Small Signal Model
PNP Transistor
Section 4.1-4.4,4.6
Schedule
9
2/11
Tuesday
Physics of a BJT
L
2/11
Tuesday
Measure Beta of a transistor
10
2/13
Thursday PNP
4.5
11
2/18
Tuesday
4.5
L
2/18
Tuesday
12
2/20
BJT in saturation mode
4.1-4.3
BJT in saturation/BJT implementation of an
NAND gate
Thursday Small Signal model [homework: small eq.
4.4,4.6
circuit, (PNP)]
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 model
4. 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
RS should be 0 for a
good battery!
DC Voltage
Source in Small
Signal Analysis
Current Source
RS 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 points
from 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
Small signal model
of Q1
If a signal changes the base-emitter
voltage by a small amount, how
much change is produced in the
collector current?
But there is something else….
Small signal model
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
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 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