Transcript 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.

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 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
VBE=800 mV
β=100
IS,Q1=3 x 10-16 A
Small Signal Equivalent Circuit
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 that
1. The DC at Vout is 0.9 V
2. gm=1 mS
Gain is approximately equal to –gmRC.
Bias current is IC=gmVt
R=(1.8V-0.9V)/26uA=34.6 Kohms
Gain 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)