3. Bipolar Junction Transistor...
Transcript of 3. Bipolar Junction Transistor...
3. Bipolar Junction Transistor (BJT)
ECE 65, Winter 2013, F. Najmabadi
Lecture notes: Sec. 3
Sedra & Smith (6th Ed): Sec. 6.1-6.4* Sedra & Smith (5th Ed): Sec. 5.1-5.4*
* Includes details of BJT device operation which is not covered in this course
A BJT consists of three regions
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (2/22)
NPN transistor
An implementation on an IC
Device construction is NOT symmetric
o “Thin” base region (between E & C)
o Heavily doped emitter
o Large area collector
Simplified physical structure
Device is constructed such that BJT does NOT act as two diodes back to back when voltages are applied to all three terminals.
BJT iv characteristics includes four parameters
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (3/22)
Six circuit variables: (3 i and 3 v)
Two can be written in terms of the other four:
NPN transistor
CEBEBC
BCE
vvviii−=
+= :KVL
:KCL
Circuit symbol and Convention for current directions
(Note: vCE = vC – vE)
BJT iv characteristics is the relationship among (iB , iC , vBE , and vCE )
It is typically derived as
),( )(
CEBC
BEB
vi givfi
==
BJT operation in the “active” mode
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (4/22)
Active mode:
0
/
/
DCE
VvSC
VvSCB
VveIi
eIii
TBE
TBE
≥=
==ββ
If the base is “thin” these electrons get near the depletion region of BC junction and are swept into the collector if vCB ≥ 0 (vBC ≤ 0 : BC junction is reverse biased!)
In this picture, ic is independent of vBC
(and vCE ) as long as
TBE VvSC eIi /=
0
0 0
DCE
CEDCEBEBC
VvvVvvv
≥≤−=−=
As Emitter is heavily doped, a large number of electrons diffuse into the base (only a small fraction combine with holes) The number of these electrons scales as TBE Vve /
BE junction is forward biased (vBE = VD0)
Base current is also proportional to
(and thus, iC ): iB = iC/β
TBE Vve /
BJT operation in saturation mode
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (5/22)
For vBC ≥ 0 BC junction is forward biased and a diffusion current will set up, reducing iC .
1. Soft saturation: vCE ≥ 0.3 V (Si)* vBC ≤ 0.4 V (Si), diffusion current is small and iC is very close to its active-mode level.
2. Deep saturation region: 0.1 < vCE < 0.3 V (Si) or vCE ≈ 0.2 V = Vsat (Si), iC is smaller than its active-mode level (iC < β iB). o Called saturation as iC is set by outside
circuit & does not respond to changes in iB.
3. Near cut-off: vCE ≤ 0.1 V (Si) Both iC & iB are close to zero.
Similar to the active mode, a large number of electrons diffuse into the base.
BE junction is forward biased (vBE = VD0)
“Deep” Saturation mode:
satCE
BC
VvSB
Vvii
eIi TBE
≈<
=
/
ββ
* Sedra & Smith includes this in the active region, i.e., BJT is in active mode as long as vCE ≥ 0.3 V.
BJT iv characteristics: iB = f(vBE) & iC = g(iB , vCE)
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (6/22)
iB
Cut-off : BE is reverse biased
0 ,0 == CB ii
Active*: BE is forward biased BC is reverse biased
BC ii β=
Saturation: BE is forward biased, BC is forward biased
1. Soft saturation:
2. Deep saturation:
3. Near cut-off:
BCCE iiv ,V 7.03.0 β≈≤≤
BCCE iiv ,V 3.01.0 β<≤≤0 ,V 1.0 ≈≤ CCE iv
* Plot includes early effect (slide 8)
BJT iC = g(iB , vCE) is a surface*
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (7/22)
Looking at surface with iB axis pointing out of the paper
* Saturation region is exaggerated in 3D picture for clarity
Early Effect modifies iv characteristics in the active mode
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (8/22)
iC is NOT constant in the active region.
Early Effect: Lines of iC vs vCE for different iB (or vBE ) coincide at vCE = − VA
+=
A
CEVvSC V
veIi TBE 1/
NPN BJT iv equations
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (9/22)
“Linear” model*
Cut-off : BE is reverse biased Active: BE is forward biased BC is reverse biased (Deep) Saturation: BE is forward biased BC is foward biased
0 ,0 == CB ii
0 0 ,0
DBE
CB
Vvii
<==
+=
==
A
CEVvSC
VvSCB
VveIi
eIii
TBE
TBE
1/
/
ββ
0
0
, 0 ,
DCEBC
BDBE
VviiiVv
≥=≥=
β
BCsatCE
VvSB
iiVv
eIi TBE
,
/
ββ
<≈
=
BCsatCE
BDBE
iiVviVv
, 0 , 0
β<=≥=
V 2.0 ,V 7.0 Si,For 0 == satD VV
* BJT Linear model is based on a diode “constant-voltage” model for the BE junction and ignores Early effect.
PNP transistor is the analog to NPN BJT
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (10/22)
PNP transistor
Compared to a NPN: 1) Current directions are reversed 2) Voltage subscripts “switched”
“Linear” model
Cut-off : EB is reverse biased Active: EB is forward biased CB is reverse biased (Deep) Saturation: EB is forward biased CB is foward biased
0 0 ,0
DEB
CB
Vvii
<==
0
0
, 0 ,
DECBC
BDEB
VviiiVv
≥=≥=
β
BCsatEC
BDEB
iiVviVv
, 0 , 0
β<=≥=
Notations
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (11/22)
DC voltages: Use “Double subscript” of BJT terminal: VCC , VBB , VEE . Resistors:
Use “subscript” of BJT terminal: RC , RB , RE .
Voltage sources are identified by node voltage!
Transistor operates like a “valve:” iC & vCE are controlled by iB
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (12/22)
Controller part: Circuit connected to BE sets iB
Controlled part: iC & vCE are set by transistor state (& outside circuit)
Cut-off (iB = 0): Valve Closed iC = 0
Active (iB > 0): Valve partially open iC = β iB
Saturation (iB > 0): Valve open iC < β iB iC is limited by circuit connected to CE terminals, increasing iB does not increase iC
Recipe for solving BJT circuits (State of BJT is unknown before solving the circuit)
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (13/22)
1. Write down BE-KVL and CE-KVL:
2. Assume BJT is OFF, Use BE-KVL to check: a. BJT OFF: Set iC = 0, use CE-KVL to find vCE (Done!)
b. BJT ON: Compute iB
3. Assume BJT in active. Set iC = β iB . Use CE-KVL to find vCE . If vCE ≥ VD0 , Assumption Correct, otherwise in saturation:
4. BJT in Saturation. Set vCE = Vsat . Use CE-KVL to find iC . (Double-check iC < β iB )
NOTE: o For circuits with RE , both BE-KVL & CE-KVL have to be solved
simultaneously.
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (14/22)
Example 1: Compute transistor parameters (Si BJT with β = 100).
CEC
BEB
vivi
+=
+×=
10 12 :KVL-CE
10 40 4 :KVL-BE3
3
incorrect Assumption V 7.0V 4V 4 0 10 40 4 :KVL-BE
V 7.0 and 0 :off-Cut Assume
0
30
→=>==→+××=
=<=
DBE
BEBE
DBEB
Vvvv
Vvi
0A 5.82 7.0 10 40 4 :KVL-BE
0 and V 7.0 :ON BE3
0
>=→+××=
≥==
µBB
BDBE
iiiVv
correct Assumption V 7.0V 75.3V 75.3 1025.8 10 12 :KVL-CE
mA 25.81025.8100
V 7.0 and :Active Assume
0
33
60
→=>==→+××=
=××==
=≥=
−
−
DCE
CECE
BC
DCEBC
Vvvv
iiVvii
β
β
BJT Transfer Function (1)
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (15/22)
CECCCC
BEBBi
viRVviRv+=
+= :KVL-CE
:KVL-BE
0 :KVL-CE0
0 :KVL-BE and 0 :off-Cut 0
CCCECECCC
C
iBEBEBi
DBEB
VvvRVi
vvvRvVvi
=→+×==
=→+×=<=
,0 ,0 Cutoffin BJT For 0
CCCECB
Di
VviiVv
===→<
0
:KVL-BE
0 and :ON BE
0
00
0
DiB
B
DiBDBBi
BDBE
VviR
VviViRv
iVv
≥→≥
−=→+×=
≥=
BJT Transfer Function (2)
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (16/22)
CECCCC
B
DiBDBE
viRVR
VviVv
+=
−==
:KVL-CE
and :ON BE 00
BC
DCCDiDCE
CCCCCECECCCC
B
DiC
DCEBc
RRVVVvVv
iR-VvviRVR
Vvi
Vvii
/
:KVL-CE
and :Active
000
0
0
β
β
β
−+≤→≥
=→+=
−×=
≥=
activein BJT /
For 000 →
−+≤≤
BC
DCCDiD RR
VVVvVβ
BJT Transfer Function (3)
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (17/22)
CECCCC
B
DiBDBE
viRVR
VviVv
+=
−==
:KVL-CE
and :ON BE 00
BC
satCCDIHiBc
C
satCCCsatCCCC
BcsatCE
RRVVVVvii
RV-ViViRV
iiVv
/
:KVL-CE
and :nSaturaatio
0 ββ
β
−+=>→<
=→+=
<=
saturationin BJT /
For 00 →<
−+ i
BC
DCCD v
RRVVV
β
BJT Transfer Function (4)
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (18/22)
saturation deepin BJT /
activein BJT /
Cutoffin BJT
0
000
0
→<−
+
→−
+≤≤
→<
iBC
satCCD
BC
DCCDiD
Di
vRRVVV
RRVVVvV
Vv
β
β
BJT transfer function on the load line
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (19/22)
CCCCCE iRVv )KVL-(CE Line Load
−=
togetherincrease & :Active 0
CB
IHiD
iiVvV ≤≤
unchanged but increases :Saturation
CB
iIH
iivV <
:offCut
0Di Vv <−
BJT as a switch
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (20/22)
Use: Logic gate can turn loads ON (BJT in saturation) or OFF (BJT in cut-off)
ic is uniquely set by CE circuit (as vce = Vsat)
RB is chosen such that BJT is in deep saturation with a wide margin (e.g., iB = 0.2 ic /β)
*Lab 4 circuit Solved in Lecture notes (problems 12 & 13)
Load is placed in collector circuit
BJT as a Digital Gate
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (21/22)
Other variants: Diode-transistor logic (DTL) and transistor-transistor logic (TTL)
BJT logic gates are not used anymore except for high-speed emitter-coupled logic circuits because of o Low speed (switching to saturation is quite slow). o Large space and power requirements on ICs
RTL NOT gate (VL = Vsat , VH = VCC)
Resistor-Transistor logic (RTL)
RTL NOR gate* RTL NAND gate*
*Solved in Lecture notes (problems 14 & 15)
BJT β varies substantially
F. Najmabadi, ECE 65, Winter2013, Intro to BJT (22/22)
Our BJT model includes three parameters: VD0 , Vsat and β o VD0 and Vsat depend on base semiconductor:
o For Si, VD0 = 0.7 V, Vsat = 0.2 V
Transistor β depends on many factors: o Strongly depends on temperature (9% increase per oC)
o Depends on iC (not constant as assumed in the model)
o β of similarly manufactured BJT can vary (manufacturer spec sheet typically gives a range as well as an average value for β )
o We will use the average β in calculations (PSpice also uses average β but includes temperature and iC dependence).
o βmin is an important parameter. For example, to ensure operation in deep saturation for all similar model BJTs, we need to set iC /iB < βmin