Methods Discussions
The D Operating Point D Bias Bias establishes the D operating point (Q-point) Quiescent point for proper operation of the amplifier. The output swings equally above and below the D of the output.
The D Operating Point contd. Graphical Analysis is defined by the value of B. is therefore implicitly dependent on B. R defines this relation.
The D Operating Point contd. The D Load Line A straight line between = (sat) (y-axis) and = (x-axis). 1 = = + R R R Linear Region All points between saturation and cutoff.
The D Operating Point contd. ariations in urrents and oltages B ± 100uA from BQ. ± 10 ma from Q. ± 2.2 from the Q.
The D Operating Point contd. lipping A signal that swings outside the active region will be clipped. BQ nput signal Q Q utoff 0 ce utoff Q
The D Operating Point contd. xample 5-1 Determine the Q-point operation. Draw the D load line. Find the maximum base current for linear operation. Solution: The Q-point is defined by and. B 10 0.7 198 μ A = BB B = = RB 47kΩ = βd B = (200)(198μA) = 39.6mA = R = 20 13.07 = 6.93
The D Operating Point contd. xample 5-1 contd. Find the maximum base current for linear operation. Solution: We are in linear operation if the transistor is NOT in cutoff nor in saturation. utoff: The collector current has its minimum D value of 0 A. Saturation: The collector has its peak value of 60.6 ma. This means that the Q-point is closer to saturation than to cutoff. Therefore, The maximum peak increase in B b(peak) c(peak) = = 21.0 ma = 105 μ A β 200 D
oltage-divider Bias Single Bias B R R 2 1+ R 2 = B B = R = R =
oltage-divider Bias contd. xample 5-2 B R 2 R = 1+ R 2 3.59 = B B = 3.59 0.7= 2.89 2.89 5.16mA = = = R 560Ω = R = 10 (5.16 ma)(1.0k Ω ) = 4.84 = = 4.84 2.89 = 1.95
oltage-divider Bias contd. deally, oltage Divider an be considered as stiff BJT does not appear as a significant load. RN(BAS) = β D R
oltage-divider Bias contd. Stability of oltage-divider Bias R = 2 TH R R + 1 2 R R / β TH TH D B RR R = 1 2 TH R + R 1 2 = + + TH = R + R / B β TH D
oltage-divider Bias contd. oltage-divider Biased PNP Transistor
oltage-divider Bias contd. xample 5-5 Find and. R = 68k Ω, R = 47k Ω, R = 1.8k Ω, R = 2.2k Ω, = 6, β = 75. 1 2 D R β R 75(2.2 k ) 165k N(BAS) D = = Ω = Ω B R R 2 N(BAS) = ( ) = 2.1 R + R R 1 2 N(BAS) = + = 1.4 B B = = 636μA R = 636μA = R = 4.86 = = 3.46
Other Bias Methods mitter Bias Due to voltage drop at R B, the base voltage drops below ground. This forces to be at approximately -1 is entered as a negative value. = R 1
Other Bias Methods contd. mitter Bias contd. xample = -1 1 15 1 1.4mA = = = R 10 kω = R = 8.4 = = 9.4
Other Bias Methods contd. mitter Bias contd. xact Analysis of emitter-biased circuits. + R + B B + R = 0 + R B + + = 0 β B R D = R + R / β B B D = + R = + B B = R
Other Bias Methods contd. mitter Bias xample 5-7 Determine how much the Q-point will change if β D changes from 100 to 200. βd = 100 15 0.7 B = = R + R / β 10k + 47k /100 B D 1.37 ma = R = 15 (1.37mA)(4.7k Ω ) = 8.56 = + R = 15 + (1.37mA)(10k Ω ) = 1.3 = = 8.56 ( 1.3) = 9.83 β = 100 D Repeat same calculations for β D. hange is very small Stable! β D = 200 1.38 ma β = = 200 D 9.71
Other Bias Methods contd. Base Bias R = 0 B B B B = R B B = β = β D B D R B B = R
Other Bias Methods contd. Base Bias contd. Q-point Stability of Base Bias = β = β D B D R B B R = β B D R B Therefore, The Q-point is very dependent on β D Base bias is unreliable. Hence, Base bias is not normally used.
Other Bias Methods contd. mitter-feedback Bias Adds an emitter resistance to the base bias circuit. Makes the circuit more predictable. f increases, increases, and reduces B. = B R + R / β B D is not directly dependent on β D.
Other Bias Methods contd. ollector-feedback Bias When is increased decreases B decreases Decrease in. Therefore, the decrease in B offsets the decrease in. Hence, the Q-point is stable. = B R + R / β B D = R ncrease temperature increases, B decreases, B increases As increases, decreases B decreases and decreases.
Discussion & Notes K K A K A A A A K K K K A K A K K A K A K A