Electronic Circuits – EE359A
Bruce McNair
B206
bmcnair@stevens.edu
201-216-5549
Lecture 5
0
Bipolar Junction Transistors
(BJT)
Small Signal Analysis
Graphical Analysis / Biasing
Amplifier, Switch and Logic Appl.
1
NPN Transistor Amplifier
Example
• NPN
•Quiescent point
VC  VCC  2.3 x3  3.1V
VBB  VBE 3  0.7
IB 

 0.023mA
RBB
100
 3  0.7 
I C   I B  100 
  2.3mA
 100 
VCC
10V
3.0kohm
R1
V1
100kohm
R2
Q1
1DEAL_BJT_NPN
3V
2
BJT as Amplifier
BJT as a voltage-controlled
current source ( a
transconductance amplifier)
BJT as a currentcontrolled current source
(a current amplifier).
3
PNP Transistor Amplifier
Example
• Voltage Gain
• Signal Waveforms
• Capacitor couples
input signal vi to
emitter
• DC bias with V+ & V-
4
DC Analysis
• Find operating pt. Q
IE 
10  VE 10  0.7

 0.93mA
RE
10
• Let =100 and =0.99
I C  0.99 I E  0.92mA
VC  10  I C RC  5.4V
• The transistor is active
• Max. signal swing
depends on bias
voltage
5
Small Signal Analysis
• Replace BJT with T
equivalent ckt.
• Why? Base is
gnded. More
convenient than
hybrid 
= 0.99
re=25mV/0.93mA= 27 
6
Small Signal Equiv Ckt
• VO/Vi
=0.99x5k/27=183
• Allowable signal
magnitude?
• But veb = vi For small
signal limit to 10mV.
Then, vc=1.833V
7
Graphical Analysis
• Find DC bias point
• Set vi=0 and draw
load line to
determine dc bias
point IB (similar to
diode ckts)
8
Graphical Construction
• Load line has a
slope of –1/RB
• iB vs vBE from
forward biased
diode eqns
Graphical construction for the
determination of the dc base
current
9
Collector Current
Graphical construction for determining the dc collector
current IC and the collector-to-emmiter voltage
10
Small Signal Graphical
Analysis
• Signal is superimposed
on DC voltage VBB
• Corresponding to each
instantaneous value of
VBB + vi(t) draw a load
line
• Intersection of the iB vBE curve with the load
lines
• Amplitude vi(t) small
so ib linear
11
Collector Currrent
• Corresponding to
each instantaneous
value of VCE + vce(t)
operating point will
be on the load line
• Amplitude vi(t) small
so ic linear
12
Bias Point vs Signal Swing
•
Bias-point location limits
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, loadline B results in an
operating point too close to
the saturation region, thus
limiting the negative swing
of vCE.
13
Basic Single Stage Amplifiers
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.
14
Common Base Amp
The common-base amplifier. (a) Circuit. (b) Equivalent
circuit obtained by replacing the BJT with its T model.
15
Common Collector
The common-collector or emitter-follower amplifier. (a)
Circuit. (b) Equivalent circuit obtained by replacing the BJT
with its T model.
16
(c) The circuit redrawn to show that ro is in parallel with RL.
(d) Circuit for determining Ro.
17
BJT Digital Logic
Basic BJT digital logic
inverter.
18
•voltage transfer
characteristic of the
inverter circuit
•RB = 10 k, RC = 1 k,
 = 50, and VCC = 5V.
19
Saturation Region
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.
20
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.
21
Common Base Characteristic
The hybrid- model,
including the resistance
r, which models the
effect of vc on ib.
22
Common-emitter characteristics.
23
Common Emitter in Saturation
Region
24
Single Stage BJT Amplifier
•Quiescent point, NPN
•VCC=VEE=10V, I=1mA,
RB=100 kW RC=8 kW, b=100
25
26
Characteristic Parameters of
Amplifiers
27
Common Emitter Amplifier
CE grounds emitter,
also called “bypass”
capacitor
28
Small Signal Analysis
• R =V R
• Replace BJT with hybrid equivalent ckt.
• Rout=RC||rO
in
i/
i
29
Emitter Resistance
30
Common Base
31
Small Signal
32
Simplified Small Signal
• Redraw with
r_o in
parallel with
R_L.
33
Simplified (continued)
34
Thevenin Equivalent
35
Thevenin Equivalent
This circuit can be used
to find gain for R_L
36
High Frequency Model
37
Transfer Function (s domain)
38
39
40
41
42
Pspice of CE with Emitter
Resistance
43
Frequency Response
44