Solutions to BJT Questions

BJTs-Biasing

Q: How do you perform a more accurate analysis if the IB-VBE characteristic of the BJT is provided.

CHECK YOURSELF-POINT

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

BJTs-Biasing

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

12.4 A

0.65 V

9.5 A

VBB =RBB x IB + VBE + IE x RE

3.33 = 33.3 x 103 x IB + VBE + 101 x IB x 2.33 x 103

VBE = 3.33 268.63 x 103 x IB

VCE = VCC ICRC IERE = 10 100 x IB x 3x103 101 x IB x 2.33 x 103 = 4.91 V

Q.1: (to be solved on white board) IB-VBE characteristic of the npn BJT used in the following circuit is given below. Emitter injection efficiency and base transport factor of the BJT are both equal to 0.995. Contribution of the thermally generated minority carriers around the B-C depletion region to base current is negligible. a) Find the collector current and collector-emitter voltage of the BJT in the following circuit, and plot the IC-VCE characteristic of the BJT on the provided graph.

0.00 0.25 0.50 0.750

25

50

75

100

B

a

s

e

C

u

r

r

e

n

t

(

A

)

Base Emitter Voltage (V)

BJTs-EXERCISE QUESTIONS

BJTs-EXERCISE QUESTIONS

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

0.7 V

+

VBE

-

0.8 V +

VBE

-

Q.3 (to be solved on white board) The following circuits are constructed by using the same Si BJT with F=R=0.9.

a) Use the Ebers Moll Model to determine IE, IB and IC in Circuit 2. b) What is the operation region of the BJT in Circuit 2. c) Show and label the electron and hole flow directions due to minority carrier injection for the BJT

of Circuit 2. Is the collector current in Circuit 2 controllable by VBE? Explain your reasoning.

Circuit 1 Circuit 2 10 mA

BJTs-EXERCISE QUESTIONS

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

+ 0.8 V -

BJTs-EXERCISE QUESTIONS

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

F = R = 0.9 , F x IES = R x ICS => IES = ICS

IF = 10 mA = IES (e0.7/0.025-1) => IES =ICS = 6.9 x 10-15 A

BJTs-EXERCISE QUESTIONS

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

VBE = VBC => IF = IR = IES (e0.8/0.025 1) = 545 mA

IE = IF 0.9 x IR = 0.1 x IF = 54.5 mA

IC = 0.9 x IF IR = -0.1 x IF = - 54.5 mA , IB = IE IC = 109 mA

Q.5 (to be solved on white board) Find the emitter current and VCE in the following circuit constructed with a BJT having a very large . Comment on the value of VCE (note that a small current is flowing through RB) .

BJTs-EXERCISE QUESTIONS

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

RB

14 K

RC

RE

5 V

10 K

-5 V

4.6 K

BJTs-EXERCISE QUESTIONS

Middle East Technical University, EE 212-Semiconductor Devices and Modeling, Lecture Notes

-5+ 4.6k x IE+0.7+ IBx10k + IE x 14 k = 5 V

=> IE = 0.5 mA

VCE=(5-14 x 0.5) (-5 + 4.6 x 0.5) = 0.7V

Negligible