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CHAPTER 18
BJT-TRANSISTORS
Function of Transistors
Main Applications of Transistors
BIPOLAR JUNCTION
TRANSISTORS (BJTs)


BJT is constructed with three doped
semiconductor regions separated by two pn
junction
There are three regions :




Emitter
Base
Collector
There are two type of BJT:


npn
pnp
Basic construction of BJT
การป้ อนไฟให้ กับ Transistor แบบ BJT

AC bias


ป้ อนสัญญาณไฟกระแสสลับ
DC bias
ป้ อนสัญญาณไฟกระแสตรง
 กระตุ้นให้ BJT อยู่ในช่ วงทางานได้


แม้ สัญญาณ input AC จะอยู่ในช่ วง ติดลบ
ขา BE: Forward bias
 ขา BC: Reverse bias
 Q-point อยู่ในช่ วงใช้ งาน Active Region

Transistor DC Biasing


The BE junction is forward-biased
The BC junction is reverse-biased
(Vbase ( B ) > Vemitter ( E) )
(Vbase ( B ) < Vemitter ( E) )
(Vcollector ( C ) > Vbase ( B ) )
(Vcollector ( C ) < Vbase ( B ) )
Transistor Operation

E

B

C
e e e
I E  I B  IC
DC current gain
Alpha   and Bata  

The Collector current is equal to  DC
the emitter current
I C   DC I E

 DC
times
has a value between
0.950 and 0.995
The collector current is equal to the base current
multiplied by  DC
I C   DC I B
 DC
has a value between
20 and 200
Transistor Voltages (DC bias)
VC  VCC  I C RC  VE  VCC  I C RC
VB  VBE  VE  VBE
VBB  I B RB  VBE
VBB  VBE
IB 
RB
2 Voltage Sources
I C   DC I B
I C   DC I E  I E
Transistor Voltages (DC bias)
Voltage divider Sources


Use only a single dc
source to provide
forward-reverse bias to
the transistor
Resistor R1 and R2 form
a voltage divider that
provides the base bias
voltage
Transistor Voltages (DC bias)
 R2 
VCC
Left : VB  
 R1  R2 
Right : VB  VBE  I E RE
VB  VBE
IE 
RE
Voltage divider Sources
 R2  
VCC   VBE

 R1  R2  


RE
VCE  VCC  I C RC  I E RE
I C   DC I E  I E
VCE  VCC  I E ( RC  RE )
I B  I C /  DC
Input Resistance at the Base
Voltage divider Sources
RIN
VB
 RB 
IB
VB  VE  I E RE
I E   DC I B
RIN 
 DC I B RE
IB
  DC RE
Base Voltage
RIN  RB   DC RE
 R2 || RIN
VB  
 R1  R2 || RIN
RIN  R2
 R2 
VCC
VB  
 R1  R2V
VE  VB  0.7

VCC

THE BIPOLAR JUNCTION
TRANSISTOR AS AN AMPLIFIER
THE BIPOLAR JUNCTION
TRANSISTOR AS AN AMPLIFIER


When both junction are forward-biased,
the transistor is in the saturation region of
its operation
When VCE exceeds 0.7 V, the basecollector junction becomes reverse-biased
and the transistor goes into the active or
linear region

When IB=0 the transistor is in the cutoff
region
Load Line Operation
Load Line: VCE vs IC
Quiescent or Q-Point
VCE  VCC  I C RC
IC 
VCC  VCE
V
1
  VCE  CC
RC
RC
RC
Active region : I C   DC I B
Saturation region : I C   DC I B
Quiescent or Q-Point
AC Sources
DC Sources
Load Line
IB = 400 uA
IB = 300 uA
IB = 200 uA
Signal Operation on the Load Line
Q-point สู งเกินไป
จ่ายไปให้ IB มากเกินไป
่ นไป
Q-point ตาเกิ
จ่าย IB น้อยเกินไป
Q-point เหมาะสม
ค่ากาลังขยาย beta มากเกินไป
BJT Transistor
AC Amplifier

Class A


Class B


วงจรทางานตลอดเวลา full cycle of AC input signal (360o)
วงจรทางาน half cycle หยุด half cycle (180o)
Class C

วงจรทางานน้ อยกว่ า half cycle (< 180o)
BJT Transistor
AC Amplifier: Class A

ขยายสัญญาณได้ 3 แบบ
Common Base Amplifier (CB): Voltage Amp.
 Common Emitter Amplifier (CE):
Power Amp.
 Common Collector Amplifier (CC): Current Amp.

การวิเคราะห์ การขยายสัญญาณ AC (i/p)

ใช้ Equivalent Circuit มาเป็ นตัวแทน BJT (AC model)

Simple model

Hybrid Π model

h parameter model
Simple Model
(small signal analysis)
E
C
C
 AC  AC
re' rc
B
BJT ( npn type)
B
Simple model
E
Simple Model

Parameter in BJT equivalent circuit
 AC ,  AC , re' , rc
 AC ,  AC
ถ้ าโจทย์ ไม่ ให้
 AC   DC
 AC   DC
re' = Dynamic AC resistance ที่ขา E-B
re' 
VT
I E ( DC )

0.026
I E ( DC )
rc = Dynamic AC resistance ที่ขา C-B
= ค่ าสู งมากระดับ MΩ ถ้ าโจทย์ ไม่ ให้ rc  
Amplifier Gain
Calculation Process

Calculating DC bias



DC current and voltage in DC bias circuit
Checking load line and Q-point
Calculating AC amplifier gain




re’ from IE(DC)
vout
Voltage gain (AV) =
vin
iout
Current gain (Ai) =
iin
Power gain (AP) = AV Ai
Common Base Amplifier
( CB Amp )
E
BJT
C
vOUT
B
vin
Common Base Amplifier
C
E
re'
 ie
rc
B
Simple model for AC small signal analysis
CB Amp. Circuit
Vcc
 R2 
VCC
Left : VB  
 R1  R2 
Right : VB  VBE  I E RE  0
VB  VBE
IE 
RE
 R2  
VCC   VBE

 R1  R2  


RE
Common Base Amplifier
( CB Amp )
vout ic Rc  AC ie Rc Rc
AV 


 '
'
'
vin
ie re
ie re
re
iout ic
ic
ic
 AC ie
Ai 
 
 '
 ' '
iin iin vin / rin ie re / rin ie re /( re // RE )
re' RE
 AC ( '
)
'
 AC (re // RE )
re  RE
RE


  AC ( '
)   AC  1
'
'
re
re
re  RE
iout  iin
AP  AV Ai  AV
Common Emitter Amplifier
( CE Amp )
B
BJT
C
vOUT
E
vin
Common Emitter Amplifier
B

'
AC e
C
 AC ib
rc
r
E
Simple model for AC small signal analysis
CE Amp. Circuit without CE
Vcc
R1
 R2 
VCC
Left : VB  
 R1  R2 
Right : VB  VBE  I E RE  0
R2
VB  VBE
IE 
RE
 R2  
VCC   VBE

 R1  R2  


RE
Common Emitter Amplifier
( CE Amp: no CE )
vout
 ic Rc
 ic Rc
 ic Rc
 Rc
AV 

 '
 '

'
vin ib ( re )  ie RE ic re  ie RE ic re  ic RE
RE
 rin 
  Rc  rin 
iout vout / rout   vout  rin 

   AV 
  


Ai 

 
iin
vin / rin  vin  rout 
 RE  rout 
 rout 
vb ib (  re' )  ie RE ib (  re' )  ic RE  ib re'   ib RE
rin  rb  


ib
ib
ib
ib
rout  Rc //( rc  RE )  Rc
rin   (re'  RE )
Common Emitter Amplifier
( CE Amp: no CE )
  Rc  rin   Rc   AC (re'  RE )   AC (re'  RE )
   
 

Ai  
Rc
RE
 RE  rout   RE 


 AC (re'  RE )
RE

 AC RE
Rc
  AC
  Rc 
 AC
AP  AV Ai  
 RE 
CE Amp. Circuit with CE
Vcc
 R2 
VCC
Left : VB  
 R1  R2 
Right : VB  VBE  I E RE  0
VB  VBE
IE 
RE
 R2  
VCC   VBE

 R1  R2  


RE
Common Emitter Amplifier
( CE Amp: with CE
AV 
from E-GND
)
vout  ic (rc // Rc )   AC ib (rc // Rc )  (rc // Rc )  Rc



 '
'
'
'
vin
ib ( re )
ib (  AC re )
re
re
Higher voltage gain
 rin 
  Rc  rin 
iout vout / rout   vout  rin 

   AV 
   ' 

Ai 

 
iin
vin / rin  vin  rout 
 rout 
 re  rout 
vin  AC re'ib
rin 

  AC re'
iin
ib
rout  rc // Rc  Rc
Common Emitter Amplifier
( CE Amp: with CE
from E-GND
  Rc   AC re' 
   AC
Ai   ' 
 re  Rc 
  Rc 
AP  AV Ai   ' (  AC )
 re 
gainDB
 vout 

 20 log 10 
 vin 
)
Common Collector Amplifier
( CC Amp )
B
E
BJT
vOUT
C
vin
Common Emitter Amplifier
B

E
'
AC e
r
rc
 AC ib
C
Simple model for AC small signal analysis
CC Amp. Circuit
Vcc
 R2 
VCC
Left : VB  
 R1  R2 
Right : VB  VBE  I E RE  0
VB  VBE
IE 
RE
 R2  
VCC   VBE

 R1  R2  


RE
Common Collecter Amplifier
( CC Amp)
vout
ie RE
ie RE
RE
RE
AV 

 '
 '

1
'
vin ib ( re )  ie RE ie re  ie RE re  RE RE
 rin 
 rin 
iout vout / rout  vout  rin 

  AV 
  1

Ai 

 
iin
vin / rin  vin  rout 
 rout 
 rout 
 vb 
 ib (  AC re' )  ie RE 
vin

rin 
 RB // rb  RB //    RB // 
iin
ib
 ib 


 ib (  AC re' )  ib ( AC RE ) 
  RB //(  AC RE )
 RB // 

ib

Common Emitter Amplifier
(CC Amp)
rout  rc // RE  RE
 RB  AC RE

 RB //  AC RE   RB   AC RE
 
Ai  1
RE
RE

 




   AC RB
 RB   AC RE


  Rc   AC RB
AP  AV Ai   ' (
)
 re  RB   AC RE
THE BJT AS A SWITCH
Conditions in cutoff
VCE ( cutoff )  VCC
Conditions in saturation
I C ( sat)
VCC

RC
I B (min) 
I C sat 
 DC
BJT PARAMETERS AND RATINGS
IF the temperature goes up,  DC goes up, and
vice versa.
BJT PARAMETERS AND RATINGS

IC 
Maximum Transistor Ratings
PD max 
VCE
PD(max) = maximum power dissipation