ESE 372 / Spring 2013 / Lecture 15
Last time: BJT CE and CB amplifiers biased by current source
Assume FA regime, then
IB 
VB
VC
VE
IE
, I QC  α  I E , VBE  0.7V
β 1
Calculate VCE and confirm it is > 0.2-0.3V,
then BJT can be replaced with model for
small signal analysis
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ESE 372 / Spring 2013 / Lecture 15
Common Emitter
Biased by current source and without RE
Output from
Collector
Input to
Base
i
inverting
ti
Vin
Must have bypass
cap to get AC gain.
The circuit can
have both voltage
and current gains:
Thevenin form, can be replaced with
Norton form with current source -gmVin.
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ESE 372 / Spring 2013 / Lecture 15
Common Base
Biased by current source
Output from
Collector
Input to
Emitter
Start with bias DC analysis – make sure BJT is in FA,
then calculate small signal parameters for AC analysis.
*ignore rO for simplicity, then:
noninverting
Vin
No need in
b
bypass
cap
Small !
Short circuit current gain
The circuit is current
buffer: delivers current
from source to load
* when BJT output impedance rO
can not be neglected – the circuit
is said to perform an impedance
transformation.
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ESE 372 / Spring 2013 / Lecture 15
Common Collector
Biased by current source
Input to
Base
Output from
Emitter
Start with bias DC analysis – make sure BJT is in FA,
then calculate small signal parameters for AC analysis.
No need in
bypass cap
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ESE 372 / Spring 2013 / Lecture 15
Common Collector amplifier
Again.
g
No need for CE !
Also
RB and CC1 can be eliminated
Bias current IE will
determine gm, rπ and rO
Redraw equivalent circuit in more
convenient form
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ESE 372 / Spring 2013 / Lecture 15
Common Collector amplifier
+
When
i.e. no voltage gain !
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ESE 372 / Spring 2013 / Lecture 15
Common Collector amplifier
Input impedance
Impedance transformation
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ESE 372 / Spring 2013 / Lecture 15
Common Collector amplifier
Output impedance
Impedance
transformation
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ESE 372 / Spring 2013 / Lecture 15
Common Collector Amplifier = Emitter follower
Input to
Base
Output from
Emitter
No need in
bypass cap
Short circuit current gain is
almost the same as in case
of CE amp, namely β+1.
Start with bias DC analysis – make sure BJT is in FA,
then calculate small signal parameters for AC analysis.
Often RB >> RS and rO >> RL, then
Impedances
transformed
The circuit is voltage
buffer: delivers voltage
g
from source to load
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ESE 372 / Spring 2013 / Lecture 15
Frequency response of Common Emitter amplifier
Low frequencies, i.e. BJT itself is fast enough
1. DC bias – make sure BJT is in FA - AC analysis.
Before – assumed coupling caps are big
enough to act as a short circuit for any
f
frequency
off AC signal.
i
l
Now – assume they have finite values.
1. Assume C1 is finite while C2 and CE are still infinite.
Depends on frequency.
Frequency independent.
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ESE 372 / Spring 2013 / Lecture 15
Frequency response of Common Emitter amplifier
Role of the input coupling cap C1
Depends on frequency.
Voltage gain found before
GV0 –
net voltage
gain found
before for
infinite caps.
p
Input voltage divider
found before
before.
High
Pass
Filter
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ESE 372 / Spring 2013 / Lecture 15
Frequency response of Common Emitter amplifier
Role of the output coupling cap C2
2. Assume C2 is finite while C1 and CE are still infinite.
Again High Pass Filter but with
3dB frequency defined by C2
GV0 – net voltage gain found
before for infinite caps.
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ESE 372 / Spring 2013 / Lecture 15
Frequency response of Common Emitter amplifier
Role of the bypass cap CE
3. Assume CE is finite while C1 and C2 are still infinite.
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ESE 372 / Spring 2013 / Lecture 15
Low Frequency response of Common Emitter amplifier
We have identified three HPF.
Ti f  
f L1 
f L2
j  f f Li
1  j  f f Li
1
2  π  C1  R in  R S 
1

2  π  C 2  R out  R L 
f L3 
1
C
2  π  E  rπ  R S || R B 
β 1
C1  C 2  C E  1μμ
R in  R S ~ kOhm  f L1  100Hz
R out  R L ~ 10kOhm  f L2  20Hz
rπ  R S || R B ~ kOhm  f L3  kHz
L
Low
ffrequency cutoff
t ff is
i determined
d t
i d by
b CE
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ESE 372 / Spring 2013 / Lecture 15
Bandwidth of Common Emitter amplifier
High frequency 3dB determines
amplifier bandwidth.
Amplifier bandwidth is determined by BJT high frequency capabilities –
determined by internal parasitic capacitances Cπ and Cμ.
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ESE 372 / Spring 2013 / Lecture 15
Short circuit current gain at high frequencies
Common emitter current
gain defined earlier.
Negligible since << β0 for
not extreme frequencies.
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ESE 372 / Spring 2013 / Lecture 15
Short circuit current gain at high frequencies
Unity gain bandwidth fT.
Looks like it is supposed to
i
improve
with
ith bi
bias currentt b
because
However it does not. Why?
18