Small Signal Model PNP Transistor

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Small Signal ModelPNP Transistor

• Section 4.4

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BJT in the active region
Electrons cross the forward biased BE junction
and are swept reverse biased BC junction.
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Large Signal Model of a BJT
Called large signal model because this model is
applicable even if VBE changes from 300 mV to
800 mV
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Experiments
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Transconductance
If a signal changes the base-emitter voltage by a
small amount, how much change is produced in the
collector current?
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Illustration of Transconductance
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But there is something else.
A change in VBE creates a change in base current!
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Example 4.10
Signal Generated By a microphone
Small Signal Equivalent Circuit
VBE800 mV ß100 IS,Q13 x 10-16 A
Question If a microphone generates a 1 mV
signal, how much change is observed in the
collector and base current?
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A Simple Amplifier
Determine the output signal level if the
microphone produces a 1 mV signal.
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AC Ground

• The voltage produced by a voltage source is
  constant.
• The small signal model is concerned only with
  changes in quantities.
• Therefore, a DC voltage source must be replaced
  with a ground in small signal analysis.

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Example
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Early Effect
A larger reverse bias voltage leads to a larger
BC depletion region. The effective base width
(WB) is reduced. The slope of the electron
profile increases. IC increases as VCE is
increased.
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Early Effect
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James M. Early
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Modeling of Early Effect
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What Doesnt Change with Early Effect?
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Modification of the Small Signal Model
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Summary
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A Simple Cadence Example
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Assumption

• Assume that
• The DC at Vout is 0.9 V
• gm1 mS

Gain is approximately equal to gmRC. Bias
current is ICgmVt R(1.8V-0.9V)/26uA34.6
Kohms Gain is -34.6.
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DC Bias of the Amplifier
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Sweep the Base Voltage to Get the IC26 uA
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Display the Transconductance
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Display Transconductance
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Verify Transconductance (1)
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Verify Transconductance (2)
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Transconductance
VB IC
562.5 mV 25.64 uA
563.5 mV 26.64 uA

?VBE 1 mV
?IC 1 uA
gm?VBE/?IC 1 mS
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Introduce a Small Signal
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Calculate Peak to Peak Voltage
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Peak to Peak Voltage67.78 mV
67.78 mV/233.9
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PNP Transistors
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Structures of BJT Transistors
(NPN transistor)
(PNP transistor)
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Voltage and Current Polarities of NPN and PNP
transistors
A fat voltage between collector and emitter
voltage places a transistor in the active region!
A skinny voltage between collector and emitter
voltage places a transistor in the active region!
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Large-Signal Model of BJT Transistors
(NPN)
(PNP)
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Equations
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Small Signal Model of BJT Transistors
(NPN)
(PNP)