Lecture 9

Bipolar Junction Transistor (BJT)

BJT 1-1 Sunday 22/10/2017

Outline

Continue BJT Configurations BJT Biasing DC analysis

• Fixed-bias circuit

BJT 1-2

BJT Configurations

BJT 1-3

• The emitter is common to both input (base-emitter) and output (collector-emitter)

• The input is on the base and the output is on the collector

Common-Emitter configuration

BJT Configurations

BJT 1-4

Common-Emitter configuration

Collector (output) Characteristics Base (input) Characteristics

BJT Configurations

BJT 1-5

• The input is on the base and the output is on the emitter • The characteristics are similar to those of the common-

emitter configuration, except the vertical axis is IE

Common-Collector configuration

I-V characteristics of BJT

BJT 1-6

Load line

I-V characteristics of BJT (cont’d)

BJT 1-7

Operating Limits for Each BJT Configuration

BJT 1-8

• VCE is at maximum and IC is at minimum (IC_min= ICEO) in the cutoff region • IC is at maximum and VCE is at minimum (VCE_min = VCEsat = VCEO) in the

saturation region • The transistor operates in the active region between saturation and

cutoff

Power Dissipation in BJT

Common-base

Common-emitter

Common-collector

BJT 1-9

CCBCmax IVP

CCECmax IVP

ECECmax IVP

Example (1)

BJT 1-10

Analyze the BJT circuit, shown in the figure, to get the values of: IB, IC, IE,α,VCE, VCB, VBE and max. power dissipated in the BJT

Solution We assume an operating mode

for the BJT (active mode VBE=0.7V)

The base-emitter KVL equation is: 5.7 − 10IB −VBE − 2IE = 0

Also, IE =(β+1)IB=100IB

Then, 5.0 – 210 IB =0

IB = 23.8 μA

IC =βIB= 2.356 mA

IE = 2.38 mA

α = IC/IE= 0.9899 BJT 1-11

Solution (cont’d) VBE=0.7V from assumption

VC = 10.7 – IC . 1 kΩ = 8.34 V

VE = 0 + IE . 2 kΩ = 4.76 V

So, VCE=VC - VE = 3.58 V

By applying KVL to the BJT transistor: VCE = VCB + VBE

Then, VCB = VCE – VBE = 2.88 V

Max. power = VCE IC = 8.43 mWatt

Last thing to check assumption: VCE = 3.58 V > 0.3 V ok (no saturation)

IB = 23.8 μΑ > 0.0 ok (no cutoff) BJT 1-12

BJT Biasing

Definition: The DC voltages applied to a transistor in

order to turn it ON so that it can amplify the AC signal

BJT 1-13 No Bias

Good Bias

BJT Circuits at DC

The BJT operating mode depends on the

voltages at EBJ and BCJ

Simplified models and classifications are

needed to speed up the hand-calculation

analysis

BJT 1-14

DC analysis of BJT circuits

Step 1: assume the operating mode

Step 2: use the conditions or model for circuit analysis

Step 3: verify the solution

Step 4: repeat the above steps with another assumption if necessary

BJT 1-15

DC Biasing Circuits

Fixed-bias circuit

Emitter-stabilized bias circuit

Voltage divider bias circuit

DC bias with voltage feedback circuit

BJT 1-16

Fixed-bias circuit

BJT 1-17

Base-Emitter Loop

From Kirchhoff’s voltage law:

+VCC – IBRB – VBE = 0

Solving for base current:

B

BECCB

R

VVI

Collector-Emitter Loop

/R

VVII

B

BECCBC

Collector current:

From Kirchhoff’s voltage law: CCCCCE RIVV

Example (2)

BJT 1-18

Design the following circuit so that Ic = 2 mA and Vc= 5 V. For this particular transistor, β =100 and VBE=0.7 V

Vc

Solution To design the circuit, we need to determine

values of RC and RE

We assume BJT works in active mode

Ic = 2 mA = (15- Vc)/ RC then RC = 5 kΩ

VB = 0 V, from assumption VBE = 0.7 V

VE = VB – VBE = 0 – 0.7 = - 0.7 V

RE = (VE -(-15))/ IE

IE = (β +1)/ β Ic = 2.02 mA

RE = 14.3/2.02 = 7.07 kΩ

BJT 1-19

Load Line Analysis The end points of the

load line are:

BJT 1-20

ICsat

IC = VCC / RC

VCE = 0 V

VCEcutoff

VCE = VCC

IC = 0 mA

The Q-point is the operating

point that sets the values

of VCE,Q and IC,Q

VCE,Q

IC,Q

Circuit Values Affect the Q-Point

BJT 1-21

Circuit Values Affect the Q-Point

BJT 1-22

Circuit Values Affect the Q-Point

BJT 1-23

Example (3)

BJT 1-24

Given the load line and the defined Q-point, as shown in figure 2-a, determine the required values of VCC, RC, and RB for a fixed-bias configuration as depicted at figure 2-b.

Figure 2-a

Figure 2-b

Solution

BJT 1-25

sat

sat

BJT 1-26

Lecture Summary

Covered material Continue BJT

Biasing DC analysis

• Fixed-bias circuit

Material to be covered next lecture

Continue BJT Continue DC analysis

• More examples

Introduction to ac signal analysis