Bipolar Junction Transistor

transcript

Bipolar JunctionTransistor

Electronic CircuitsElectronic Circuits

CHO, Yong HeuiCHO, Yong Heui

EM Wave EM Wave LabLab

1. Physical operation

Device structure

Mode EBJ CBJ

Cutoff Reverse Reverse

Active Forward Reverse

Rev. Act. Reverse Forward

Saturation Forward Forward

Device structure

Operation in active mode

Operation in forward active mode

Minority carrier profile

Collector current

For short Emitter

IB2 = Qn/τb

Qn = AE·q·1/2·np(0)·W = (AE·q·W·ni2/2NA)evBE/VT

IB = IB1 + IB2 = IC/βDp

2Dnτbβ = +[ ]

-1: common emitter current gain

Base current

: common base current gain

α = αF, β = βF : at forward active

Emitter current

Equivalent model

Ebers-Moll model

Ebers-Moll Model at saturation Mode

Ebers-Moll model

Saturation Mode

Saturation mode

PNP transistor

2. I-V

Example

2. I-V

Common-base

iC = ISeVBE/VT·(1+vCE/VA)

∂vCE

ro ≡vBE=constant

2. I-V

Common-emitter

βdc = ICQ/IBQ

βac = ΔiC/ΔiB : vCE=constant

2. I-V

Current gain

βforced = ICsat/IB

βforced < βF

2. I-V

Saturation voltage

∂vCE

∂iCRCEsat ≡ iB=IB

iC=ICsat

2. I-V

Saturation resistance

∂vCE

∂iCRCEsat ≡ iB=IB

iC=ICsat

2. I-V

Saturation resistance

vO = vCE = VCC - RCiC

vO = VCC - RC·ISevI/VT : at active

ICsat = (VCC – VCEsat)/RC

0 < vI < 0.5 : Cutoff (switch off)

: at saturation

Av ≡dvo

dvi vI=VBE

= -(1/VT)·ISeVBE/VT·RC

= -ICRC/VT = -VRC/VT

VRC = VCC - VCE

For max gain and swing

3. Amplifier

Large signal

vCE = VCC-RCiC

iC = VCC/RC – vCE/RC

vBE = VBB-RBiB

iB = VBB/RB – vBE/RB

3. Amplifier

Graphical analysis

3. Amplifier

Graphical analysis

IB(EOS)

3. Amplifier

Bias point

Operation as switch

4. BJT circuits

Example

4. BJT circuits

Example

4. BJT circuits

Example

4. BJT circuits

Example

RE : negative feedback

5. Bias

Discrete-circuit biasing

Biasing using a collector-to-base feedback resistor

VCB = IBRB = IERB/(β+1)

Small RB small swing

5. Bias

Discrete-circuit biasing

I = IREF = (VCC + VEE – VBE)/R

Large RB 사용가능

5. Bias

Current source biasing

6. Small-signal operation

Bias point

Transconductance

Small signal Emitter resistance : re

Voltage Gain

vc = -icRC = gmvbeRC = (-gmvbe)RC Av = vc/vbe = -gmRC = -ICRC/VT

Input resistance

Hybrid model

T model

Example

Av = vc/vbe = -gm(RC//rO)

Output resistance

Bypass Capacitor : CE

Coupling Capacitor : CC1, CC2

At DC analysis

7. BJT amplifier

Common-emitter amplifier

Rib = rπ Rin = vi/ii = RB//Rib ≒ rπ vsigvi =Rin

Rin+Rsig

vsig= rπ

rπ+Rsig

vi = vπvO = -gmvπ(RC∥RL∥rO)

Av = -gm(RC∥RL∥rO) Avo = -gm(RC∥rO) ≒ -gmRC (rO≫RC)

Rin+Rsig

AvGv =

rπ+Rsig

≒ - gm(Rc∥RL∥rO) ≒ Av (rπ≫Rsig)

Rout = RC∥rO ≒ RC

Ais = ios/ii = -gmRin ≒ -gmrπ= -β

7. BJT amplifier

Small signal circuit

vo = -ic(RC∥RL) = -αie(RC∥RL) = -α(RC∥RL)

Rin = RB//Rib Rib = vi/ib = (β+1)(re+Re) ≒ rπ(1+gmRe)

rO = ∞

Av ≒ -(RC∥RL)/(re+Re)

Avo = -αRC/(re+Re) ≒ -gmRC/(1+gmRe) Rout = RC∥rO ≒ RC

Ais = ios/ii = -αie/(vi/Rin) = -αRinie/vi = -α(RB∥Rib)/(re+Re) = -α(β+1)(re+Re)/(re+Re) = -β

ie = vi/(re+Re)

7. BJT amplifier

Emitter resistance

Rin+Rsig

AvGv =

Rin+Rsig

= -α(RC∥RL)

re+Re ≒ -

β(RC∥RL)

Rsig+(β+1)(re+Re)

: less sensitive to β

vπ = vi re

11+gmRe

≒ vi

Rib is increaed by the factor (1+gmRe)

The voltage gain is reduced by the factor (1+gmRe)

For the same nonlinear distortion, vi can be increased by the factor (1+gmRe)

Overall voltage gain is less sensitive to β

High frequency response is improved.

Effects of negative feedback through Re

7. BJT amplifier

Emitter resistance

vo = -αie(RC∥RL) = α(RC∥RL) ie = -vi/reRin = re

Av = gm(RC∥RL) Avo = gmRC Rout = RC

Ais = ios/ii = -αie/(-ie) = αvi

Rin+Rsig

re+Rsig

AvGv = = gm(RC∥RL)

re+Rsig

α(RC∥RL)

re+Rsig

7. BJT amplifier

Common base amplifier

Rib = vb/ib = (β+1)[re+(ro∥RL)]

Rin = RB∥Rib

7. BJT amplifier

Emitter follower

vo/vsig ≒RL

[Rsig/(β+1)] +re+RL

RB≫Rsig, ro≫RL

Rout = ro∥Rsig∥RB

re + β+1[ ] Rsig∥RB

re + β+1≒

Gvo =ro

[(Rsig∥RB)/(β+1)] +re+roRsig+R

7. BJT amplifier

Emitter follower

8. BJT capacitance

Internal capacitance

hfe : CE short-circuit current gain

Ib = Vπ· (1/rπ//sCπ//sCμ) Ic = Vπ· (gm - sCμ)

hfe ≡ Ic/Ib = 1/rπ+ sCπ+ sCμ

gm - sCμ

1 + s(Cπ+Cμ)rπ

gmrπ≒1 + s/ωβ

β0=: STC LPF

(Cπ+Cμ)rπ

1ωβ = ωT = β0·ωβ =

Cπ+Cμ

gm fT = 2π(Cπ+Cμ)

8. BJT capacitance

High frequency model

High level Injection

8. BJT capacitance

Collector current

RB//rπ

RB//rπ+Rsig

AM = - gm(Rc∥RL∥rO)

9. Frequency response

Frequency bands

High frequency model

Iμ = sCμ(Vπ-Vo)

= sCμ(Vπ+ gmRL’Vπ)

= sCμ(1 + gmRL’)Vπ

= sCeqVπ

Ceq = sCμ(1 + gmRL’)

1 + s/ω0

1V’sigVπ =

ω0 = 1/CinRsig’

High frequency response

1 + s/ω0

RB + Rsig

AM =1 + s/ω0

rπ+rx+(RB∥Rsig)

rπ·gmRL’

High frequency response

ss+ ωP2

ss+ ωP3

= - AM

High Freq. gain X 3 poles

Low frequency response

(RB//rπ)+Rsig+ sCC1

RB//rπVsigVπ =Vo = - gmVπ(Rc∥RL)

(RB//rπ)+Rsig

RB//rπ gm(Rc∥RL)

CC1[(RB//rπ)+Rsig]1

s ωP1

= - AM= -

Capacitance effect

(RB//Rsig)+(β+1)(re sCE

VsigIb =Vo = - βIb(Rc∥RL)

VsigωP2

= - AM= -

RB + Rsig

CE[re+

RB//Rsig

β+1 ]

RB + Rsig

β(Rc∥RL)

(RB//Rsig)+(β+1)re

Capacitance effect

(RB//rπ)+Rsig

RB//rπVsigVπ = Vo = - gmVπ

(RB//rπ)+Rsig

RB//rπ gm(Rc∥RL)

CC2(RC+RL)1

s ωP3

= - AM= -

Capacitance effect

Bipolar Junction Transistor

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