Transcript Small Signal Model Section 4.4 BJT in the active region Electrons cross the forward biased BE junction and are swept reverse biased BC.

Small Signal Model
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
Problem
IS,Q1=5 x 10-17 A
VBE=800 mV
β=100
What is the voltage at VX?
Problem
What value of RC will take Q1 out of the active region?
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 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