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EE 2109 Electronics-I

Dr. Mostafa Zaman Chowdhury

Chapter 4:

DC Biasing–BJTs

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Biasing

Biasing: The DC voltages applied to a transistor in order to turn

it on so that it can amplify the AC signal.

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Operating Point

The DC input establishes an

operating or quiescent point

called the Q-point.

Which point is best?

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The Three States of Operation

• Active or Linear Region Operation

Base–Emitter junction is forward biased

Base–Collector junction is reverse biased

• Cutoff Region Operation

Base–Emitter junction is reverse biased

• Saturation Region Operation

Base–Emitter junction is forward biased

Base–Collector junction is forward biased

DC Biasing Circuits

Fixed-bias circuit

Emitter-stabilized bias circuit

Collector-emitter loop

Voltage divider bias circuit

DC bias with voltage feedback

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Fixed Bias

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The Base-Emitter Loop

From Kirchhoff’s voltage law:

Solving for base current:

+VCC – IBRB – VBE = 0

B

BECCB

R

VVI

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Collector-Emitter Loop

Collector current:

From Kirchhoff’s voltage law:

BIIC

CCCCCE RIVV

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Saturation

When the transistor is operating in saturation, current through the transistor

is at its maximum possible value.

CR

CCV

CsatI

V 0CEV

Load Line Analysis

ICsat

IC = VCC / RC

VCE = 0 V

VCEcutoff

VCE = VCC

IC = 0 mA

where the value of RB sets the value of

IB

that sets the values of VCE and IC

The Q-point is the operating point:

The end points of the load line are:

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Circuit Values Affect the Q-Point

more …

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Circuit Values Affect the Q-Point

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Circuit Values Affect the Q-Point

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Emitter-Stabilized Bias Circuit

Adding a resistor (RE) to

the emitter circuit

stabilizes the bias circuit.

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Base-Emitter Loop

From Kirchhoff’s voltage law:

0R1)I(-RI-V EBBBCC

0 RI-V-RI-V EEBEEECC

EB

BECCB

1)R(R

V-VI

Since IE = ( + 1)IB:

Solving for IB:

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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

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Improved Biased Stability

Stability refers to a circuit condition in which the currents and voltages

will remain fairly constant over a wide range of temperatures and

transistor Beta () values.

Adding RE to the emitter improves the stability of a transistor.

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Saturation Level

VCEcutoff: ICsat:

The endpoints can be determined from the load line.

mA 0 I

V V

C

CCCE

ERCR

CCV

CI

CE V 0V

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Voltage Divider Bias

This is a very stable bias

circuit.

The currents and voltages

are nearly independent of

any variations in .

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Approximate Analysis

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

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Voltage Divider Bias Analysis

Transistor Saturation Level

EC

CCCmaxCsat

RR

VII

Load Line Analysis

Cutoff: Saturation:

mA0I

VV

C

CCCE

V0VCE

ERCR

CCV

CI

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Exact Analysis

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DC Bias with Voltage Feedback

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, .

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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:

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Collector-Emitter Loop

Applying Kirchoff’s voltage law:

RE 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)

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Base-Emitter Bias Analysis

Transistor Saturation Level

EC

CCCmaxCsat

RR

VII

Load Line Analysis

Cutoff: Saturation:

mA 0I

VV

C

CCCE

V 0VCE

ER

CR

CCV

CI

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Transistor Switching Networks

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

switches.

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Switching Circuit Calculations

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:

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Switching Time

Transistor switching times:

dron ttt

fsoff ttt

tr=rise time

td=delay time

ts=storage time

tf=forward time

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PNP Transistors

The analysis for pnp transistor biasing circuits is the same

as that for npn transistor circuits. The only difference is that

the currents are flowing in the opposite direction.

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