Post on 09-Nov-2015
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AMPLIFIERS
TRANSISTORS AND
I. TRANSISTOR FUNDAMENTALS
Transistor Developed in December 23, 1947
in Bell Laboratories
By John Bardeen, William Shockley, and Walter Brattain
Basically a resistor that amplifies electrical impulses as they are from its input to its output terminals
Basic Types
1. Bipolar Junction Transistor (BJT)
It is a three layer semiconductor device consisting of either two N-type and one P-type layers of materials or two P-type and one N-type layers of semiconductor materials.
Three Regions of BJT
Base Region to which carriers flow from emitter to collector.
1017 dopants/ cm3 Moderately doped
Three Regions of BJT
Emitter Region from which carriers flow 1019 dopants/ cm3 Heavily doped
Three Regions of BJT
Collector Region to which carriers flow 1015 dopants/ cm3 Lightly doped Largest
BJT Structure and Construction
Metal contacts
Epitaxial Planar Structure
Substrate Collector
Base Emitter
BJT Structure and Construction collector collector
base base
emitter emitter
npn-type pnp-type
n
n
n p p
p
Transistor Currents and Configuration
Common Base Configuration
In this circuit, the input signal is applied at the emitter, the output is taken at the collector and the base is the common terminal.
This has very low input impedance.
Vi Vo
VEE VCC
E C
B Ie Ic
RE RC
Transistor Currents and Configuration
Alpha () In the dc mode, the levels of IC and IE due to
majority carriers are related by a quantity called
alpha and defined by the following equation:
= Ic Ie
Transistor Currents and Configuration Common Emitter
Configuration The input is applied to
the base, the amplified output is taken from the collector and the emitter is the common terminal.
The circuit is the one generally used for transistors because this has the best combination of current and voltage gains.
Vi
Vo
Ic
Ie
Ib
RE
RB
VBB
VCC
Ie = Ib + Ic
Transistor Currents and Configuration
Beta () the ratio of collector current to the base current .
= Ic Ib
Transistor Currents and Configuration
Common Collector Configuration
This circuit has the input applied to the base, the output taken at the emitter terminal and the collector is the common terminal.
Impedance matching.
Vi
Vo
Ic
Ie
Ib
RE
RB
VBB
VCC
Transistor Currents and Configuration
Gamma () the ratio of collector current to the base current .
= Ie Ib
Comparison of Amplifier Configurations
Characteristic Common Base Common Emitter Common Collector
Power Gain moderate highest moderate
Voltage Gain highest moderate less than 1
Current Gain lowest than1 moderate highest Input
Impedance lowest moderate highest
Output Impedance highest moderate highest
Phase Inversion none
180o out of phase none
Application RF amplifier universal isolation
Transistor Biasing
Bias An electrical, mechanical or magnetic force
applied to a device to establish a desired electrical or mechanical reference level for its operation.
Is a DC voltage or current that sets the operating point for amplifying the AC signal
Transistor Biasing Fixed Bias
Is taken from a battery or power supply
Vi Vo RB
VCC
C C
RC
Transistor Biasing Self Bias
The amplifier produces its own DC voltage from an IR drop across a resistor in the return circuit of the common terminal.
Self bias is probably the type of bias used most often because it is economical and has stabilizing effect on the DC level of the output current.
Can be emitter stabilized or collector stabilized.
Transistor Biasing Self Bias
Vi Vo RB
VCC
C C
RC
RE Emitter
Stabilized
Transistor Biasing Self Bias
Vi Vo
RB
VCC
C C
RC
Collector Stabilized
Transistor Biasing Voltage-Divider Bias
The most stable type of circuit biasing.
Vi Vo RL
VCC
C C
RC
RE R2
Transistor Biasing Signal Bias
Vo C C
RC
RE
RB
VCC
Regions of Transistor Action SA
TUR
ATIO
N
RL VCC
ACTIVE
CUT- OFF
BRE
AKDO
WN
Q-POINT
LOADLINE
VCE VCC
IB
IB
IB
IB
IB
IC
Regions of Transistor Action Active region Base-emitter junction is forward biased and the
collector-base junction is reversed biased.
Transistors active operation as an amplifier. Saturation region Both junctions are forward biased. Switch on operation for the transistor.
Cut off region Both junctions are reverse biased. Switch off operation for the transistor.
Loadline and Q-Point
Loadline - Is a straight line drawn on the collector
curves between the cut-off and saturation points of the transistor.
Q-point (Quiescent point ) - Is the operating point of the transistor with
the time varying sources out of the circuit.
Review Question: Given the circuit below, draw the DC loadline
Analysis: At cut-off, IC = 0 thus VCE = VCC At saturation, VCE = 0 thus IC = VCC / RC
VBB = 3V
VCC = 25V
10K
1K
Ic
VCE
DC Loadline
25 mA
25 V
BJT Small Signal Analysis
Vo Vi
Ii
hi
hr Vo
hi Ii hi
Io
Transistor Hybrid Equivalent Circuit
BJT Small Signal Analysis
H - Parameters 1. hi short circuit input impedance
hi = Vi
Ii
2. hr open circuit reverse voltage gain (voltage feedback ratio)
(Vo = 0)
hr = Vi
Vo
(Ii = 0)
BJT Small Signal Analysis
H - Parameters 3. hf short circuit forward current gain
hf =
Io
Ii
(Vo = 0)
4. ho open circuit output admittance
ho = Io
Vo
(Ii = 0)
2. Field Effect Transistor (FET)
Unipolar device because they operate only with one type of charge carrier.
Voltage controlled device where the voltage between two of the terminals (gate and source) controls the current through the device.
Major feature is very high input resistance.
a. Junction Field Effect Transistor (JFET)
Operates with a reverse-biased PN junction to control current in the channel .
Square law device because of the relation of ID and VGS
ID = IDSS VGS
VGS(OFF) JFET/ D-MOSFET transfer characteristics
1 - 2
can be n-channel or p-channel
n-ch
anne
l
p-ch
anne
l
p p n n
drain
gate
source
drain
gate
source
D D G G
S S
Types of JFET, its structure
and parts
JFET Symbol
n-channel p-channel
Operation of JFET
JFET is always operated with the gate-source PN junction reversed biased.
Reverse biasing of the gate source junction with the negative voltage produces a depletion region along the PN junction which extends into the n-channel and thus increases its resistance by restricting the channel width as shown in the preceding figure.
Operation of JFET
n-ch
ann
el
p p
drain
gate
source
VGS
VDS
Operation of JFET
Pinch off Region
Bre
akdo
wn
Reg
ion
Ohm
ic R
egio
n
Vp pinch off voltage Va avalanche breakdown voltage
DC Biasing for JFET
1. Fixed Bias - a separate power source.
Vin
RG
RL
VDD
VGS
VGG
-
+
-
+
ID
DC Biasing for JFET
2. Self Bias
Vin
RG
RL
VDD
VGS
-
+
RS VS
+
ID
DC Biasing for JFET
3. Source Bias
Vin
RG
RL
VDD
VGS RS
+
ID
- VSS
-
+
DC Biasing for JFET
4. Voltage Divider
Vin
R2
RL
VDD
VGS RS
+
ID
VS
-
+
R1
b. Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
Second category of the field effect transistor
Because of the presence of an insulated gate, then it is sometimes called IGFETs
MOSFETs differs from JFET in that it has no PN junction structure.
It has two basic types: D MOSFET and E - MOSFET
Depletion MOSFET (D - MOSFET)
The drain and source are diffused into substrate material and connected by a narrow channel adjacent to the insulated gate
It can be operated two in modes, the depletion mode or the enhancement mode and sometimes called depletion/enhancement mode MOSFET
Depletion MOSFET (D - MOSFET)
It can be operated with a zero, positive or negative gate-source voltage.
Normally operated in the depletion mode. When configured as switch, it is normally-
on.
D D
G G
S S
n-channel p-channel
p-substrate n-substrate
drain
gate
source
gate
drain
source
n-channel D-MOSFET p-channel D-MOSFET
SiO2 SiO2
Depletion Mode Negative gate to
source voltage is applied
n-channel is depleted of some electrons hence decreasing channel conductivity.
Enhancement
Mode Positive gate voltage
is applied.
More conduction electrons are attracted to the channel thus enhancing channel conductivity.
Enhancement MOSFET (E - MOSFET)
Operates only in the enhancement mode Has no depletion mode It has no structural channel It has no IDSS parameter For an n-channel type of this device, a positive
gate voltage above threshold induces a channel by creating a layer of negative charges (inversion layer) in the substrate portion that is adjacent to the SiO2 layer.
Enhancement MOSFET (E - MOSFET)
An n-channel E-MOSFET has a positive VGS while a p-channel E-MOSFET has a negative VGS.
The conductivity of its channel is enhanced by increasing the gate to source voltage.
For gate voltage below the threshold, there is no channel to be formed.
If configured as switch, this device is normally off LD MOSFET, VMOSFET and TMOSFET are E-
MOSFET technologies developed for higher power dissipation.
D
G
S
p-substrate
SiO2 n
n
No permanent
channel
D
S
G n
n +
+
-
- Inversion
layer
Basic construction Operation drain
gate
source source
drain gate
n-channel p-channel
II. AMPLIFIERS
Electronic devices capable of amplification or increasing the amplitude of power, current or voltage at its output.
Circuits designed to increase the amplitude of level of an electronic signal.
Used as boosters.
AMPLIFIER input output
Classification of Amplifier
1. According to Function a. Voltage Amplifier
- Voltage controlled source
- Op-amps are voltage amplifier
b. Current Amplifier
- current controlled source
- BJTs are current amplifier c. Power Amplifier
- Boost the power level of the signal
Classification of Amplifier
2. According to Configuration a. Common Base Amplifier
- Transistor amplifier where input is applied at the emitter and output is taken from the collector terminal.
- The base is common to both input and output.
- maximum current gain is 1
- No phase inversion from input to output .
Classification of Amplifier
2. According to Configuration b. Common Collector Amplifier (emitter
follower)
- Transistor amplifier where input is applied at the base, output is taken from the emitter terminal.
- Maximum voltage gain is 1.
- Capacitors must have a negligible reactance at the frequency of operation.
- No phase inversion from input to output.
Classification of Amplifier
2. According to Configuration c. Common Emitter Amplifier
- Transistor amplifier wherein the input is applied at the base and the output is taken from the collector terminal.
- There is a phase inversion from input to output.
Classification of Amplifier 3. According to Class of Operation
Class A Class B Class C Class AB Efficiency 50 % 78.5 % 100 % Between A & B Conduction
Angle 360O 180O Below 360O
Slightly greater than
180O
Distortion Low High Extreme Moderate Bias (Base
Emitter) Linear portion
Above Cut-off
Below Cut-off Cut-off
Input Output Output Output Output
Classification of Amplifier
4. According to Frequency a. DC Amplifier
- amplifies DC signal.
b. Audio Amplifier
- amplifies signal whose frequency is within the audio range (20 Hz 20 KHz).
c. RF Amplifier
- amplifies signal whose frequency is within the radio frequency range.
Classification of Amplifier 4. According to Frequency d. IF Amplifier
- amplifies signal whose frequency is in between the carrier and the modulating frequency.
e. Video Amplifier
- a wide band amplifier that amplifies video signal.
- video signal refers to the frequency range of the picture information which arises from the television scanning process.
Classification of Amplifier
5. According to the signal being amplified a. Small Signal Amplifiers
- Amplifier that utilizes only the very linear portion of the
b. Large Signal Amplifiers
- Amplifier that utilizes almmost the full rated output power
Classification of Amplifier
6. According to method of coupling a. Direct Coupling
- Amplifiers connected or coupled without any passive
b. Capacitive Coupling
- Amplifiers are connected or coupled by the used
Classification of Amplifier
6. According to method of coupling c. Inductive Coupling
- Amplifiers are connected or coupled by the use of inductor transformer.
d. Transformer Coupling
- Most often, inductor is not used as coupling device instead transformer is used.
Classification of Amplifier
7. Power Amplifiers a. Push-Pull Amplifiers
- Amplifier with two similar circuits operating in phase.
- On amplifies the half of the cycle and the remaining half is being amplified by the other amplifier.
Classification of Amplifier
7. Power Amplifiers b. Complementary-Symmetry Amplifiers
- Push-pull amplifiers using complementary transistors such as pair of pnp and npn.
c. Quasi-Complementary Amplifiers
- Push-pull amplifiers using the same transistors at the output but the driver is using complementary transistors.
Compound Configurations
a. Cascade Connection - a cascade connection is a series
connection with the output of one stage then applied as input to the second stage.
- The cascade connection provides a multiplication of the gain of each stage for a larger overall gain.
AV = AV1AV2AV3AVn AV(dB) = 20Log(AV)
Compound Configurations
b. Cascode Connection - a cascode connection has one transistor on
top of (in series with) another.
- This arrangement is design to provide high input impedance with low voltage gain to ensure that the input Miller capacitance is minimum.
Compound Configurations
c. Darlington Connection - The main feature of Darlington connection
is that the composite transistor acts as a single unit with a current gain that is the product of the current gains of the individual transistors.
- It is a circuit meant to boost input resistance.
Compound Configurations
c. Darlington Connection
1 2 D = 1 2
Compound Configurations
d. Feedback Pair - The feedback pair connection is a two
transistor circuit that operates like the Darlington circuit.
- It uses a pnp transistor driving an npn transistor.