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EE105 Fall 2007 Lecture 9, Slide 1 Prof. Liu, UC Berkeley
Lecture 9
OUTLINE• BJT Amplifiers (cont’d)– Common-base topology– CB core– CB stage with source resistance– Impact of base resistance
Reading: Chapter 5.3.2
ANNOUNCEMENTS• Friday discussion section (103) has been moved back to 5 Evans.• HW#5 is now posted.• List of frequently misunderstood/forgotten points has been updated.
EE105 Fall 2007 Lecture 9, Slide 2 Prof. Liu, UC Berkeley
Common Base (CB) Amplifier• The base terminal is biased at a fixed voltage; the input
signal is applied to the emitter, and the output signal sensed at the collector.
EE105 Fall 2007 Lecture 9, Slide 3 Prof. Liu, UC Berkeley
Small-Signal Analysis of CB Core• The voltage gain of a CB stage is gmRC, which is
identical to that of a CE stage in magnitude and opposite in phase.
Cmv RgA
EE105 Fall 2007 Lecture 9, Slide 4 Prof. Liu, UC Berkeley
Tradeoff between Gain and Headroom • To ensure that the BJT operates in active mode, the
voltage drop across RC cannot exceed VCC-VBE.
T
BECCC
T
Cv V
VVR
V
IA
EE105 Fall 2007 Lecture 9, Slide 5 Prof. Liu, UC Berkeley
Simple CB Stage Example
2.172230130
1 Cmv RgA
VCC = 1.8VIC = 0.2mAIS = 5x10-17 A = 100
k7.67 ,k3.22
2010 Choose
if 354.1
21
211
121
2
RR
RR
VAII
IIVRR
RVV
CCB
BCCb
EE105 Fall 2007 Lecture 9, Slide 6 Prof. Liu, UC Berkeley
Input Impedance of a CB Stage• The input impedance of a CB stage is much smaller
than that of a CE stage.
Am
in Vg
R if 1
EE105 Fall 2007 Lecture 9, Slide 7 Prof. Liu, UC Berkeley
CB Stage with Source Resistance• With the inclusion of a source resistance, the input
signal is attenuated before it reaches the emitter of the amplifier; therefore, the voltage gain is lowered. – This effect is similar to CE stage emitter degeneration.
S
m
Cv
Rg
RA
1
EE105 Fall 2007 Lecture 9, Slide 8 Prof. Liu, UC Berkeley
Practical Example of a CB Stage• An antenna usually has low output impedance;
therefore, a correspondingly low input impedance is required for the following stage.
EE105 Fall 2007 Lecture 9, Slide 9 Prof. Liu, UC Berkeley
Output Impedance of a CB Stage• The output impedance of a CB stage is equal to RC in
parallel with the impedance looking into the collector.
12
1
||
||)||(1
outCout
EOEmout
RRR
rRrrRgR
EE105 Fall 2007 Lecture 9, Slide 10 Prof. Liu, UC Berkeley
Output Impedance: CE vs. CB Stages• The output impedances of emitter-degenerated CE and
CB stages are the same. This is because the circuits for small-signal analysis are the same when the input port is grounded.
EE105 Fall 2007 Lecture 9, Slide 11 Prof. Liu, UC Berkeley
Av of CB Stage with Base Resistance (VA = ∞)
• With base resistance, the voltage gain degrades.
BC
outP
BCm
outBP
Cm
outCmout
RrR
vv
RrRgr
vRr
r
vv
Rg
vvRvgv
E
inBC
out
Cm
outm
E
inPm R
vRrR
v
Rg
vg
rR
vvvg
r
v
1 :P nodeat KCL
1
11
BE
m
C
BE
C
in
out
RR
g
R
RRr
R
v
v
EE105 Fall 2007 Lecture 9, Slide 12 Prof. Liu, UC Berkeley
Voltage Gain: CE vs. CB Stages• The magnitude of the voltage gain of a CB stage with
source and base resistances is the same as that of a CE stage with base resistance and emitter degeneration.
EE105 Fall 2007 Lecture 9, Slide 13 Prof. Liu, UC Berkeley
Rin of CB Stage with Base Resistance (VA = ∞)
• The input impedance of a CB stage with base resistance is equal to 1/gm plus RB divided by (+1). This is in contrast to a degenerated CE stage, in which the resistance in series with the emitter is multiplied by (+1) when seen from the base.
1
1
1
1
B
m
B
x
xin
xxB
m
xm
R
g
Rr
i
vR
ivRr
rg
r
ivgr
vKCL
xB
vRr
rv
EE105 Fall 2007 Lecture 9, Slide 14 Prof. Liu, UC Berkeley
Input Impedance Seen at Emitter vs. Base
Common Base Stage Common Emitter Stage