Transcript TRANSISTORx

TRANSISTOR
INTRODUCTION TO TRANSISTORS
 The discovery of the first transistor in 1948 by a team of
physicists at the Bell Telephone Laboratories sparked an
interest in solid-state research that spread rapidly.
 Transistor demonstrated amplification in solids possible.
 Before the transistor, amplification was achieved only with
electron tubes.
 In many cases, transistors are more desirable than tubes
coz they are small, rugged, require no filament power, and
operate at low voltages with comparatively high
efficiency.
 The development of transistors made possible the
miniaturization of electronic circuits.
INTRODUCTION TO TRANSISTORS
• Transistors have infiltrated virtually every area
of science and industry, from the family car to
satellites, Military etc.
• The ever increasing uses for transistors have
created an urgent need for sound and basic
information regarding their operation.
TRANSISTOR FUNDAMENTALS
• Semiconductor devices that have-three or more
elements are called TRANSISTORS.
• The term transistor was derived from the words
TRANSfer and resISTOR
• Term adopted coz it best describes the operation
of the transistor - the transfer of an input signal
current from a low-resistance circuit to a highresistance circuit.
• Basically, the transistor is a solid-state device that
amplifies by controlling the flow of current
carriers through its semiconductor materials.
TRANSISTOR FUNDAMENTALS
Same theory to explain operation of transistor
used with the PN-junction diode.
The three elements of the two-junction
transistor are
• (1) the EMITTER, which gives off, or emits,"
current carriers (electrons or holes);
• (2) the BASE, which controls the flow of current
carriers;
• (3) the COLLECTOR, which collects the current
carriers.
NPN Transistor Bias
The voltage on the collector must be
more positive than the base:
In summary, the base of the NPN transistor must be positive with respect to the emitter, and
the collector must be more positive than the base.
Introduction
• Recall forward-biased(FB) PN junction is EQU to a lowresistance circuit element.
• A reverse-biased (RB) PN junction is EQU to a highresistance circuit element.
• Ohm's law formula for power(P = I2R) & assuming
current is held constant, can conclude that the power
developed across a high resistance is greater than that
developed across a low resistance.
• If crystal were to contain two PN junctions (one FB and
the other RB), a low-power signal could be injected into
the FB junction and produce a high-power signal at the
RB junction = Power Gain across Crystal
• This concept, is the basic theory behind how the
transistor amplifies.
NPN FORWARD-BIASED JUNCTION
• Important point to bring out at this time is the
fact that the N material on one side of the
forward-biased junction is more heavily doped
than the P material.
• This results in more current being carried across
the junction by the majority carrier electrons
from the N material than the majority carrier
holes from the P material.
• Therefore, conduction through the forwardbiased junction is mainly by majority carrier
electrons from the N material (emitter)
The forward biased junction in the
NPN Transistor
NPN FORWARD-BIASED JUNCTION
• With the emitter-to-base junction biased in the
forward direction, electrons leave the negative
terminal of the battery and enter the N material
(emitter).
• Since electrons are majority current carriers in the N
material, they pass easily through the emitter, cross
over the junction, and combine with holes in the P
material (base).
• For each electron that fills a hole in the P material,
another electron will leave the P material (creating a
new hole) and enter the positive terminal of the
battery.
NPN REVERSE-BIASED JUNCTION
• The second PN junction (base-to-collector), or reverse-biased
junction as it is called, blocks the majority current carriers from
crossing the junction.
• There is a very small current that does pass through this
junction.
• This current is called minority current, or reverse current.
• This current was produced by the electron-hole pairs.
• The minority carriers for the reverse-biased PN junction are
the electrons in the P material and the holes in the N material.
• These minority carriers actually conduct the current for the
reverse-biased junction when electrons from the P material
enter the N material, and the holes from the N material enter
the P material.
• However, the minority current electrons play the most
important part in the operation of the NPN transistor.
Reverse biased junction in the NPN
Transistor
NPN REVERSE-BIASED JUNCTION
• At this point you may wonder why the second PN junction
(base-to-collector) is not forward biased like the first PN
junction (emitter-to-base).
• If both junctions were forward biased, the electrons would
have a tendency to flow from each end section of the N P N
transistor (emitter and collector) to the center P section (base).
• In essence, we would have two junction diodes
possessing a common base, thus eliminating any
amplification and defeating the purpose of the
transistor.
• A word of caution is in order at this time.
• If you should mistakenly bias the second PN junction in the
forward direction, the excessive current could develop enough
heat to destroy the junctions, making the transistor useless.
• Therefore, be sure your bias voltage polarities are correct before
making any electrical connections.
NPN JUNCTION INTERACTION
NPN JUNCTION INTERACTION
• The bias batteries in the above figure have been
labeled VCC for the collector voltage supply, and VBB
for the base voltage supply.
• Also notice the base supply battery is quite small,
as indicated by the number of cells in the battery,
usually 1 volt or less.
• However, the collector supply is generally much
higher than the base supply, normally around 6
volts.
• This difference in supply voltages is necessary to
have current flow from the emitter to the collector.
Common–Base Current Gain
 The ratio of collector current to emitter current is called α, which is
also named as h-parameter hFB.
 This parameter is commonly known as common base gain.
α = IC/IE
(2)
 The typical value of α ranges from from 0.95 to 0.99.
 For a good transistor, its α value is close to one.
 If the collector leakage current is included then the collector
current is given by

IC = αIE + ICBO
(3)
 The ratio of collector current to base current is β , which also
denoted as h parameter-hFE.
 This parameter is commonly known as common emitter gain.
β = IC/IB
(4)
The Common Emitter Configuration.
• As well as being used as a semiconductor switch to
turn load currents "ON" or "OFF" by controlling the
Base signal to the transistor in ether its saturation or
cut-off regions, NPN Transistors can also be used in its
active region to produce a circuit which will amplify
any small AC signal applied to its Base terminal with
the Emitter grounded.
• If a suitable DC "biasing" voltage is firstly applied to
the transistors Base terminal thus allowing it to always
operate within its linear active region, an inverting
amplifier circuit called a single stage common emitter
amplifier is produced.
The Common Emitter Configuration
• One such Common Emitter Amplifier configuration of an NPN
transistor is called a Class A Amplifier.
• A "Class A Amplifier" operation is one where the transistors Base
terminal is biased in such a way as to forward bias the Base-emitter
junction.
• The result is that the transistor is always operating halfway
between its cut-off and saturation regions, thereby allowing the
transistor amplifier to accurately reproduce the positive and
negative halves of any AC input signal superimposed upon this DC
biasing voltage.
• Without this "Bias Voltage" only one half of the input waveform
would be amplified.
• This common emitter amplifier configuration using an NPN
transistor has many applications but is commonly used in audio
circuits such as pre-amplifier and power amplifier stages.
The Common Emitter Configuration
• A family of curves known as the Output
Characteristics Curves, relates (Ic) to (Vce) when
different values of (Ib) are applied to the transistor
for transistors with the same β value.
• A DC "Load Line" to show all the possible operating
points when different values of base current are
applied.
• It is necessary to set the initial value of Vce correctly
to allow the output voltage to vary both up and
down when amplifying AC input signals and this is
called setting the operating point or Quiescent Point,
Q-point.
Single Stage Common Emitter
Amplifier Circuit
Output Characteristics Curves for a
Typical Bipolar Transistor
Common-Emitter CE Configuration
• The most important factor to notice is the effect
of Vce upon the collector current Ic when Vce is
greater than about 1.0 volts.
• We can see that Ic is largely unaffected by
changes in Vce above this value and instead it is
almost entirely controlled by the base current, Ib.
• When this happens we can say then that the
output circuit represents that of a "Constant
Current Source".
• By using the output characteristics curves in
our example above and also Ohm´s Law, the
current flowing through the load resistor, (RL),
is equal to the collector current, Ic entering
the transistor which inturn corresponds to the
supply voltage, (Vcc) minus the voltage drop
between the collector and the emitter
terminals, (Vce) and is given as:
Common-Emitter CE Configuration
• Transistor connected with emitter as the
common or ground is called common-emitter
configuration as shown in Fig below
dc Analysis C-E Config.
• there are three currents and three voltages,
which are base current IB, emitter current IE,
collector current IC, base-to-emitter voltage
VBE, collector-to-base voltage VCB, and
collector-to-emitter voltage VCE.
• For any other dc biasing configuration,
there always have these currents and
voltages.
Common-emitter current and voltage
C-E Config.
C-E Config
dc Operating Point
• The dc operating point is referred to Q-point (quiescent
point).
• It is a point on the transistor characteristic curve.
• If one chooses collector current IC versus collector-toemitter voltageVCE characteristics curve then Q-point is the
point on the curve determined by collector current IC and
collector-to-emitter voltageVCE for a fixed value of base
current IB derived from the biasing of circuit.
• Using the transistor biasing circuit shown in Fig. overleaf,
the Q-point on the characteristics curve can be determined
by finding the values of IC and VCE for a given base current
IB determined by the circuit.
• The line joining the Q-point is known as dc load line.
Biasing circuit for determining Q-point
Biasing circuit for determining Q-point
Q-point and dc load line