ESE 372 / Spring 2013 / Lecture 16
Last time: BJT CE low frequency response
Need FA
VBE ≈ 0.7V
VCE > 0.3V
Equivalent circuit for low frequency small signal analysis
Ti f  
j  f f Li
1  j  f f Li
f L1 
1
2  π  C1  R in  R S 
f L2 
1
2  π  C 2  R out  R L 
f L3 
1
2 π
CE
 rπ  R S || R B 
β 1
Coupling and bypass capacitors result into
high pass filters (as could be expected).
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ESE 372 / Spring 2013 / Lecture 16
Role of the bypass cap CE
3. Assume CE is finite while C1 and C2 are still infinite.
1A
ESE 372 / Spring 2013 / Lecture 16
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 16
Frequency dependence of short circuit current gain
Common emitter current
gain defined earlier.
Negligible since << β0 for
not extreme frequencies.
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ESE 372 / Spring 2013 / Lecture 16
Frequency dependence of short circuit current gain
Unity gain bandwidth fT.
Looks like it is supposed to
i
improve
with
ith bi
bias currentt b
because
4
ESE 372 / Spring 2013 / Lecture 16
Frequency dependence of short circuit current gain
Unity gain bandwidth fT.
Looks like it is supposed to
i
improve
with
ith bi
bias currentt b
because
However it does not. Why?
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ESE 372 / Spring 2013 / Lecture 16
Frequency dependence of common base current gain
3dB frequency for α is equal to fT.
Hence at fT electrons from
emitter can not reach collector.
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ESE 372 / Spring 2013 / Lecture 16
Frequency dependence of common base current gain
3dB frequency for α is equal to fT.
C BC VCB  
C BC0
1  VCB
Vbi 
BC-junction depletion
region capacitance
C BE VBE  
C BE0
1  VBE
EB-junction depletion
region capacitance
Vbi 
1
2
Base transport time – time of
flight of electrons from emitter
to collector.
Hence at fT electrons from
emitter can not reach collector.
*There
There are also several parasitic caps
associated with technology limitations
7
1
2
ESE 372 / Spring 2013 / Lecture 16
Base transport time and diffusion capacitance
time of flight of electrons from emitter to collector.
*Need thin base for
high speed operation
Effective velocity of
diffusion electrons
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ESE 372 / Spring 2013 / Lecture 16
Base transport time and diffusion capacitance
time of flight of electrons from emitter to collector.
*Need thin base for
high speed operation
Electron charge stored in base when current IC is flowing
Charge storage
capacitance
Pn-junction depletion region
capacitances and other parasitic caps
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ESE 372 / Spring 2013 / Lecture 16
Unity gain bandwidth
Total time delay
Minimum possible
time delay
Ultimate limit for BJT speed
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ESE 372 / Spring 2013 / Lecture 16
Bandwidth of Common Emitter amplifier
H
Amplifier bandwidth (fH) is determined by BJT internal parasitic capacitances Cπ and Cμ.
Charge storage
capacitance
Pn-junction depletion region
capacitances and other parasitic caps
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ESE 372 / Spring 2013 / Lecture 16
Unity gain bandwidth
Unity gain bandwidth fT.
junctions
junctions
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
We are interested in high frequency cutoff, i.e.
coupling and bypass capacitors can be replaced
by short circuit for frequencies >> fL.
We have got capacitive
coupling between input
and output.
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
Observe increase of iμ with transconductance gm.
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
nominator
fT 
1
 1GHz, C  0.1 pF
2  π  TF
  C  0.001  1
g m  0.04
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
nominator
fT 
1
 1GHz, C  0.1 pF
2  π  TF
  C  0.001  1
g m  0.04
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
nominator
fT 
1
 1GHz, C  0.1 pF
2  π  TF
  C  0.001  1
g m  0.04  1
denominator
for
C  C 10
and f  fT

  C  RL  1
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
nominator
fT 
1
 1GHz, C  0.1 pF
2  π  TF
  C  0.001  1
g m  0.04  1
denominator
for
C  C 10
and f  fT

  C  RL  1
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
Resistor in series
with input cap
equivalent input cap
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ESE 372 / Spring 2013 / Lecture 16
Net voltage gain bandwidth of CE BJT amplifier
“Miller” capacitor
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ESE 372 / Spring 2013 / Lecture 16
Example
* confirm FA regime first …
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ESE 372 / Spring 2013 / Lecture 16
Example – cont.
Even << fβ = 8MHz
<< fT
Reduced gain – improved BW
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ESE 372 / Spring 2013 / Lecture 16
Miller Effect
Miller transformation
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ESE 372 / Spring 2013 / Lecture 16
Miller Effect in CE amplifier
If iμ is not very high (small Cμ), then
Low pass
filter
Negligible, hence
open circuit
Total equivalent input cap
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ESE 372 / Spring 2013 / Lecture 16
CB amplifier does not suffer from Miller effect
Neglect rO and build
equivalent circuit
Observe no capacitor coupling
output and input – No Miller effect.
27
ESE 372 / Spring 2013 / Lecture 16
CB amplifier does not suffer from Miller effect
Neglect rO and build
equivalent circuit
Observe no capacitor coupling
output and input – No Miller effect.
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