Lecture 9

Bipolar Junction Transistor (BJT)

BJT 1-1

Outline

Continue BJT BJT Biasing DC analysis

• Fixed-bias circuit • Emitter-stabilized bias circuit • Voltage divider bias circuit • DC bias with voltage feedback circuit

BJT switching time

BJT 1-2

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 1-3 No Bias

Good Bias

BJT Circuits at DC

The BJT operation mode depends on the voltages at EBJ and BCJ

The I-V characteristics are strongly nonlinear

Simplified models and classifications are needed to speed up the hand-calculation analysis

BJT 1-4

DC analysis of BJT circuits

Step 1: assume the operation 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 1-5

DC Biasing Circuits

Fixed-bias circuit

Emitter-stabilized bias circuit

Voltage divider bias circuit

DC bias with voltage feedback circuit

BJT 1-6

Fixed-bias circuit

BJT 1-7

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 (1)

BJT 1-8

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

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, 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 1-9

Load Line Analysis The end points of the

load line are:

BJT 1-10

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 and IC

Circuit Values Affect the Q-Point

BJT 1-11

Circuit Values Affect the Q-Point

BJT 1-12

Circuit Values Affect the Q-Point

BJT 1-13

Example (2)

BJT 1-14

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 1-15

Emitter-Stabilized Bias Circuit

Adding a resistor (RE) to the emitter circuit

The addition of the emitter resistor to the dc bias of the BJT provides improved stability the dc bias currents

and voltages remain closer to where they were set by the circuit when outside conditions (e.g., temperature) change

BJT 1-16

Emitter-Stabilized Bias Circuit

BJT 1-17

Base-Emitter Loop

From Kirchhoff’s voltage law:

0R1)I(-RI-V EBBBCC

0 RI-V-RI-V EEBEEECC

EB

BECC

EB

BECCB

RR

V-V

1)R(R

V-VI

Since IE = ( + 1)IB:

Solving for IB:

EB

BECCBC

R/R

V-VII

Emitter-Stabilized Bias Circuit

BJT 1-18

Collector-Emitter Loop

From Kirchhoff’s voltage law:

0 CC

VC

RC

I CE

V E

RE

I

Since IE IC:

)R (RI– V V ECCCCCE

Also:

EBEBRCCB

CCCCECEC

EEE

V V RI– V V

RI - V V V V

RI V

Voltage Divider Bias Circuit

This is a very stable bias circuit.

The currents and voltages are nearly independent of any variations in .

BJT 1-19

Approximate Analysis of Voltage Divider Bias Circuit

BJT 1-20

Where IB << I1 and I1 I2 :

Where RE > 10R2:

From Kirchhoff’s voltage law:

21

CC2B

RR

VRV

E

EE

R

VI

BEBE VVV

EECCCCCE RI RI V V

)R (RIV V

II

ECCCCCE

CE

Condition to be tested

Problem

Design the bias network for the silicon npn BJT circuit such that IE = 1mA, R1=22k Ω and R2=2.2kΩ, if the transistor has β equals 100.

BJT 1-21

DC Bias with Voltage Feedback Circuit Another way to

improve the stability of a bias circuit is to add a feedback path from collector to base

In this bias circuit the Q-point is only slightly dependent on the transistor beta,

BJT 1-22

DC Bias with Voltage Feedback Circuit

BJT 1-23

Base-Emitter Loop

)R(RR

VVI

ECB

BECCB

From Kirchhoff’s voltage law:

0RI–V–RI–RI– V EEBEBBCCCC

Where IB << IC:

CI

BI

CI

CI'

Knowing IC = IB and IE IC, the loop

equation becomes:

0RIVRIRI– V EBBEBBCBCC

Solving for IB:

DC Bias with Voltage Feedback Circuit

BJT 1-24

Collector-Emitter Loop

Applying Kirchoff’s voltage law:

IE + VCE + I’CRC – VCC = 0

Since IC IC and IC = IB:

IC(RC + RE) + VCE – VCC =0

Solving for VCE:

VCE = VCC – IC(RC + RE)

Transistor Switching Networks

Transistors with only the DC source applied can be used as electronic switches

BJT 1-25

Switching Circuit Calculations

BJT 1-26

C

CCCsat

R

VI

dc

CsatB

II

Csat

CEsatsat

I

VR

CEO

CCcutoff

I

VR

Saturation current:

To ensure saturation:

Emitter-collector resistance

at saturation and cutoff:

C

CECE

I

VR

Switching Time

BJT 1-27

Transistor switching times:

dron ttt

fsoff ttt

Times in range of nano-seconds

Example

Note: npn BJT has faster switching time than pnp BJT

BJT 1-28

Lecture Summary

Covered material Continue BJT

Biasing DC analysis

• Fixed-bias circuit • Emitter-stabilized bias circuit • Voltage divider bias circuit • DC bias with voltage feedback

BJT switching time

Material to be covered next lecture

Continue BJT Continue DC analysis

• More examples

Introduction to ac signal analysis