BJT CE
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Transcript of BJT CE
ELECTRONICS LABORATORY I
1
by Fatemeh Khorramshahi
Outline
BJTCommon Emitter
○ Determining quiescent conditions○ Calculating small signal performance
Voltage GainInput ImpedanceOutput ImpedanceCut-off frequency
Common CollectorCommon Base
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npn transistor
simplified structure
Note: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping.
B
C
E
Schematic Symbol
3
• Curve TracerProvides a graph of the characteristic curves.
• Digital multimeters (DMM)Some DMMs measure βDC or hfe.
• Ohmmeter
Transistor Testing
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Small signal models of bjt
b
e
hoe
hie
hrevce hfeib vbe
ib ic
vce
c
e
+ _
+ +
_ _
h-parameter model
Hybrid-π model
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Common Emitter (CE)
DC analysis AC analysis
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Collector Characteristics Base Characteristics
Common-Emitter Characteristics
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Features of Common Emitter
High voltage gain High current gain Medium input impedance due to high
current gain High output impedance. For HF capacitive
loading will need to be resonated reducing bandwidth.
Bad HF & bandwidth as falling beta with frequency reduces gain.
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Input Impedance
IN
ININ i
vr
iIN iB
iRB
Input impedance, rIN, is the ratio of the small signal input voltage and the small signal input current
BRBIN iii
B
INRB R
vi
mINC
B
gvii
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Input Impedance
iIN iB
iRB
mIN
B
INBRBIN
gv
R
viii
mB
mBIN
ININ g
RgRi
vr
||//1
1
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Output Impedance
One way to measure rOUT is: Short the input to 0 V Output now looks like just rOUT
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Output Impedance (cont)
00 CIN iv
Applying Kirchoff’s current law:
RCOUTOUTRCC iiiii 0
RC
OUTC
RC
C
i
vR
i
v
By Ohm’s law:
CCRC
OUT
OUT
OUTOUT RR
iv
iv
r
VCC
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Capacitors
Capacitor COUT is needed to remove the d.c. component of the collector voltage
Capacitor CIN is needed to allow the base voltage to be offset from 0V
In both cases this is known as coupling
Both capacitors are chosen to look like short circuits at operating frequencies
Their reactance will, however, become significant at low frequencies
VCC
0 V
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Frequency response Midband:
The frequency range of interest for amplifiers Large capacitors can be treated as short circuit and small capacitors can be
treated as open circuit Gain is constant and can be obtained by small-signal analysis
Low-frequency band: Gain drops at frequencies lower than fL
Large capacitors can no longer be treated as short circuit The gain roll-off is mainly due to coupling and by-pass capacitors For calculation we use dominant pole approximation
○ If there is a dominant pole, the cutoff frequency is determined mainly by this pole.
High-frequency band: Gain drops at frequencies higher than fH
Small capacitors can no longer treated as open circuit The gain roll-off is mainly due to parasitic capacitances of the MOSFETs and
BJTs
Considering the effect of each capacitor separately
Considering only Cc1:
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Considering the effect of each capacitor separately
Considering only CE:
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By using Norton equivalent circuit
Considering the effect of each capacitor separately
Considering only Cc2:
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Determining the lower 3-dB frequency
Coupling and by-pass capacitors result in a high-pass frequency response with three poles
The lower 3-dB frequency is simply the highest-frequency pole if the poles are sufficiently separated
The highest-frequency pole is typically ωp2 due to the small resistance of RE
An approximation of the lower 3-dB frequency is given by
Selecting values for the coupling and by-pass capacitors These capacitors are typically required for discrete amplifier designs CE is first determined to satisfy needed fL
CC1 and CC2 are chosen such that poles are 5 to 10 times lower than fL 18r
fC
CCC
102
12,1
Common Base (CB)
Current gain of approximately 1 (alpha) Low input impedance
(due to low current gain) High output impedance High voltage gain
(if input impedance matched) Good HF & bandwidth as falling beta
with frequency matters less.
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Common Collector (CC)
Voltage gain of almost exactly 1 High current gain High input impedance
(due to high current gain) Low output impedance (Good for
unknown loads) Good HF & bandwidth as falling beta
with frequency matters less.
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