1. Lecture 10:Load Line & BJT Biasing CSE251

2. DC Biasing To establish a constant dc collector current in the BJT. Biasing is required to operate the transistor in the linear region. The DC bias point in i C -v CE plane should be such that maximum output signal swing is achieved. Ideally the I C and V CE in a biased circuit should remain constant. Practical circuit bias point is affected by the change in  and temperature changes. Q point should be insensitve to variations in temperature and current gain beta.

3. replace the capacitors with an open-circuit equivalent because the reactance of a capacitor for dc is ∞ Ω The dc supply V cc can be separated into two supplies Fixed bias circuit DC equivalent Fixed Base Bias

4. Write KVL equation in the clockwise direction of the loop : +V CC – I B R B – V BE =0 Solving the equation for the current I B results : Base-emitter loop Forward Bias of Base-emitter

5. The magnitude of the I C is related directly to I B through I C = β I B Apply KVL in the clockwise direction around the indicated close loop results: V CE +I C R C -V CC =0 V CE = V CC -I C R C Recall that : V CE = V C - V E In this case, V E = 0V, so V CE =V C V BE =V B -V E Than V E =0V, V BE =V E Collector–emitter loop Collector-emitter loop

6. Determine the value of Q-point for this figure. Also find the new value of Q-point if  change to 150.

7. Voltage divider bias

8. Step 1: The input side of the network can be redrawn for DC analysis. Step 2: Analysis of Thevenin equivalent network to the left of base terminal

9. Step 2(a): Replaced the voltage sources with short-circuit equivalent and gives the value of R TH

10. Step 2(b): Determining the E TH by replacing the voltage sources and open circuit Thevenin voltage. Then apply the voltage-divider rule.

11. Step 3: The Thevenin network is then redrawn and I BQ can be determined by KVL

12. Book Boyelstad Fixed bias circuit: section 4.3. exmaple 4.1 Load-line analysis: page 150-152,157. example 4.3. Voltage-divider bias: section 4.5 pages 157-160. example 4.7.