Two Stage Amplifier Design

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Two Stage Amplifier Design

• ENGI 242
• ELEC 222

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HYBRID MODEL PI
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HYBRID MODEL PI PARAMETERS

• Parasitic Resistances
• rb rbb 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? rbe dynamic emitter resistance
  magnitude varies to give correct low frequency
  value of Vbe for Ib
• r? rbc collector base resistance accounts
  for change in recombination component of Ib due
  to change in Vc which causes a change in base
  storage
• c? Cbe dynamic emitter capacitance due to
  Vbe stored charge
• c? Cbc collector base transistion
  capacitance (CTC) plus Diffusion capacitance (Cd)
  due to base width modulation
• gmV? gmVbe Ic equivalent current generator

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Hybrid Pi Relationships
? gm r?
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Hybrid Pi Relationships
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Design of a Two Stage Amplifier
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Two Stage Amplifier Design Specifications

• Design a two stage common emitter amplifier with
  partial emitter bypass for the following
  specifications
• VCC 20V VE .1VCC
• RE1A .25RE1 VC1 .6VCC IC1 2mA
• RE2A .4RE2 VC2 .55VCC IC2 2.5mA
• R2 .1?RE1 R4 .1?RE2 RL 10k?
• fCL1 16Hz fCL2 13Hz fCL3 12Hz
• fCL4 67Hz fCL5 8Hz
• For both stages
• ? 140 ?CB 150ps VA 100V
• C? ? 8pF fT 150MHz rb 19?

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Hybrid Pi Model
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Low Critical Frequencies

• There is one low critical frequency for each
  coupling and bypass capacitor
• We start by determining the (Thevenin) impedance
  seen by each capacitor
• Then we construct a RC high pass filter (output
  across Z)
• We may then calculate the critical frequency by
  letting
• XC Z and solving for either fCL or C
• and fCL fCL1 fCL2 fCL3 fCL4 fCL5

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Hybrid Pi Model Input First Stage
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Hybrid Pi Model Output First Stage
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Hybrid Pi Model Input Second Stage
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Hybrid Pi Model Output Second Stage
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Hybrid Pi Model Emitter Bypass First Stage
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Hybrid Pi Model Emitter Bypass Second Stage
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fCL1
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fCL2
Determine the Thevenin Impedance seen by C2
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fCL3
Determine the Thevenin Impedance seen by C3
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fCL4
Determine the Thevenin Impedance seen by CE1
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fCL5
Determine the Thevenin Impedance seen by CE2
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Schematic of Design
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Simulation Profile
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Probe Plot Y Axis Settings
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Probe Plot X Axis X Grid Settings
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Frequency Response
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Frequency Response