Section 5.5 Biasing of BJT Amplifier circuit

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Section 5.5Biasing of BJT Amplifier circuit
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Biasing of BJT Amplifier circuit

• Biasing to establish constant DC Collector
  current Ic should be
• Calculatable
• Predictable
• Insensitive to temp. variations
• Insensitive to large variations in ß
• To allow max. output signal swing with no
  distortion

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Figure 5.43 Two obvious schemes for biasing the
BJT (a) by fixing VBE (b) by fixing IB.
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Figure 5.44 Classical biasing for BJTs using a
single power supply

• Typical Biasing
• Single power supply
• Voltage Divider Network
• RE in Emitter Circuit

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Typical Biasing
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Figure 5.44 Classical biasing for BJTs using a
single power supply
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Classical Discrete-circuit Bias arrangement
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Base Emitter Loop
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Classical Discrete-circuit Bias arrangement

• For stable Ic, IE must be stable as IC aIE
• To make IE insensitive to VBE (temp.) ß
  variations

VBB gtgt VBE REgtgt RB/(ß1)
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Classical Discrete-circuit Bias arrangementVBB
gtgt VBE
But for higher gain VRC should be more Larger
signal Swing (before cutoff) Av-VRC / VT VCB
be large VCE is large for larger signed swing
(before saturation) Compromise Role of thumb
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Classical Discrete-circuit Bias arrangement
REgtgt RB/(ß1)
For Stable IE - Negative Feed Back through
RE If IE increases somehow, VRE increases, hence
VE increases correspondingly, VBB VBE VE
VBE decreases for maintaining constant
VBB Reduces collector (Emitter) current. Stable IE
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Figure 5.45 Biasing the BJT using two power
supplies.
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Two Power Supplies Version
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Two Power Supplies Version
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Two Power Supplies Version
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Figure 5.46 (a) A common-emitter transistor
amplifier biased by a feedback resistor RB.
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A common-emitter transistor amplifier biased by
a feedback resistor RB.
RB provide negative Feedback
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A common-emitter transistor amplifier biased by
a feedback resistor RB.
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A common-emitter transistor amplifier biased by
a feedback resistor RB.
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A BJT biased using a constant-current source I.
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Biasing using a constant current source

• Current in Emitter means
• Constant IC IC a IE
• Independent of RB ß value thus RB can be made
  large to
• Increase Input resistance
• Large signal swing at collector

• Q1 acts as Diode CBJ is short circuits

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Biasing using a constant current source
Q1 acts as Diode CBJ is short circuits

• VCC-IREFR-VBEVEE0
• I IREF(VCC-VBEVEE)/R
• Since Q1 Q2 have VBE is same
• I constant till Q2 in Active Mode (Region) can
  be guaranteed by
• Voltage at collector V gt (-VEEVBE)
• Current Mirror

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Biasing using a constant current source

• IE is independent of ß RB
• RB can be made large thus increasing
• input resistance
• Simple Design
• Q1 Q2 are matched pair
• Q1 is Diode collector- Base connected
• ß high IB can be neglected
• a 1 IC IE

I IREF(VCC-VBEVEE)/R