Lecture 10

Bipolar Junction Transistor (BJT)

BJT 1-1 Sunday 29/10/2017

Outline

Continue BJT Biasing DC analysis

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

Transistor Switching Networks • BJT switching time

BJT 1-2

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 1-3

Emitter-Stabilized Bias Circuit

BJT 1-4

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 1-5

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

Very stable bias circuit

The currents and voltages are nearly independent of any variations in

BJT 1-6

Approximate Analysis of Voltage Divider Bias Circuit

BJT 1-7

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

Base-Emitter Loop

Collector-Emitter Loop

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 1-8

DC Bias with Voltage Feedback Circuit

BJT 1-9

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:

CC BEC

BC E

V VI

R(R R )

DC Bias with Voltage Feedback Circuit

BJT 1-10

Collector-Emitter Loop

Applying Kirchoff’s voltage law:

IERE + VCE + I’CRC – VCC = 0

Since IC IC and IE ≈ IC:

IC(RC + RE) + VCE – VCC =0

Solving for VCE:

VCE = VCC – IC(RC + RE)

Example (1) A bipolar transistor having IS = 5×10-16 A is

biased in the active region with VBE =750 mV. If the current gain (or β) varies from 50 to 200 due to manufacturing variations, calculate the minimum and maximum terminal currents of the device

BJT 1-11

Solution For a given VBE, the collector current remains

independent of β

The base current varies from IC/200 to IC/50:

the emitter current experiences only a small variation because (β+ 1)/β is near unity for large β:

BJT 1-12

Example (2)

Determine the dc level of IB and VC for the shown BJT circuit

BJT 1-13

Solution For the dc mode,

the capacitors-related

branches become

open-circuits.

Also, RB =R1+R2

BJT 1-14

Transistor Switching Networks

Transistors with only applied DC sources at the input can be used as electronic switches

BJT 1-15

Cut-off

Saturation

Cut-off

Switching Circuit Calculations

BJT 1-16

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 1-17

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 1-19

Lecture Summary

Covered material Continue BJT Biasing

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

Transistor Switching Networks • BJT switching time

Material to be covered next lecture

Continue BJT Continue DC analysis

• More examples

Introduction to AC signal analysis