Download BJT-1-examples

BJT’s
Emitter-Base
Junction
C
iC
n
p
E
iE
n
iB
B
Collector-Base
Junction
Emitter
Base
Collector
Active Base Region
Figure 5.1(a) - Simplified cross-section of an npn transistor with currents that occur during
"normal" operation
p
+
n
to r
ec
o ll
+
C
p
+
n +
p
r
it te
n
Em
ne
pi ta
x
y
iC
p
n +
p
+
i
se
Ba
E
xy
pi ta
ne
+
n b
u ri
p
+
p
i
ed
B
l ay
er
p
p
Active Transistor
R egion
+
p
p
Figure 5. 1(b) - Cross-section of an integrated npn bipolar junction transistor
iC
Collector (C)
n
vBC
iC
Collector
iB
iB
p
Base (B)
Base
v
iE
n
BE
Emitter
Emitter (E)
(b)
(a)
iE
Figu r e 5 . 2 - (a ) Id e a lize d n p n tr a n s is tor s tr u c tu r e for a ge n e r a l b ia s c on d ition
(b ) Cir c u it s ym b ol for th e n p n tr a n s is tor
Forward Characteristics
C
iC
n
iF
Collector
iF
iB
B
v
BE
iF
F
p
Base
Emitter
iF
F
n
iE
E
Figure 5. 3 - npn transistor with vBE applied and vBC = 0.
Reverse Characteristics
C
iC
n
v
Collector
BC
iB
B
p
iR
R
iR
Base
Emitter
n
iE
E
F igu r e 5 .4 - Tr a n s is tor with vBC a p p lied a n d vBE = 0 .
Table 5.1
Common-Emitter and Common-Base
Current Gain Comparison
F or R
0.1
0.5
0.9
0.95
0.99
0.998
F 
F
R
or  R 
1  F
1  R
0.11
1
9
19
99
499
ENGR 311 – Electronic Devices and Circuits
October 26, 2000
Transistor Model: Current Amplifier
A Summary For Clarification (assume npn for the following general rules/properties – for pnp reverse
polarities)
Rules / Properties
1 – The collector must be positive than the emitter.
2 – The base-emitter and base-collector circuits behave like diodes. Normally the base-emitter diode is
conducting and the base-collector diode is reverse-biased
3 – When 1 and 2 are obeyed Ic is proportional to Ib (Ic = beta . Ib)
Both Ib and Ic follow to the emitter.
Note: the collector current is not due to forward conduction of the base-collector diode; that diode is reversebiased. Just think of it as “transistor action.”
Property 3 gives the transistor its usefulness: a small current flowing into the base controls a much larger
current flowing into the collector.
Note the effect of property 2. This means you can’t go sticking a voltage across the base-emitter terminals,
because an enormous current will flow if the base is more positive than the emitter by more than about 0.6 to
0.8 volt. This rule also implies that an operating transistor has Vb = ~ Ve + 0.6 (Vb = Ve + Vbe) (for an npn).
Let me emphasize again that you should not try to think of the collector current as diode conduction. It isn’t,
because the collector-base diode normally has voltages applied across it in a reverse direction. Furthermore,
collector current varies very little with collector voltage (it behaves like a not-too-great current source), unlike
forward diode conduction, where the current rises very rapidly with applied voltage.
Current flow
The forward bias on the base-emitter junction will cause current flow across this junction. Current will consist
of two components: electrons injected from the emitter into the base, and holes from the base into the emitter.
The electrons injected from the emitter into the base are minority carriers in the p-type base region. Because
the base is usually very thin the excess minority carriers (electron) concentration in the base will have an
almost straight-line profile. The electrons will reach the boundary of the collector-base depletion region.
Because the collector is more positive than the base these electrons will be swept across the CB junction region
into the collector. They are then “collected” to constitute the collector current. By convention the direction of
ic will be opposite to that of the electron flow; thus ic will flow into the collector terminal.
Ic – Vce Characteristic for an npn Transistor
Ic- Vbe Characteristics
Biasing
For common emitter amplifier
ENGR 311 - BJTs – Exercises - October 29, 2001
Examples
Example 1 - Beta = 100, vBE = 0.7V at
iC = 1mA. Design circuit so that a
current of 2mA flows through the
collector and a voltage of +5V appears at
the collector.
Example 2 - In the circuit below vC = 0.7V. If Beta = 50, find IE, IB, IC and
VC.
Solution
Example 3 – In the circuit below, Vb =
1V, VE = 1.7V. What are alfa and beta
for this transistor? What voltage VC do
you expect at the collector.
Example 4 - Beta = 100 – Determine all
node voltages and branch currents.
Example 5
Determine the voltages at all nodes and current through all branches.
Assume beta 1 and beta2 = 200. Assume Q1 is in the active mode.
ENGR 311 - Graphical Representation of Transistor Characteristics - October 31, 2001
Conceptual circuit for measuring the iC-vCE characteristics of the BJT. (b) The iC-vCE characteristics of a practical BJT.
The iC-vCB characteristics for an npn transistor in the active mode
Determine the voltages at all nodes and the currents at all branches in the circuit below.
Solution
The Transistor As An Amplifier – DC Conditions
(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 Collector Current and The Transconductance
The Base Current and the Input Resistance at the Base
Transistor as An Amplifier - Small Signal Approximation
Transconductance (gm), Input Resistance at the Base (r), Input Resistance at the Emitter (re), Voltage Gain
Exercise 4.22 and 4.23
Small-Signal Equivalent Circuits Models
Amplifier Circuit Without DC Sources
Hybrid-
The T Model
Application of the Small-Signal Equivalent Circuits
1
2
3
4
5
Example 4.9
DC Analysis
Small-Signal Analysis
Example 4.11
Determine voltage gain in the circuit below
DC Analysis
Small-Signal Model
Small-Signal Analysis Directly on Circuit
Graphical Analysis
Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is
superimposed on the dc voltage VBB.
Biasing The BJT For Discrete-Circuit Design
Basic Single-Stage BJT Amplifier Configurations
Minority-Carrier Transport In the Base Region
+
+
iB
v
v
BE
IF
F
N
i
BC
I
REC
IR
R
N
P
(p , n bo
)
i
bo
E
C
iT
(a)
Emitter
Base
Collector
Space Charge Regions
n( x)
n(0)
dn
i qAD n
T
dx
n(W)
(b)
0
x
W
F igu re 5.15 - (a ) Cu rren t s in t h e ba s e region of a n pn t ra n s is t or
(b) Min orit y ca rrier con cen t ra t ion in t h e ba s e of t h e n pn t ra n s is t or
iT = qADn. dn/dx = -qADnn. (nbo/Wb). [exp(vbe/Vt) – exp(vbc/Vt)]
Is = qADn.(nbo/Wb) = (qADn.ni^2 )/Nab.Wb
nbo = equilibrium electron density
A = cross-sectional area of the base region
Wb = base width
Dn = diffusity (cm^2/s)
Nab = doping concentration in base of transistor’
ni = intrinsic-carrier concentration (10^10.cm^3)
nbo = ni^2 / Nab
Base Transit Time
To turn the BJT minority-carrier charge must be introduced into the base to establish the gradient.
The forward transit time tau-f represents the time constant associated with storing the required charge Q in the
base region and is defined by
Q/IT
Diffusion Capacitance
For the base-emitter voltage and hence the collector current in the BJT to change, the charge stored in the base
region also must change.
n( x)
n(0)
Q
n(W) = n bo
n bo
x
0
W
F igu r e 5 . 1 6 (a ) - E xc e s s m in or ity c h a r ge Q s tor e d in th e b ip ola r b a s e r e gion
n( x)
n(0, V BE2
)
Q
n(0, V BE1
)
Q
n bo
n(W) = n
bo
x
0
W
F igu r e 5 .1 6 (b ) - S t o r ed ch a r ge Q ch a n ges a s vBE ch a n ges
This change in charge with vbe can be modeled by a capacitance CD
CD = (Ic/VT). f
Frequency Dependence of the Common-Emitter Current Gain
iB
C
C
B
iC
B
i =  i
+
C
F B
v
iB
BE
v
i = I S exp (
C
-
i
BE
)
VT
i = (  + 1) i B
E
F
E
E
E
Figure 5.20 - Simplified model for the npn transistor for the forward-active region
iB
iC
C
B
+
-
0.7 V
i =  i
v
C
BE
F B
iE
E
Figure 5. 21 - Further simplification of the npn model for the forward-active region
Beta-cutoff Frequency
10
3
Common-Emitter Current Gain
10 2
10 1
10 0
f
T
10 -1
10 4
10 5
10 6
10 7
10 8
10 9
Frequency (Hz)
Figure 5.22 - Common-e mitte r curre nt ga in  vs . fre que ncy
Transconductance
Relates changes in ic to changes in vbe
gm = dic/dvbe (@Q-point)
gm = Ic /VT
CD = gm.f
The Early Effect (James Early form Bell Labs)
Experimentally demonstrated that when the output curves are extrapolated back to a point of zero collector
current, the curves all intersect at a constant voltage point vce = -VA
I = 100 uA
B
4.0mA
C
o
l
l
e
c
t
o
r 2.0mA
I = 80 uA
B
I = 60 uA
B
C
u
r
r
e
n
t
I = 40 uA
B
I = 20 uA
B
0A
-V
A
-15V
-10V
-5V
0V
5V
10V
15V
Collector-Emitter Voltage
F igu r e - 5 . 3 0 Tr a n s is t or ou t p u t c h a r a c t e r is t ic s id e n t ifyin g t h e e a r ly volt a ge V A
Modeling the Early Effect
ic
Betaf
ib
Tolerances in Bias Circuits
Worst Case Analysis
V = +12 V
IC
CC
22k 
R
2
R
R
C
EQ
R
C
22k 
IB
36 k 
V
12 k 
VEQ
Q1
R
4V
18 k 
R
E
+12 V
16 k 
R
1
16 k 
CC
IE
E
Study the operation of the transistor considering tolerances (worst case anaysis) in the circuit. Assume that the
12V power supply has a 5% tolerance and the resistors have 10% tolerance. Assume also that the voltage drop
in REQ can be neglected, and beta is large.
VEQ (max, min)
IC (max, min)
VCE (max, min)
Monte Carlo Analysis
Perform Monte Carlo Analysis on previous circuit assuming the random values to
Vcc, R1, R2, Rc, Re, and beta. (Use Excel and/or Pspice).
Calculate
VEQ
REQ
IB
IC
IE
VC = VCC – IC.RC – IE.RE
Electronic Devices and Circuits – 11/5/00
Monte Carlo Analysis – Using Pspice
Probe Output
Ic(Q), Ib(Q), Vce