Small Signal Model PNP Transistor
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Transcript of Small Signal Model PNP Transistor
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Small Signal ModelPNP Transistor
Section 4.1-4.4,4.6
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Schedule
9 2/11 Tuesday Physics of a BJT 4.1-4.3
L 2/11 Tuesday Measure Beta of a transistor
10 2/13 Thursday
PNP 4.5
11 2/18 Tuesday BJT in saturation mode 4.5
L 2/18 Tuesday BJT in saturation/BJT implementation of an NAND gate
12 2/20 Thursday
Small Signal model [homework: small eq. circuit, (PNP)]
4.4,4.6
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Overview
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Review
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Small Signal Model
Section 4.4
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Schematic of an Audio Amplifier
Microphone produces a small signal.
How does the amplifier circuit respond to a small change in the input signal?How is the analysis performed? Small signal model
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Small Signal Analysis (For a Circuit You have not Seen Before)
1. Replace each ideal DC voltage source with a small signal ground.
2. Replace each ideal DC current source with an open circuit.
3. Replace each transistor by its small signal model4. Analyze the small signal equivalent circuit.
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Small Signal Analysis (For a Circuit You have not Seen Before)
1. Analyze the Circuit by Inspection
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Voltage Source
DC Voltage Source in Small Signal Analysis
RS should be 0 for a good battery!
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Current SourceRS should be infinity for a good battery!
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Small Signal Model
(NPN) (PNP)
Statements that are always true for both NPN and PNP.1.rπ is between B and E. 2.the direction of the dependent current source always pointsfrom the collector to emitter.3. ro is always between B and C.
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Question
Replace Q1 and Q2 by their small equivalent circuit.
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Answer
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Question
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Answer
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Derivation of the Small Signal Model
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Change in the Collector Current Due to a Small Change in Base-Emitter
Voltage
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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Derivation of Transconductance
If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current?
Small signal modelof Q1
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But there is something else….
A change in VBE creates a change in base current!
Small signal model
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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
Small Signal Model
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Summary
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Output Resistance Due to 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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Slides to Cover During the Lab
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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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