Transcript BJT Small Signal Models - Brookdale Community College

Transistor Modeling
ENGI 242
ELEC 222
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Hybrid Equivalent Circuit for BJT
V1
I2
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=
h11 h12
h21 h22
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I1
V2
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h-parameter Model for Common Emitter
Parameters from the spec sheet (x = lead based on circuit configuration):
h11 = hix
h12 = hrx
h21 = hfx
h22 = hox
hrx and hfx are dimensionless ratios
hix is an impedance <>
hox is an admittance <S>
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Short Circuit Input Impedance
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Open Circuit Reverse Transfer Ratio
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Short Circuit Forward Transfer Ratio
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Open Circuit Output Admittance
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Transistor Modeling
Hybrid Model Pi
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ELEC 222
HYBRID MODEL PI
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HYBRID MODEL PI PARAMETERS
• Parasitic Resistances
• rb = rb’b = ohmic resistance – voltage drop in base region
caused by transverse flow of majority carriers, 50 ≤ rb ≤
500
• rc = rce = collector emitter resistance – change in Ic due to
change in Vc, 20 ≤ rc ≤ 500
• rex = emitter lead resistance
– important if IC very large, 1 ≤ rex ≤ 3
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HYBRID MODEL PI PARAMETERS
• Parasitic Capacitances
• Cje0 = Base-emitter junction (depletion layer) capacitance,
0.1pF ≤ Cje0 ≤ 1pF
• C0 = Base-collector junction capacitance, 0.2pF ≤ C0 ≤
1pF
• Ccs0 = Collector-substrate capacitance, 1pF ≤ Ccs0 ≤ 3pF
• Cje = 2Cje0 (typical)
• 0 =.55V (typical)
• F = Forward transit time of minority carriers, average of
lifetime of holes and electrons, 0ps ≤ F ≤ 530ps
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HYBRID MODEL PI PARAMETERS
• r = rb’e = dynamic emitter resistance – magnitude varies to
give correct low frequency value of Vb’e for Ib
• r = rb’c = collector base resistance – accounts for change in
recombination component of Ib due to change in Vc which
causes a change in base storage
• c = Cb’e = dynamic emitter capacitance – due to Vb’e
stored charge
• c = Cb’c = collector base transistion capacitance (CTC)
plus Diffusion capacitance (Cd) due to base width
modulation
• gmV = gmVb’e = Ic – equivalent current generator
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Hybrid Pi Relationships
gm =
VT =
gm =
r =
IC
VT
kT
= 26mV @ 300K
q
IC
26mV
(26mV) ()
26mV
=
IC
IB
 = gm r 
ic =
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β vπ
= gm vπ
rπ
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Hybrid Pi Relationships
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HYBRID MODEL PI MID BAND
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HYBRID MODEL PI HIGH FREQ.
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Common Emitter Amplifier
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Common Emitter Amplifier – DC Bias Model
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Common Emitter Amplifier - Complete Hybrid PI
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Mid Band Hybrid PI Common Emitter
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Equivalent Circuit to find ZO
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High Frequency Hybrid PI CE Amp
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Common Emitter Amplifier
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CE Amplifer Example output
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Common Emitter Amplifier
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CE Amplifer Example output
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Emitter Follower
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Emitter Follower
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Emitter Follower
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Emitter Follower
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Emitter Follower
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Common Base
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Common Base
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