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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
EE105 Fall 2007 Lecture 9, Slide 15 Prof. Liu, UC Berkeley
Input Impedance Example• To find RX, we have to first find Req, treat it as the base
resistance of Q2 and divide it by (+1).
1
1
1
11
12 B
mmx
R
ggR
1
1
1
B
meq
R
gR
