77
ter 4 – Bipolar Junction Transistors (BJTs) Introduction p://engr.calvin.edu/PRibeiro_WEBPAGE/courses/engr311/311_frames.html
Chapter 4 – Bipolar Junction Transistors (BJTs)
Introduction
http://engr.calvin.edu/PRibeiro_WEBPAGE/courses/engr311/311_frames.html
A simplified structure of the npn transistor.
Physical Structure and Modes of Operation
A simplified structure of the pnp transistor.
Physical Structure and Modes of Operation
Physical Structure and Modes of Operation
Mode EBJ CBJ
Active Forward ReverseCutoff Reverse ReverseSaturation Forward Forward
Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown.)
Operation of The npn Transistor Active Mode
Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; vBE 0 and vCB 0.
Operation of The npn Transistor Active Mode
The Collector Current
The Base Current
Physical Structure and Modes of Operation
i C I S e
v BE
V T
i B
i C
I S
e
v BE
V T
iE iC iB 1
iC
1
IS e
vBE
VT
iC IE
1
Operation of The npn Transistor Active Mode
Large-signal equivalent-circuit models of the npn BJT operating in the active mode.
Equivalent Circuit Models
The Constant n
The Collector-Base Reverse Current
The Structure of Actual Transistors
Current flow in an pnp transistor biased to operate in the active mode.
The pnp Transistor
Two large-signal models for the pnp transistor operating in the active mode.
The pnp Transistor
Circuit Symbols and Conventions
C
B
E
C
B
E
Circuit Symbols and Conventions
Example 4.1 VCC 15 IC1 0.001 100 VBE 0.7 VEE 15 VT 0.025
Design circuit such that
VC 5 IC2 0.002
RCVCC VC
IC2 RC 5 10
3
Since VBE=0.7V at IC=1mA, the value of VBE at IC=2mA is
VBE 0.7 VT ln2
1
VBE 0.717
VE VBE VE 0.717
1 IE
IC2
IE 2.02 10
3
REVE VEE( )
IE RE 7.071 10
3
i C I S e
v BE
V TIB
IC2
IB 2 10
5
E
BC
Example 4.1
IBIC2
IB 2 10
5
Example 4.1
Summary of the BJT I-V Relationships in the Active Mode
iC IS e
vBE
VT iB
iC
IS
e
vBE
VT iE
iC
IS
e
vBE
VT
Note : for pnp transitor, replace vBE for vEB
iC iE iB 1 iEiE
1
iC iB iE 1 iB
iE
1VT 25mV
Exercise 4.8
Exercise 4.9
The Graphical Representation of the Transistor Characteristics
The Graphical Representation of the Transistor Characteristics
Temperature Effect (10 to 120 C)
The iC-vCB characteristics for an npn transistor in the active mode.
Dependence of ic on the Collector Voltage
Dependence of ic on the Collector Voltage
(a) Conceptual circuit for measuring the iC-vCE characteristics of the BJT. (b) The iC-vCE characteristics of a practical BJT.
Dependence of ic on the Collector Voltage – Early Effect
I C I S e
v BE
VT 1v CE
V A
VA – 50 to 100V
Dependence of ic on the Collector Voltage – Early Effect
Nested DC Sweeps
Example
Example
Example
Monte Carlo Analysis – Using PSpice
Monte Carlo Analysis – Using PSpice
Monte Carlo Analysis – Using PSpice
Probe Output Ic(Q), Ib(Q), Vce
Monte Carlo Analysis – Using PSpice
(a) Conceptual circuit to illustrate the operation of the transistor of an amplifier.
(b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.
The Transistor As An Amplifier
The Collector Current and The Transconductance
The Base Current and the Input Resistance at the Base
The Emitter Current and the Input Resistance at the Emitter
Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimpose din
the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Ic = gm
vbe, where gm is the slope of the ic - vBE curve at the bias point Q.
The Transistor As An Amplifier
Two slightly different versions of the simplified hybrid- model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source ( a transconductance amplifier) and that in (b) represents the BJT as a current-controlled current source (a current amplifier).
Small-Signal Equivalent Circuit Models
Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance re rather than the base resistance r featured in the hybrid- model.
Small-Signal Equivalent Circuit Models
Signal waveforms in the circuit of Fig. 4.28.
Fig. 4.30 Example 4.11: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.
Fig. 4.34 Circuit whose operation is to be analyzed graphically.
Fig. 4.35 Graphical construction for the determination of the dc base current in the circuit of Fig. 4.34.
Fig. 4.36 Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE in the circuit of
Fig. 4.34.
Fig. 4.37 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc
voltage VBB (see Fig. 4.34).
Fig. 4.38 Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which
is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to
the saturation region, thus limiting the negative swing of vCE.
Fig. 4.44 The common-emitter amplifier with a resistance Re in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced
with its T model (c) The circuit in (b) with ro eliminated.
Fig. 4.45 The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model.
Fig. 4.46 The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. (c) The circuit in (b) redrawn to show that ro is in parallel with RL. (d) Circuit for determining Ro.
An npn resistor and its Ebers-Moll (EM) model. ISC and ISE are the scale or saturation currents of diodes DE (EBJ) and DC (CBJ).
More General – Describe Transistor in any mode of operation.
Base for the Spice model.
Low frequency only
A General Large-Signal Model For The BJT: The Ebers-Moll Model
iDE ISE e
vBE
VT1
iDC ISC e
vBC
VT1
ISC > ISE (2-50)
A General Large-Signal Model For The BJT: The Ebers-Moll Model
IDE ISE e
vBE
VT1
IDC ISE e
vBC
VT1
Fforwarded of the transistor source (close to 1)
Rreverse of the transistor source (0.02 - 0.5
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Terminal Currents
F ISE R ISC IS
iE iDE R iDC iC iDC R iDE
iB 1 F iDE 1 R iDC
F
F
1 FiE
IS
Fe
vBE
VT1
IS e
vBC
VT1
R
R
1 RiC
IS
Fe
vBE
VT1
IS
Re
vBC
VT1
iB
IS
Fe
vBE
VT1
IS
Re
vBC
VT1
iE
IS
Fe
vBE
VT IS 11
F
iC IS e
vBE
VT IS1
R1
iB
IS
Fe
vBE
VT IS1
F
1
R
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Forward Active Mode
Since vBC is negative and its magnitudeIs usually much greater than VT the Previous equations can be approximatedas
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Normal Saturation
Collector current will be forced IB forced F
In saturation both junctions are forwarded biased. Thus VBE and VBCare positive and their values greater than VT.Making these approximations and substituting
iB IBand iC forced IB
results in two equations that can be solved to obtain VBE and VBC.The saturatuion voltage can be obtained as the difference between the two:
VCEsat VT ln
1forced 1
R
1forced
F
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Reverse Mode
I1
I2IB
Note that the currents indicated have positive values. Thus, since ic = -I2 and iE = -I1, both iC and IE will be negative. Since the roles of the emitter and collector are interchanged, the transistor in the circuit will operate in the active mode (called the reverse active mode) when the emitter-base junction is reverse-biased. In such a case
I1 = beta_R . IB
This circuit will saturate (reverse saturation mode) when the emitter-base junction becomes forward-biased.
I1/IB < beta_R
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Reverse Saturation
VECsat VT ln
11
F
I1
IB
1
F
1I1
IB
1
R
We can use the EM equations to find the expression of VECSat
From this expression, it can be seen that the minimum VECSat is obtained when I1 = 0. This minimum is very close to zero.
The disadvantage of the reverse saturation mode is a relatively long turnoff time.
VE 4.56VE VCC I1 RC
I1 4.4 104I1 R IB
RC 1000a) for RC = 1 K, assume that the transitor is in the reverse active mode. thus
IB 4.4 103IB
VI VBRB
VB 0.6From VBC = 0.6
Calculate approximate values ofe VE for the following cases:RC = 1K, 10K, 100K
F 50R 0.1
VBC 0.6VCC 5VI 5RB 1000
For the circuit below, let
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Example
the BJT is sauratedI1 R IBSince
mVVECsat 3.5VECsat VT ln
11
F
I1
IB
1
F
1I1
IB
1
R
VT 25a better estimate for VE is to use the equation below (4.115)
I1 5 104I1
VCC 0RC
Since VECsat is liekly to be very small, we can assume VE = 0, and
RC 10000b) For RC = 100K, assume reverse saturation mode
Since VE = VB, the BJT is still in the reverse active mode.
VE 0.6VE VCC I1 RC
I1 4.4 104I1 R IB
RC 10000b) For RC = 10K, assume reverse active mode
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Example
The transport model of the npn BJT. This model is exactly equivalent to the Ebers-Moll model. Note that the saturation currents of the diodes are given in parentheses and iT is defined by Eq. (4.117).
A General Large-Signal Model For The BJT: The Ebers-Moll Model – Transport Model npn BJT
Basic BJT digital logic inverter.
Basic BJT Digital Logic Inverter.
vi high (close to power supply) - vo lowvi low vo high
Sketch of the voltage transfer characteristic of the inverter circuit of Fig. 4.60 for the case RB = 10 k, RC = 1 k, = 50, and VCC =
5V. For the calculation of the coordinates of X and Y refer to the text.
Basic BJT Digital Logic Inverter.
(a) The minority-carrier concentration in the base of a saturated transistor is represented by line (c). (b) The minority-carrier charge stored in the base can de divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results in driving the transistor deeper into saturation.
The Voltage Transfer Characteristics
The ic-vcb or common-base characteristics of an npn transistor. Note that in the active region
there is a slight dependence of iC on the value of vCB. The result is a finite output resistance
that decreases as the current level in the device is increased.
Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common Base
Avalanche
Saturation
Slope
The hybrid- model, including the resistance r, which models the effect of vc on ib.
Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common Base
Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail.
Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common-Emitter
An expanded view of the common-emitter characteristics in the saturation region.
Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common-Emitter
The Transistor Beta
Transistor Breakdown
Internal Capacitances of a BJT
Cde F
IC
VT Base charging or Diffusion capacitance
Base Emitter Junction capacitanceCje
Cje0
1VBE
V0e
m
m - 0.2 - 0.5 grading coefficient
CC0
1VCB
V0c
m Collector Base Juntion Capacitance
C Cde Cje
r x
The Cut-Off Frequency
The Spice BJT Model and Simulation Examples
The Spice BJT Model and Simulation Examples
The Spice BJT Model and Simulation Examples
.model Q2N2222-X NPN(
Is=14.34f
Xti=3
Eg=1.11
Vaf=74.03
Bf=200
Ne=1.307
Ise=14.34f
Ikf=.2847
Xtb=1.5
Br=6.092
Nc=2
Isc=0
Ikr=0
Rc=1
Cjc=7.306p
Mjc=.3416
Vjc=.75
Fc=.5
Cje=22.01p
Mje=.377
Vje=.75
Tr=46.91n
Tf=411.1p
Itf=.6
Vtf=1.7
Xtf=3
Rb=10)
*National pid=19
case=TO18 88-09-07 bam creation
The Spice BJT Model and Simulation Examples
The Spice BJT Model and Simulation Examples
BJT Modeling - Idealized Cross Section of NPN BJT
Sunday, Marc h 08, 1998
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T1BIFILAR XFMR2 x 10t AWG32 ONAMIDON FT37-61
T3TRIFILAR XFMR3 x 10t AWG32 ONAMIDON FT37-61
T4TRIFILAR XFMR3 x 12t AWG32 ONAMIDON FT37-61
T5PRI: 360t AWG40SEC: 800t AWG40ON AMIDONPC1408-77 POT CORE
T2PRI: 650t AWG40SEC: 50t AWG32ON AMIDONPC1408-77 POT CORE
T8: BIFILAR CHOKE2 x 8t AWG26 ONAMIDON FT50-61
T9: PRI: 2 x 8t AWG 26SEC: 7t AWG 26ON AMIDON T68-6
T7: PRI: 36t AWG 32SEC: 2 x 9t AWG 32ON AMIDON T50-2
T6: PRI: 36t AWG 32SEC: 4t AWG 32ON AMIDON T50-6
The Spice BJT Model and Simulation Examples
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