Bipolar Junction Transistor (BJT) Lecture notes: Sec. 3 Sedra & - - PowerPoint PPT Presentation

Bipolar Junction Transistor (BJT)

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

• F. Najmabadi, ECE65, Winter 2012

A BJT consists of three regions

• F. Najmabadi, ECE65, Winter 2012

Simplified physical structure

NPN transistor

An implementation on an IC

• Device construction is NOT symmetric
• “Thin” base region (between E & C)
• Heavily doped emitter
• Large area collector
• Device is constructed such that

BJT does NOT act as two diodes back to back (when voltages are applied to all three terminals).

• Six circuit variables: (3 i and 3 v)
• Two can be written in terms of the
• ther four:

BJT iv characteristics includes four parameters

• F. Najmabadi, ECE65, Winter 2012

NPN transistor

CE BE BC B C E

v v v i i i − = + = : 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

) , ( ) (

CE B C BE B

v i g i v f i = =

Active mode:

/ / D CE V v S C V v S C B

V v e I i e I i i

T BE T BE

≥ = = = β β

BJT operation in the “active” mode

• F. Najmabadi, ECE65, Winter 2012
• 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

T BE V

v S C

e I i

/

=

D CE CE D CE BE BC

V v v V v v v ≥ ≤ − = − =

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

T BE V

v

e

/

BE junction is forward biased (vBE = VD0)

• Base current is also proportional to

and therefore, iC : iB = iC/β

T BE V

v

e

/

BJT operation in saturation mode

• F. Najmabadi, ECE65, Winter 2012
• 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)
• r vCE ≈ 0.2 V = Vsat (Si), iC is smaller than

its active-mode level (iC < β iB).

• 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:

sat CE B C V v S B

V v i i e I i

T BE

≈ < =

/

β β

* 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 includes four parameters

• F. Najmabadi, ECE65, Winter 2012

NPN transistor

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

) , ( ) (

CE B C BE B

v i g i v f i = = Simplified physical structure

BJT iv characteristics: iB = f(vBE) & iC = g(iB , vCE)

• F. Najmabadi, ECE65, Winter 2012

iB

Cut-off : BE is reverse biased

, = =

C B

i i

Active: BE is forward biased BC is reverse biased

B C

i i β =

Saturation: BE is forward biased, BC is forward biased

• 1. Soft saturation:
• 2. Deep saturation:
• 3. Near cut-off:

B C CE

i i v , V 7 . 3 . β ≈ ≤ ≤

B C CE

i i v , V 3 . 1 . β < ≤ ≤ , V 1 . ≈ ≤

C CE

i v

Early Effect modifies iv characteristics in the active mode

• F. Najmabadi, ECE65, Winter 2012
• 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 CE V v S C

V v e I i

T BE

1

/

NPN BJT iv equations

• F. Najmabadi, ECE65, Winter 2012

“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 reverse biased

, = =

C B

i i ,

D BE C B

V v i i < = =         + = = =

A CE V v S C V v S C B

V v e I i e I i i

T BE T BE

1

/ /

β β , ,

D CE B C B D BE

V v i i i V v ≥ = ≥ = β

B C sat CE V v S B

i i V v e I i

T BE

,

/

β β < ≈ =

B C sat CE B D BE

i i V v i V v , , β < = ≥ = V 2 . , V 7 . Si, For = =

sat D

V V

PNP transistor is the analog to NPN BJT

• F. Najmabadi, ECE65, Winter 2012

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 reverse biased

,

D EB C B

V v i i < = = , ,

D EC B C B D EB

V v i i i V v ≥ = ≥ = β

B C sat EC B D EB

i i V v i V v , , β < = ≥ =

Notations

• F. Najmabadi, ECE65, Winter 2012

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, ECE65, Winter 2012

Controller part: Circuit connected to BE sets iB Controlled part: iC & vCE are set by transistor state (&

• utside 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 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)

• 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:

• For circuits with RE , both BE-KVL & CE-KVL have to be solved

simultaneously.

• F. Najmabadi, ECE65, Winter 2012

• F. Najmabadi, ECE65, Winter 2012

Example 1: Compute transistor parameters (Si BJT with β = 100).

CE C BE B

v i v i + = + × = 10 12 : KVL

• CE

10 40 4 : KVL

• BE

3 3

incorrect Assumption V 7 . V 4 V 4 10 40 4 : KVL

• BE

V 7 . and :

• ff
• Cut

Assume

3

→ = > = = → + × × = = < =

D BE BE BE D BE B

V v v v V v i A 25 . 8 7 . 10 40 4 : KVL

• BE

and V 7 . : ON BE

3

> = → + × × = ≥ = = µ

B B B D BE

i i i V v correct Assumption V 7 . V 75 . 3 V 75 . 3 10 25 . 8 10 12 : KVL

• CE

mA 25 . 8 10 25 . 8 100 V 7 . and : Active Assume

3 3 6

→ = > = = → + × × = = × × = = = ≥ =

− − D CE CE CE B C D CE B C

V v v v i i V v i i β β

BJT Transfer Function (1)

• F. Najmabadi, ECE65, Winter 2012

CE C C CC BE B B i

v i R V v i R v + = + = : KVL

• CE

: KVL

• BE

: KVL

• CE

: KVL

• BE

and :

• ff
• Cut

CC CE CE C CC C i BE BE B i D BE B

V v v R V i v v v R v V v i = → + × = = = → + × = < = , , Cutoff in BJT For

CC CE C B D i

V v i i V v = = = → < : KVL

• BE

and : ON BE

D i B B D i B D B B i B D BE

V v i R V v i V i R v i V v ≥ → ≥ − = → + × = ≥ =

BJT Transfer Function (2)

• F. Najmabadi, ECE65, Winter 2012

CE C C CC B D i B D BE

v i R V R V v i V v + = − = = : KVL

• CE

and : ON BE

B C D CC D i D CE C C CC CE CE C C CC B D i C D CE B c

R R V V V v V v i R

• V

v v i R V R V v i V v i i / : KVL

• CE

and : Active β β β − + ≤ → ≥ = → + = − × = ≥ = active in BJT / For → − + ≤ ≤

B C D CC D i D

R R V V V v V β

BJT Transfer Function (3)

• F. Najmabadi, ECE65, Winter 2012

CE C C CC B D i B D BE

v i R V R V v i V v + = − = = : KVL

• CE

and : ON BE

B C sat CC D IH i B c C sat CC C sat C C CC B c sat CE

R R V V V V v i i R V

• V

i V i R V i i V v / : KVL

• CE

and : n Saturaatio β β β − + = > → < = → + = < = saturation in BJT / For → < − +

i B C D CC D

v R R V V V β

BJT Transfer Function (4)

• F. Najmabadi, ECE65, Winter 2012

saturation deep in BJT / active in BJT / Cutoff in BJT → < − + → − + ≤ ≤ → <

i B C sat CC D B C D CC D i D D i

v R R V V V R R V V V v V V v β β

BJT transfer function on the load line

• F. Najmabadi, ECE65, Winter 2012

C C CC CE

i R V v ) KVL

• (CE

Line Load − = together increase & : Active

C B IH i D

i i V v V ≤ ≤ unchanged but increases : Saturation

C B i IH

i i v V < :

• ff

Cut

D i

V v < −

BJT as a switch

• 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)

• F. Najmabadi, ECE65, Winter 2012

Load is placed in collector circuit *Lab 4 circuit Solved in Lecture notes (problems 12 & 13)

BJT as a Digital Gate

• 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

• Low speed (switching to saturation is quite slow).
• Large space and power requirements on ICs
• F. Najmabadi, ECE65, Winter 2012

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

• Our BJT model includes three parameters: VD0 , Vsat and β
• VD0 and Vsat depend on base semiconductor:
• For Si, VD0 = 0.7 V, Vsat = 0.2 V
• Transistor β depends on many factors:
• Strongly depends on temperature (9% increase per oC)
• Depends on iC (not constant as assumed in the model)
• β of similarly manufactured BJT can vary (manufacturer spec sheet

typically gives a range as well as an average value for β )

• We will use the average β in calculations (PSpice also uses average β but

includes temperature and iC dependence).

• 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

• F. Najmabadi, ECE65, Winter 2012