small signal model pnp transistor
DESCRIPTION
Small Signal Model PNP Transistor. Section 4.4. BJT in the active region. Electrons cross the forward biased BE junction and are swept reverse biased BC junction. Large Signal Model of a BJT. Called “large” signal model because this model is applicable even if V BE - PowerPoint PPT PresentationTRANSCRIPT
Small Signal ModelPNP Transistor
Section 4.4
BJT in the active region
Electrons cross the forward biased BE junction and are swept reverse biased BC junction.
Large Signal Model of a BJT
Called “large” signal modelbecause this model is applicable even if VBE
changes from 300 mV to 800 mV
Experiments
Transconductance
If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current?
Illustration of Transconductance
But there is something else….
A change in VBE creates a change in base current!
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
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
Summary
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
PNP Transistors
Structures of BJT Transistors
(NPN transistor) (PNP transistor)
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!
Large-Signal Model of BJT Transistors
(NPN) (PNP)
Equations
Small Signal Model of BJT Transistors
(NPN) (PNP)