Small Signal Model PNP Transistor
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Transcript of Small Signal Model PNP Transistor
Small Signal ModelPNP Transistor
Section 4.1-4.4,4.6
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
Overview
Review
Small Signal Model
Section 4.4
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
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.
Small Signal Analysis (For a Circuit You have not Seen Before)
1. Analyze the Circuit by Inspection
Voltage Source
DC Voltage Source in Small Signal Analysis
RS should be 0 for a good battery!
Current SourceRS should be infinity for a good battery!
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.
Question
Replace Q1 and Q2 by their small equivalent circuit.
Answer
Question
Answer
Derivation of the Small Signal Model
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?
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
But there is something else….
A change in VBE creates a change in base current!
Small signal model
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?
A Simple Amplifier
Determine the output signal level if the microphone produces a 1 mV signal.
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.
Example
Small Signal Model
Summary
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.
Early Effect
James M. Early
Modeling of Early Effect
What Doesn’t Change with Early Effect?
Modification of the Small Signal Model
Slides to Cover During the Lab
A Simple Cadence Example
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.
DC Bias of the Amplifier
Sweep the Base Voltage to Get the IC=26 uA
Display the Transconductance
Display Transconductance
Verify Transconductance (1)
Verify Transconductance (2)
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
Introduce a Small Signal
Calculate Peak to Peak Voltage
Peak to Peak Voltage=67.78 mV
67.78 mV/2=33.9