PEElectrical Review Course Class 4 Transistors

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PE-Electrical Review Course - Class 4
(Transistors)
Class 4 - Transistors Objectives This review
session is designed to review material and
provide practical examples such that the student
will be able to
1) Be familiar with transistor symbols,
characteristics, terminology, and typical
values. 2) Be familiar with basic transistor
biasing circuits and be able to calculate the
Q-point. 3) Determine if the Q-point is stable
with respect to variations in transistor
specs. 4) Be familiar with different
small-signal transistor models, including the
h-parameter model and the hybrid- p model. 5)
Be familiar with different transistor amplifier
configurations, including CE, CB, and CC for
BJTs and CS, CG, and CD for FETs. 6) Be able
to calculate gains (voltage, current, and power)
and impedances (input and output) for each
amplifier configuration and to be familiar with
typical values for each configuration. 7) Be
able to calculate gains and impedances for
multi-stage amplifiers. 8) Be familiar with the
causes of frequency response limitation in
amplifiers and be able to find the bandwidth of
an amplifier. 9) Refer to the review book or to
other textbooks for specialized amplifier
circuits, such as power, Darlington, and
differential amplifiers.
Reading material 1) EE Ref. Manual, 6th Ed.,
Camara, Chapter 43 Electronic Components
Appendix 43.B Semiconductor symbols and
abbreviations 2) Handout Extra Problems for
Week 4
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PE-Electrical Review Course - Class 4
(Transistors)
Transistors Reviewing this area might be more
difficult than other areas in Electrical
Engineering due to the great familiarity required
with symbols, characteristics, typical values,
common circuits, and a huge number of formulas
which can easily be misused if not understood in
their proper context. (Note that Ch. 43 in the
review text begins with a list of definitions for
well over 100 variables and symbols.)
Background In reviewing the large amount of
material in this area, the student may want to
focus on the areas which are most useful (i.e.,
most commonly occur on the PE exam). As a
result, this review course will focus on
transistors and will not review semiconductor
physics (n-type material, p-type materials,
doping, majority and minority carriers, physical
construction, etc) or diodes and diode circuits.
If students wishes to review this material on
their own, the review text contains information
on these subjects.
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PE-Electrical Review Course - Class 4
(Transistors)
Types of transistors The following 8 types of
transistors are commonly encountered
1) Bipolar Junction Transistor (BJT) A) npn
B) pnp
2) Field-Effect Transistor (FET) A) Junction
Field-Effect Transistor (JFET) a) n-channel b) p
-channel
B) Metal-Oxide-Semiconductor FET (MOSFET)
(or Insulated-Gate FET (IGFET)) a) Depletion-mod
e MOSFET 1) n-channel 2) p-channel
b) Enhancement-mode MOSFET 1) n-channel 2) p-
channel
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PE-Electrical Review Course - Class 4
(Transistors)
JFETs
Symbol n-channel JFET D Drain G Gate S
Source (Note the circle around the transistor
is optional)

• Discuss
• Current relationship using KCL
• Voltage relationship using KVL
• Typical values
• n-channel vs. p-channel

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PE-Electrical Review Course - Class 4
(Transistors)
Output characteristics n-channel JFET (typical)
Note Two key specifications for the JFET are
IDSS and VP
JFETs have two regions of operation 1) Ohmic
(VDS lt VGS - VP) 2) Saturation (or Beyond
Pinchoff) (VDS gt VGS - VP)
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PE-Electrical Review Course - Class 4
(Transistors)

• Ohmic Region n-channel JFET (typical)
• Ohmic Region defined by VDS lt VGS - VP

In the ohmic region the JFET acts like a
voltage-controlled resistance (called a Voltage
Variable Resistor or VVR), where
Notes 1) Not the most commonly used region 2)
Discuss applications
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PE-Electrical Review Course - Class 4
(Transistors)

• Saturation Region (or Beyond Pinchoff Region)
  n-channel JFET (typical)
• Saturation Region defined by VDS gt VGS - VP

The behavior of the JFET in the saturation region
is modeled by the transfer characteristic (see
graph below) and by the transfer characteristic
equation.
Note The saturation region is the most common
region (JFET used as an amplifier)
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PE-Electrical Review Course - Class 4
(Transistors)
Depletion-mode MOSFETs
Symbol n-channel depletion-mode MOSFET
(Current and voltage designations and
relationships are the same as for a JFET)
Depletion-mode MOSFETs are very similar to JFETs
except for one key difference The
depletion-mode MOSFET can operate in two
modes A) depletion-mode - operates almost
exactly as a JFET - defined by VGS lt 0 for an
n-channel device - current flows through a
physical channel in the device B) enhancement-mod
e - defined by VGS gt 0 for an n-channel
device - the physical channel is enhanced by
VGS to allow for increased ID
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PE-Electrical Review Course - Class 4
(Transistors)
Output characteristics n-channel depletion-mode
MOSFET (typical)
As with JFETs, MOSFETs have two regions of
operation 1) Ohmic VDS lt VGS - VP 2)
Saturation (or Beyond Pinchoff) VDS gt VGS -
VP Ohmic Region As with JFETs, in the ohmic
region the MOSFET acts like a Voltage Variable
Resistor (VVR), and is described by the ohmic
region equation (below)
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PE-Electrical Review Course - Class 4
(Transistors)

• Saturation Region n-channel depletion-mode
  MOSFET
• Saturation Region defined by VDS gt VGS - VP

As with the JFET, the behavior of the
depletion-mode MOSFET in the saturation region is
modeled by the transfer characteristic (see graph
below) and by the transfer characteristic
equation.
Two key specifications for the depletion-mode
MOSFET are IDSS and VP
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PE-Electrical Review Course - Class 4
(Transistors)
Enhancement-mode MOSFETs
Symbol n-channel enhancement-mode MOSFET
Enhancement-mode MOSFETs lack the physical
channel of the depletion-mode devices, so they
can only operate in the enhancement mode. Some
minimum amount of VGS must be applied before a
significant channel can be created and
significant drain currents developed. This
minimum voltage is called VT threshold voltage
minimum VGS needed to enhance a significant
channel for IDgt0
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PE-Electrical Review Course - Class 4
(Transistors)
Output characteristics n-channel
enhancement-mode MOSFET (typical)
As with JFETs, MOSFETs have two regions of
operation 1) Ohmic VDS lt VGS - VP 2)
Saturation (or Beyond Pinchoff) VDS gt VGS -
VP Ohmic Region As with JFETs, in the ohmic
region the MOSFET acts like a Voltage Variable
Resistor (VVR), and is described by the ohmic
region equation (below). Note that this equation
is different from the equations for other FETs.
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PE-Electrical Review Course - Class 4
(Transistors)

• Saturation Region n-channel enhancement-mode
  MOSFET
• Saturation Region defined by VDS gt VGS - VT

As with other FETs, the behavior of the
enhancement-mode MOSFET in the saturation region
is modeled by the transfer characteristic (see
graph below) and by the transfer characteristic
equation. Note that this equation is different
from the equations for other FETs.
Two key specifications for the enhancement-mode
MOSFET are K and VT. (IDSS and VP do not exist
for the enhancement-mode MOSFET.)
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PE-Electrical Review Course - Class 4
(Transistors)
FET Biasing Circuits
The purpose of the biasing circuit is to insure
that the FET is operating in the saturation
region so that it can be used as an amplifier.
The biasing circuit establishes a Q-point (or
quiescent point or operating point) in the
saturation region (for amplifier use). For
example, two possible Q points are shown below.
Q1 Ohmic Region ID 2.6 mA
VDS 0.4 V VGS -0.5 V
Q2 Saturation Region ID 5.625 mA
VDS 10 V VGS -0.5 V
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PE-Electrical Review Course - Class 4
(Transistors)
FET Biasing Circuits
There are various types of biasing circuits for
FETs. Several are shown below.
A) Fixed-bias B) Fixed-bias
C) Self-bias D) Fixed-
Self-bias
Comments (discuss in class) A) Fixed-bias B)
Fixed-bias (This biasing circuit works in the
enhancement mode only.) C) Self-bias D)
Fixed- Self-bias
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PE-Electrical Review Course - Class 4
(Transistors)
Find the Q-point for the biasing circuit shown
below. The JFET has the following
specifications IDSS 4 mA VP -1.46 V
Example 1
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PE-Electrical Review Course - Class 4
(Transistors)
Find the Q-point for the biasing circuit shown
below. The JFET has the following
specifications IDSS 4 mA VP -1.46 V
Example 2
Note The addition of RSS makes the analysis
more difficult. KVL around the input loop
combined with the transfer characteristic will
yield the Q-point. An alternative is to use a
Universal JFET Curve (not covered).
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PE-Electrical Review Course - Class 4
(Transistors)
BJTs (Bipolar Junction Transistors)
Symbol npn BJT C Collector B Base E
Emitter (Note the circle around the transistor
is optional)

• Discuss
• Current relationship using KCL
• Voltage relationship using KVL
• Typical values
• npn vs. pnp

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PE-Electrical Review Course - Class 4
(Transistors)
Output characteristics npn BJT (typical)
Note The PE review text sometimes uses ?dc
instead of ?dc. They are related as follows

• Find the approximate values of bdc and adc
  from the graph.

Input characteristics npn BJT (typical)
The input characteristics look like the
characteristics of a forward-biased diode. Note
that VBE varies only slightly, so we often ignore
these characteristics and assume Common
approximation VBE Vo 0.65 - 0.7V
Note Two key specifications for the BJT are Bdc
and Vo (or assume Vo is about 0.7 V)
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PE-Electrical Review Course - Class 4
(Transistors)
When analyzing a DC BJT circuit, the BJT is
replaced by one of the DC circuit models shown
below.
DC Models for a BJT
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PE-Electrical Review Course - Class 4
(Transistors)
BJT Biasing Circuits
The purpose of the biasing circuit is to insure
that the BJT is operating in the active region so
that it can be used as an amplifier. The biasing
circuit establishes a Q-point (or quiescent point
or operating point) in the active region (for
amplifier use). For example, three possible Q
points are shown below.
Q1 Active Region IB 150 mA
IC 22.5 mA VCE 15 V bdc
IC/IB 22.5mA/150mA
150
Q2 Saturation Region IB 150 mA
IC 17 mA VCE 0.5 V rsat
0.5V/17mA 29.4 W
Q3 Cutoff Region IB 0 IC
0 VCE 10 V
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PE-Electrical Review Course - Class 4
(Transistors)
BJT Biasing Circuits
There are various types of biasing circuits for
BJTs. Several are shown below.
A) Base-bias B) Collector-bias C)
Base-bias with D) Voltage-divider bias

emitter feedback with
emitter feedback

(most popular)
Analysis Procedure A) Assume a region of
operation (generally the active region for
amplifier use) B) Replace the BJT with the
active region model C) Analyze the circuit to
determine the Q-point Note If unreasonable
values are found it typically means that the
wrong region was used. For example, if RC is too
large, the BJT may be in saturation. Analysis
with the active region model might yield a value
of VCE that is negative (which is not reasonable
for an npn BJT).
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PE-Electrical Review Course - Class 4
(Transistors)
Find the Q-point for the biasing circuit shown
below. The BJT has the following specifications
bdc 100, rsat 100 W (Vo not
specified, so assume Vo 0.7 V)
Example 3
Example 4
Repeat Example 3 if RC is changed from 1k to 2.2k.
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PE-Electrical Review Course - Class 4
(Transistors)
Voltage-Divider Biasing Circuit with Emitter
Feedback
Most popular biasing circuit. Why? Problem bdc
can vary over a wide range for BJTs (even with
the same part number) Solution Adding the
feedback resistor RE. How large should RE be?
Lets see.
Substituting the active region model into the
circuit to the left and analyzing the circuit
yields the following well known equation
ICEO has little effect and is often neglected
yielding the simpler relationship
Voltage divider biasing circuit with emitter
feedback
Replacing the input circuit by a Thevenin
equivalent circuit yields
Test for stability For a stable Q-point w.r.t.
variations in bdc choose
Why? Because then
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PE-Electrical Review Course - Class 4
(Transistors)
Find the Q-point for the biasing circuit shown
below. The BJT has the following specifications
bdc varies from 50 to 400, Vo 0.7 V, ICBO
10 nA Solution Case 1 bdc 50
Example 5
Case 2 bdc 400 Similar to Case 1 above.
Results are IC 0.659 mA, VCE 6.14
V Summary
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PE-Electrical Review Course - Class 4
(Transistors)
Small-Signal BJT Amplifiers
BJT Amplifier Models Once the DC Q-point has
been established in the active region, the BJT
can be used as an amplifier to amplifier small
(AC) signals. New small-signal circuit models
are needed to model the BJT. There are several
types of models 1) mid-frequency models - used
to find voltage, current, and power gain and Zin
and Zout. 2) low- and high-frequency models -
used to find frequency response
information Mid-frequency small-signal
models There are several possible models.
Textbooks vary in which model they prefer. The
models are, however, closely related. Some of
the most common are 1) h-parameter model 2)
hybrid-p model 3) re (or re) model
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PE-Electrical Review Course - Class 4
(Transistors)
Mid-frequency small-signal models Notes
Note The hybrid-p model will be used in this
presentation.
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PE-Electrical Review Course - Class 4
(Transistors)
Determining hybrid-p parameter values hybrid-p
values may be provided (for example, from a
specification sheet), but they vary with the
Q-point and are often calculated using Q-point
values.
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PE-Electrical Review Course - Class 4
(Transistors)
BJT Mid-frequency Analysis using the hybrid-p
model
A common emitter (CE) amplifier is shown to the
right.

• The mid-frequency circuit is drawn as follows
• the coupling capacitors (Ci and Co) and the
• bypass capacitor (CE) are short circuits
• short the DC supply voltage (superposition)
• replace the BJT with the hybrid-p model
• The resulting mid-frequency circuit is shown
  below.

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PE-Electrical Review Course - Class 4
(Transistors)
Procedure Analysis of a BJT amplifier at
mid-frequency
1) Find the DC Q-point. This will insure that
the BJT is operating in the active region and
these values are needed for the next
step. 2) Find the hybrid-p values. If the values
are not given, use the relationships on the
previous page. 3) Calculate the required values
(typically Avi, Avs, AI, AP, Zi, and Zo. Use the
formulas for the appropriate amplifier
configuration (CE, CB, CC, partially-bypassed CE,
etc).
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PE-Electrical Review Course - Class 4
(Transistors)
Example 6
Find the mid-frequency values for Avi, Avs, AI,
AP, Zi, and Zo for the amplifier shown below.
Assume that Ci, Co, and CE are large. Note that
this is the same biasing circuit used in Ex. 5,
so IC 0.634 mA. The BJT has the following
specifications bdc 50, Vo 0.7 V, ICBO
10 nA, bo 40, n 1, VA 30 V
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PE-Electrical Review Course - Class 4
(Transistors)
BJT Amplifier Configurations and Relationships
Using the hybrid-p model.
Note The biasing circuit is the same for each
amplifier.
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PE-Electrical Review Course - Class 4
(Transistors)
Small-Signal FET Amplifiers
FET Amplifier Models FET amplifier models are
very similar to BJT amplifiers with only a few
differences 1) The hybrid-p model is used
almost exclusively. 2) Many formulas for
calculating gains and impedances are similar to
those for BJTs. However, FETs have very high
input resistances so rp is infinite. 3) FET
amplifiers in general tend to have lower voltage
gains and higher input impedances. 4) The
hybrid-? model parameters (particularly gm) are
calculated differently.
Mid-frequency FET small-signal model
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PE-Electrical Review Course - Class 4
(Transistors)
FET Mid-frequency Analysis
A common source (CS) amplifier is shown to the
right.

• The mid-frequency circuit is drawn as follows
• the coupling capacitors (Ci and Co) and the
• bypass capacitor (CSS) are short circuits
• short the DC supply voltage (superposition)
• replace the FET with the hybrid-p model
• The resulting mid-frequency circuit is shown
  below.

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PE-Electrical Review Course - Class 4
(Transistors)
Procedure Analysis of an FET amplifier at
mid-frequency
1) Find the DC Q-point. This will insure that
the FET is operating in the saturation region and
these values are needed for the next
step. 2) Find gm. If gm is not specified,
calculate it using the DC values of VGS as
follows 3) Calculate the required values
(typically Avi, Avs, AI, AP, Zi, and Zo. Use the
formulas for the appropriate amplifier
configuration (CS, CG, CD, etc).
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PE-Electrical Review Course - Class 4
(Transistors)
Example 7
Find the mid-frequency values for Avi, Avs, AI,
AP, Zi, and Zo for the amplifier shown below.
Assume that Ci, Co, and CSS are large. Note that
this is the same biasing circuit used in Ex. 2,
so VGS -0.178 V. The JFET has the following
specifications IDSS 4 mA, VP -1.46 V,
rd 50 k
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PE-Electrical Review Course - Class 4
(Transistors)
FET Amplifier Configurations and Relationships
Note The biasing circuit is the same for each
amplifier.
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PE-Electrical Review Course - Class 4
(Transistors)
Which amplifier should be used? In order to
answer this question, it is necessary to be
familiar with typical values for each of the
amplifier configurations. The tables below list
some typical values. However, note that each
value might vary considerably.

• Highlight some of the key features that
  distinguish each amplifier configuration.
• Point out the similarities between the BJT and
  FET amplifier configurations.

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PE-Electrical Review Course - Class 4
(Transistors)
Multi-stage Amplifiers
In general, multistage amplifiers are used when a
single-stage amplifier cannot deliver all of the
desired features. Keep in mind the following
principles 1) Avi (Avi1)(Avi2) 2) Rs for
stage 2 Zo stage 1 3) RL for stage 1 Zi for
stage 2
Label the type of amplifier to be used in each
case below in order to satisfy the design
specifications?
Example 8
A) Design specifications Avi 100, Zo
30 B) Design specifications Avi
100, Zi 100 k
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PE-Electrical Review Course - Class 4
(Transistors)
Find Avi1 v2/v1 , Avi2 v3/v2 , Avi v3/v1 ,
Zi1, Zi2, Zo1, and Zo2 for the CE-CE multistage
amplifier shown below.
Example 9
Note that the biasing circuits are identical and
are from Ex. 5, so IC 0.634 mA. Each BJT has
the same specifications bdc 50, Vo 0.7 V,
ICBO 10 nA, bo 40, n 1, VA 30 V
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PE-Electrical Review Course - Class 4
(Transistors)
Frequency Response of Amplifiers
The voltage gain of an amplifier is typically
flat over the mid-frequency range, but drops
drastically for low or high frequencies. A
typical LM response is shown below.

• For a CE BJT (shown on lower left)
• low-frequency drop-off due to CE, Ci and Co
• high-frequency drop-off due to Cp and Cm
• (combined for form Ctotal)
• Each capacitor forms a break (simple pole or
  zero)
• with a break frequency of the form f
  1/(2pREqC),
• where REq is the resistance seen by the
  capacitor
• CE usually yields the highest low-frequency
  break
• which establishes fLow.
• For a CS FET (not shown - similar to BJT)
• low-frequency drop-off due to CSS, Ci and Co
• low-frequency drop-off due to Cds, Cgs and Cgd

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PE-Electrical Review Course - Class 4
(Transistors)
BJT High-frequency model
The BJT high-frequency hybrid-p model is shown to
the right. The capacitance Cm is replaced by a
Miller capacitance so that the two capacitors
can be added in parallel. I.e., Ctotal Cp
CMiller The break frequencies can now be
calculated using the following
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PE-Electrical Review Course - Class 4
(Transistors)
Example 10
Find fHigh, fLow, and the bandwidth, BW, for the
amplifier used in Example 6 if Ci 10 mF, Co
10 mF, CE 50 mF, Cp 20 pF, Cm 2 pF, and rx
20 ohms.
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PE-Electrical Review Course - Class 4
(Transistors)
Example 10 (continued)
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PE-Electrical Review Course - Class 4
(Transistors)
Special Amplifiers

• The focus of this review has been basic BJT and
  FET amplifiers. There are also a number of
  special amplifiers that would require
  considerable time to present. It would be
  difficult for the student to prepare adequately
  to be able to handle all special amplifiers.
  The best approach seems to be to review the
  basics well and then use a good reference book,
  such as the one for this course, if a special
  amplifier is encountered on the exam.
• Special amplifiers would include
• Differential amplifiers
• Darlingtons
• Power amplifiers
• Push-pull amplifiers