small signal model pnp transistor section 4.4. bjt in the active region electrons cross the forward...
TRANSCRIPT
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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 modelbecause 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 CircuitVBE=800 mVβ=100IS,Q1=3 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 Doesn’t 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 that1.The DC at Vout is 0.9 V2.gm=1 mS
Gain is approximately equal to –gmRC.Bias current is IC=gmVt
R=(1.8V-0.9V)/26uA=34.6 KohmsGain is -34.6.
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DC Bias of the Amplifier
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Sweep the Base Voltage to Get the IC=26 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 Voltage=67.78 mV
67.78 mV/2=33.9
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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)