انثى من زمن النقاء
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 | | موضوع: ترانزستور الوصلة ثنائي القطبية الإثنين فبراير 22, 2010 11:21 am | |
| ترانزستور الوصلة
ثنائي القطبية
Bipolar Junction Transistor BJT
المحتويات:
فكرة عامة عن تاريخ الترانزستور
مقدمة باللغة العربية عن ترانزستورات الوصلة ثنائية القطبية
شرح مفصّل باللغة الإنجليزية عن:
Bipolar Transistor Basics
The NPN Transistor
The PNP Transistor
Transistor as a Switch
الترانزستورات Transistors تم التغلب على جميع عيوب الصمام الإلكتروني باختراع الترانزستور في عام 1947م وذلك على يد ثلاثة من الفيزيائيين الأميركيين العاملين في مختبرات بيل الأمريكية وهم جون باردين (John Bardeen) و ولتر براتين (Walter Brattain) و وليم شوكلى (William Shockley) والذين حصلوا على جائزة نوبل في عام 1956م تقديرا لجهودهم على هذا الإنجاز العظيم. والترانزستور عنصر إلكتروني فعال (activedevice) مصنوع من مواد شبه موصلة كالجرمانيوم والسيليكون وله ثلاثة أقطاب كما هو الحال مع الصمام الثلاثي ولكن بدون دائرة تسخين. [center] [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
ويتميز الترانزستور على الصمام الإلكتروني بصغر حجمه الذي لا يتجاوز إذا ما
صنع منفردا حجم حبة الحمص أما إذا كان في دوائر متكاملة فإنه بالإمكان
تصنيع ملايين الترانزستورات على شريحة لا تتجاوز مساحتها السنتيمتر المربع
الواحد مما أدى إلى تقليص بالغ في أحجام وأوزان الأجهزة الكهربائية.
ويتميز كذلك بأنه يعمل على جهد كهربائي منخفض لا يتجاوز عدة فولتات وبقلة
استهلاكه للطاقة الكهربائية التي تقاس بالميللي واط في الترانزستورات
المنفردة والميكروواط وحتى النانوواط في الدوائر المتكاملة مما أدى إلى
تصنيع أجهزة كهربائية مختلفة تعمل بالبطاريات الصغيرة ولفترات طويلة من
الزمن.
ويتميز بصلادته فهو جسم مصمت من مواد شبه موصلة حيث لا توجد في داخله أجزاء
متحركة ولذلك فهو لا يتأثر بالصدمات والإهتزازات الميكانيكية كما هو الحال
مع الصمام الإلكتروني ولذا يمكن وضعه في الأجهزة المحمولة. ويتميز كذلك
بطول عمره التشغيلي الذي يمتد لعشرات السنوات وبإمكانية عمله على نطاق واسع
من درجات الحرارة وبإمكانية إنتاجه بكميات كبيرة جدا وبأسعار منخفضة جدا.
ومع اختراع الترانزستور الذي يعده العلماء أعظم اختراع في القرن العشرين
تجددت أمال المهندسين في صنع معدات وأجهزة إلكترونية صغيرة الحجم وقليلة
الاستهلاك للطاقة كالحواسيب الرقمية والتلفزيونات الملونة والراديوات
الصغيرة والهواتف المحمولة والآلات الحاسبة المكتبية واليدوية. وكذلك فإنه
لا يوجد الآن ما يحول دون وضع المعدات والأجهزة الإلكترونية في مختلف
أنواع المركبات والصواريخ العابرة للقارات وفي المركبات الفضائية والأقمار
الصناعية حيث أنها لا تحتل حيزا كبيرا ويمكنها أن تعمل على البطاريات.
ترانزستورات الوصلة ثنائية القطبية Bipolar
Junction Transistors (BJT)
يتم تصنيع هذا النوع من الترانزستورات من خلال تطعيم ثلاث مناطق متجاورة
على بلورة نقية من السيليكون بحيث يكون التطعيم إما على شكل (سالب_موجب_
سالب)(NPN) أو على شكل (موجب_سالب_موجب) (PNP).
ويتم توصيل أقطاب معدنية بهذه المناطق الثلاثة حيث يسمى القطب الموصول
بالمنطقة الوسطى بالقاعدة (****) بينما تسمى الأقطاب الموصولة بالمنطقتين
الخارجيتين بالباعث (Emitter) والمجمع (Collector).
ويطلق على هذه النوع من الترانزستورات بالترانزستور ثنائي القطبية
(bipolar) وذلك بسبب وجود وصلتين فيه وكذلك بسبب مساهمة الفجوات
والإلكترونات في حمل التيار الذي يسري داخل الترانزستور.
يتطلب عمل هذا النوع من الترانزستورات وجود وصلتين يكون في الغالب وضع
الإنحياز لأحدهما أمامي وللأخرى عكسي مما يعني أن الوصلة المنحازة أماميا
ستسمح بمرور التيار بينما لا تسمح الوصلة المنحازة عكسيا بمروره. ولكن إذا
ما تم تصنيع الترانزستور بحيث يكون عرض منطقة القاعدة قليل جدا بحيث أن
المنطقة المنضبة للوصلة المنحازة عكسيا تغطي معظم أجزائها فإن الإلكترونات
أو الفجوات التي تصل إلى منطقة القاعدة من تيار الوصلة المنحازة أماميا
سيقع في أسر المجال الكهربائي للمنطقة المنضبة المنحازة عكسيا وسيمر تيارا
عاليا فيها رغم أنها منحازة عكسيا. وكلما قل عرض منطقة القاعدة كلما زادت
نسبة عدد الإلكترونات أو الفجوات التي يتم اقتناصها من قبل الوصلة
المنحازة عكسيا من العدد الكلي المتولد في الوصلة المنحازة أماميا.
إن هذه الآلية في طريقة عمل الترانزستور تمكن تيارا ضعيفا يمر في القاعدة
من التحكم بتيار قوي يمر بين الباعث والمجمع ويطلق على نسبة تيار المجمع أو
الباعث على تيار القاعدة بكسب الترانزستور (transistor gain).
ويمكن زيادة كسب الترانزستور من خلال تقليل عرض منطقة القاعدة ويمكن الحصول
على كسب قد يصل لعدة مئات. إن العيب الرئيسي للترانزستور ثنائي القطبية هو
أن القاعدة تستخدم التيار الكهربائي للتحكم بعمل الترانزستور مما يستدعي
استخدام دائرة كهربائية خارجية دقيقة لضبط قيمة تيار القاعدة والذي قد
يؤدي أي انحراف في قيمته إلى تغيير مكان نقطة التشغيل التي يعمل عندها
الترانزستور.
إن الترانزستورات من نوع (NPN) أكثر شيوعا في الاستخدام من الترانزستورات
من نوع (PNP) وذلك لاستجابتها العالية وذلك بسبب أن سرعة حركة الإلكترونات
في المناطق السالبة أعلى بكثير من سرعة حركة الفجوات في المناطق الموجبة.
ويتم تصنيع الترانزستور من نوع (NPN) بالطريقة السطحية من خلال تطعيم منطقة
محددة بذارات مانحة لتنتج منطقة سالبة بعمق معين على سطح شذرة من
السيليكون وفي داخل هذه المنطقة السالبة يتم تطعيم جزء منها بذرات مستقبلة
لتحولها إلى منطقة موجبة وفي داخل هذه المنطقة الموجبة يتم تطعيم جزء منها
بذرات مانحة لتحولها إلى منطقة سالبة وبهذا تتكون ثلاث مناطق منطقتين
سالبتين بينهما منطقة موجبة ويتم وصل القاعدة بالمنطقة الموجبة والباعث
بالمنطقة السالبة الأقرب من السطح والمجمع بالمنطقة السالبة الأبعد عن
السطح.
وبسبب أن مساحة وصلة الباعث أقل منها بكثير من مساحة وصلة المجمع في عملية
التصنيع هذه فإن الكسب في تيار الباعث منه أكبر بكثير منه في حالة تيار
المجمع ولذا يجب أن يراعى ذلك عند تصميم المضخمات فالترانزستور غير متماثل
في مثل هذه الطريقة من التصنيع.
ويتم تصنيع أنواع لا حصر لها من الترانزستورات بعضها يعمل عند الترددات
المنخفضة وبعضها عند الترددات العالية وبعضها لأغراض القدرات المنخفضة
وبعضها للقدرات العالية وذلك لتلبي حاجة التطبيقات المختلفة.
ويحمل كل ترانزستور على سطحه رمزا مكون من عدد من الأحرف والأرقام ويمكن
استخلاص بعض المعلومات من هذه الرموز كنوع مادة الترانزستور إن كانت من
السيليكون أو الجرمانيوم أو كمدى الترددات التي يعمل عندها ومقدار الجهد أو
التيار أو القدرة الكهربائية التي يتحملها.
[size=12]Bipolar Transistor
Bipolar Transistor Basics
If we now join together two individual diodes end to end giving two
PN-junctions connected together in series, we now have a three layer,
two junction, three terminal device forming the basis of a Bipolar
Junction Transistor, or BJT for short. This type of transistor is
generally known as a Bipolar Transistor, because its basic construction
consists of two PN-junctions with each terminal or connection being
given a name to identify it and these are known as the Emitter, **** and
Collector respectively.
The word Transistor is an acronym, and is a combination of the words
Transfer Varistor used to describe their mode of operation way back in
their early days of development. There are two basic types of bipolar
transistor construction, NPN and PNP, which basically describes the
physical arrangement of the P-type and N-type semiconductor materials
from which they are made. Bipolar Transistors are "CURRENT" Amplifying
or current regulating devices that control the amount of current flowing
through them in proportion to the amount of biasing current applied to
their **** terminal. The principle of operation of the two transistor
types NPN and PNP, is exactly the same the only difference being in the
biasing (**** current) and the polarity of the power supply for each
type.
Bipolar Transistor
Construction
The construction and circuit symbols for both the NPN and
PNP bipolar transistor are shown above with the arrow in the circuit
symbol always showing the direction of conventional current flow between
the **** terminal and its emitter terminal, with the direction of the
arrow pointing from the positive P-type region to the negative N-type
region, exactly the same as for the standard diode symbol.
There are basically three possible ways to connect a Bipolar Transistor
within an electronic circuit with each method of connection responding
differently to its input signal as the static characteristics of the
transistor vary with each circuit arrangement.
• 1. Common **** Configuration - has Vol***e Gain but no Current
Gain.
• 2. Common Emitter Configuration - has both Current and Vol***e
Gain.
• 3. Common Collector Configuration - has Current Gain but no
Vol***e Gain.
The Common **** Configuration
As its name suggests, in the Common **** or Grounded **** configuration,
the **** connection is common to both the input signal AND the output
signal with the input signal being applied between the **** and the
emitter terminals.
The corresponding output signal is taken from between the **** and the
collector terminals as shown with the **** terminal grounded or
connected to a fixed reference vol***e point. The input current flowing
into the emitter is quite large as its the sum of both the **** current
and collector current respectively therefore, the collector current
output is less than the emitter current input resulting in a Current
Gain for this type of circuit of less than "1", or in other words it
"Attenuates" the signal.
The Common **** Amplifier Circuit
This type of amplifier configuration is a non-inverting vol***e
amplifier circuit, in that the signal vol***es Vin and Vout are
In-Phase. This type of arrangement is not very common due to its
unusually high vol***e gain characteristics. Its Output characteristics
represent that of a forward biased diode while the Input characteristics
represent that of an illuminated photo-diode.
Also this type of configuration has a high ratio of Output to Input
resistance or more importantly "Load" resistance (RL) to "Input"
resistance (Rin) giving it a value of "Resistance Gain". Then the
Vol***e Gain for a common **** can therefore be given as:
Common **** Vol***e Gain
The Common **** circuit is generally only used in single s***e amplifier
circuits such as microphone pre-amplifier or RF radio amplifiers due to
its very good high frequency response.
The Common Emitter
Configuration
In the Common Emitter or Grounded Emitter configuration, the input
signal is applied between the ****, while the output is taken from
between the collector and the emitter as shown.
This type of configuration is the most commonly used circuit for
transistor ****d amplifiers and which represents the "normal" method of
connection. The common emitter amplifier configuration produces the
highest current and power gain of all the three bipolar transistor
configurations.
This is mainly because the input impedance is LOW as it is connected to
a forward-biased junction, while the output impedance is HIGH as it is
taken from a reverse-biased junction.
The Common Emitter Amplifier Circuit
In this type of configuration, the current flowing out of the transistor
must be equal to the currents flowing into the transistor as the
emitter current is given as Ie = Ic + Ib. Also, as the load resistance
(RL) is connected in series with the collector, the Current gain of the
Common Emitter Transistor Amplifier is quite large as it is the ratio of
Ic/Ib and is given the symbol of Beta, (β).
Since the relationship between these three currents is determined by
the transistor itself, any small change in the **** current will result
in a large change in the collector current.
Then, small changes in **** current will thus control the current in
the Emitter/Collector circuit.
By combining the expressions for both Alpha, α and Beta, β the
mathematical relationship between these parameters and therefore the
current gain of the amplifier can be given as:
Where: "Ic" is the current flowing into the collector terminal, "Ib" is
the current flowing into the **** terminal and "Ie" is the current
flowing out of the emitter terminal.
Then to summarise, this type of bipolar transistor configuration has a
greater input impedance, Current and Power gain than that of the common
**** configuration but its Vol***e gain is much lower. The common
emitter is an inverting amplifier circuit resulting in the output signal
being 180o out of phase with the input vol***e signal.
The Common Collector Configuration
In the Common Collector or Grounded Collector configuration, the
collector is now common and the input signal is connected to the ****,
while the output is taken from the Emitter load as shown.
This type of configuration is commonly known as a Vol***e Follower or
Emitter Follower circuit. The Emitter follower configuration is very
useful for impedance matching applications because of the very high
input impedance, in the region of hundreds of thousands of Ohms, and it
has relatively low output impedance.
The Common Collector Amplifier Circuit
The Common Emitter configuration has a current gain equal to the β value
of the transistor itself. In the common collector configuration the
load resistance is situated in series with the emitter so its current is
equal to that of the emitter current.
As the emitter current is the combination of the collector AND ****
currents combined, the load resistance in this type of amplifier
configuration also has both the collector current and the input current
of the **** flowing through it. Then the current gain of the circuit is
given as:
This type of bipolar transistor configuration is a non-inverting
amplifier circuit in that the signal vol***es of Vin and Vout are
"In-Phase". It has a vol***e gain that is always less than "1" (unity).
The load resistance of the common collector amplifier configuration
receives both the **** and collector currents giving a large current
gain (as with the Common Emitter configuration) therefore, providing
good current amplification with very little vol***e gain.
Bipolar Transistor Summary
The behaviour of the bipolar transistor in each one of the above circuit
configurations is very different and produces different circuit
characteristics with regards to Input impedance, Output impedance and
Gain and this is summarised in the table below.
Transistor Characteristics
The static characteristics for Bipolar Transistor amplifiers can be
divided into the following main groups.
with the characteristics of the different transistor configurations
given in the following table:
[/size] The NPN Transistor
In the previous tutorial we saw that the standard Bipolar Transistor or
BJT, comes in two basic forms. An NPN (Negative-Positive-Negative) type
and a PNP (Positive-Negative-Positive) type, with the most commonly used
transistor type being the NPN Transistor.
We also learnt that the transistor junctions can be biased in one of
three different ways - Common ****, Common Emitter and Common Collector.
In this tutorial we will look more closely at the "Common Emitter"
configuration using NPN Transistors and an example of its current flow
characteristics is given below.
An NPN Transistor Configuration
Note: Conventional current flow.
We know that the transistor is a "CURRENT" operated device and that a
large current (Ic) flows freely through the device between the collector
and the emitter terminals. However, this only happens when a small
biasing current (Ib) is flowing into the **** terminal of the transistor
thus allowing the **** to act as a sort of current control input.
The ratio of these two currents (Ic/Ib) is called the DC Current Gain
of the device and is given the symbol of hfe or nowadays Beta, (β). Beta
has no units as it is a ratio. Also, the current gain from the emitter
to the collector terminal, Ic/Ie, is called Alpha, (α), and is a
function of the transistor itself.
As the emitter current Ie is the product of a very small **** current
to a very large collector current the value of this parameter α is very
close to unity, and for a typical low-power signal transistor this value
ranges from about 0.950 to 0.999.
α and β Relationships
By combining the two parameters α and β we can produce two mathematical
expressions that gives the relationship between the different currents
flowing in the transistor.
The values of Beta vary from about 20 for high current power transistors
to well over 1000 for high frequency low power type bipolar
transistors.
The equation for Beta can also be re-arranged to make Ic as the
subject, and with zero **** current (Ib = 0) the resultant collector
current Ic will also be zero, (β x 0). Also when the **** current is
high the corresponding collector current will also be high resulting in
the **** current controlling the collector current.
One of the most important properties of the Bipolar Junction Transistor
is that a small **** current can control a much larger collector
current. Consider the following example.
Example No1.
An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate
the **** current Ib required to switch a resistive load of 4mA.
Therefore, β = 200, Ic = 4mA and Ib = 20µA.
One other point to remember about NPN Transistors. The collector
vol***e, (Vc) must be greater than the emitter vol***e, (Ve) to allow
current to flow through the device between the collector-emitter
junction.
Also, there is a vol***e drop between the **** and the emitter terminal
of about 0.7v for silicon devices as the input characteristics of an
NPN Transistor are of a forward biased diode. Then the **** vol***e,
(Vbe) of an NPN Transistor must be greater than this 0.7 V otherwise the
transistor will not conduct with the **** current given as.
Where:
Ib is the **** current, Vb is the **** bias vol***e, Vbe is the
****-emitter volt drop (0.7v) and Rb is the **** input resistor.
Example No2.
An NPN Transistor has a DC **** bias vol***e, Vb of 10v and an input
**** resistor, Rb of 100kΩ. What will be the value of the **** current
into the transistor.
Therefore, Ib = 93µA.
The Common Emitter Configuration.
As well as being used as a switch to turn load currents "ON" or "OFF" by
controlling the **** signal to the transistor, NPN Transistors can also
be used to produce a circuit which will also amplify any small AC
signal applied to its **** terminal.
If a suitable DC "biasing" vol***e is firstly applied to the
transistors **** terminal thus allowing it to always operate within its
linear active region, an inverting amplifier circuit called a Common
Emitter Amplifier is produced.
One such Common Emitter Amplifier configuration is called a Class A
Amplifier.
A Class A Amplifier operation is one where the transistors ****
terminal is biased in such a way that the transistor is always operating
halfway between its cut-off and saturation points, thereby allowing the
transistor amplifier to accurately reproduce the positive and negative
halves of the AC input signal superimposed upon the DC Biasing vol***e.
Without this "Bias Vol***e" only the positive half of the input
waveform would be amplified. This type of amplifier has many
applications but is commonly used in audio circuits such as
pre-amplifier and power amplifier s***es.
With reference to the common emitter configuration shown below, a family
of curves known commonly as the Output Characteristics Curves, relates
the output collector current, (Ic) to the collector vol***e, (Vce) when
different values of **** current, (Ib) are applied to the transistor for
transistors with the same β value. A DC "Load Line" can also be drawn
onto the output characteristics curves to show all the possible
operating points when different values of **** current are applied.
It is necessary to set the initial value of Vce correctly to allow the
output vol***e to vary both up and down when amplifying AC input signals
and this is called setting the operating point or Quiescent Point,
Q-point for short and this is shown below.
The Common Emitter Amplifier Circuit
Output Characteristics Curves for a Typical Bipolar Transistor
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.
You can see that Ic is largely unaffected by changes in Vce above this
value and instead it is almost entirely controlled by the **** current,
Ib.
When this happens we can say then that the output circuit represents
that of a "Constant Current Source". It can also be seen from the common
emitter circuit above that the emitter current Ie is the sum of the
collector current, Ic and the **** current, Ib, added together so we can
also say that " Ie = Ic + Ib " for the common emitter configuration.
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 vol***e, (Vcc) minus the vol***e drop between
the collector and the emitter terminals, (Vce) and is given as:
Also, a Load Line can be drawn directly onto the graph of curves above
from the point of "Saturation" when Vce = 0 to the point of "Cut-off"
when Ic = 0 giving us the "Operating" or Q-point of the transistor.
These two points are calculated as:
Then, the collector or output characteristics curves for Common Emitter
NPN Transistors can be used to predict the Collector current, Ic, when
given Vce and the **** current, Ib. A Load Line can also be constructed
onto the curves to determine a suitable Operating or Q-point which can
be set by adjustment of the **** current.
The PNP Transistor
The PNP Transistor is the exact opposite to the NPN Transistor device we
looked at in the previous tutorial. Basically, in this type of
transistor construction the two diodes are reversed with respect to the
NPN type, with the arrow, which also defines the Emitter terminal this
time pointing inwards in the transistor symbol.
Also, all the polarities are reversed which means that PNP Transistors
"sink" current as opposed to the NPN transistor which "sources" current.
Then, PNP Transistors use a small output **** current and a negative
**** vol***e to control a much larger emitter-collector current.
The construction of a PNP transistor consists of two P-type
semiconductor materials either side of the N-type material as shown
below.
A PNP Transistor Configuration
Note: Conventional current flow.
The PNP Transistor has very similar characteristics to their NPN bipolar
cousins, except that the polarities (or biasing) of the current and
vol***e directions are reversed for any one of the possible three
configurations looked at in the first tutorial, Common ****, Common
Emitter and Common Collector. Generally, PNP Transistors require a
negative (-ve) vol***e at their Collector terminal with the flow of
current through the emitter-collector terminals being Holes as opposed
to Electrons for the NPN types. Because the movement of holes across the
depletion layer tends to be slower than for electrons, PNP transistors
are generally more slower than their *****alent NPN counterparts when
operating.
To cause the **** current to flow in a PNP transistor the **** needs to
be more negative than the Emitter (current must leave the ****) by
approx 0.7 volts for a silicon device or 0.3 volts for a germanium
device with the formulas used to calculate the **** resistor, ****
current or Collector current are the same as those used for an
*****alent NPN transistor and is given as.
[size=21]Generally, the PNP transistor can replace NPN
transistors in electronic circuits, the only difference is the
polarities of the vol***es, and the directions of the current flow. PNP
Transistors can also be used as switching devices and an example of a
PNP transistor switch is shown below.
A PNP Transistor Circuit
The Output Characteristics Curves for a PNP transistor look very similar
to those for an *****alent NPN transistor except that they are rotated
by 180o to take account of the reverse polarity vol***es and currents,
(the currents flowing out of the **** and Collector in a PNP transistor
are negative).
Transistor Matching
You may think what is the point of having a PNP Transistor, when there
are plenty of NPN Transistors available?. Well, having two different
types of transistors PNP & NPN, can be an advan***e when designing
amplifier circuits such as Class B Amplifiers that use "Complementary"
or "Matched Pair" transistors or for reversible H-Bridge motor control
circuits.
A pair of corresponding NPN and PNP transistors with near identical
characteristics to each other are called Complementary Transistors for
example, a TIP3055 (NPN), TIP2955 (PNP) are good examples of
complementary or matched pair silicon power transistors.
They have a DC current gain, Beta, (Ic / Ib) matched to within 10% and
high Collector current of about 15A making them suitable for general
motor control or robotic applications.
Identifying the PNP Transistor
We saw in the first tutorial of this Transistors section, that
transistors are basically made up of two Diodes connected together
back-to-back. We can use this analogy to determine whether a transistor
is of the type PNP or NPN by testing its Resistance between the three
different leads, Emitter, **** and Collector.
By testing each pair of transistor leads in both directions will result
in six tests in total with the expected resistance values in Ohm's
given below.
• 1. Emitter-**** Terminals - The Emitter to **** should act like a
normal diode and conduct one way only.
• 2. Collector-**** Terminals - The Collector-**** junction should act
like a normal diode and conduct one way only.
• 3. Emitter-Collector Terminals - The Emitter-Collector should not
conduct in either direction.
Transistor Resistance Values for the PNP
transistor and NPN transistor types
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]. [size=12]The Transistor as a Switch
When used as an AC signal amplifier, the transistors **** biasing
vol***e is applied so that it operates within its "Active" region and
the linear part of the output characteristics curves are used. However,
both the NPN & PNP type bipolar transistors can be made to operate
as an "ON/OFF" type solid state switch for controlling high power
devices such as motors, solenoids or lamps.
If the circuit uses the Transistor as a Switch, then the biasing is
arranged to operate in the output characteristics curves seen previously
in the areas known as the "Saturation" and "Cut-off" regions as shown
below.
Transistor Curves
[center]
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
The pink shaded area at the bottom represents the "Cut-off" region. Here
the operating conditions of the transistor are zero input **** current
(Ib), zero output collector current (Ic) and maximum collector vol***e
(Vce) which results in a large depletion layer and no current flows
through the device.
The transistor is switched "Fully-OFF". The lighter blue area to the
left represents the "Saturation" region. Here the transistor will be
biased so that the maximum amount of **** current is applied, resulting
in maximum collector current flow and minimum collector emitter vol***e
which results in the depletion layer being as small as possible and
maximum current flows through the device.
The transistor is switched "Fully-ON". Then we can summarize this as:
• 1. Cut-off Region - Both junctions are Reverse-biased, **** current is
zero or very small resulting in zero Collector current flowing, the
device is switched fully "OFF".
• 2. Saturation Region - Both junctions are Forward-biased, **** current
is high enough to give a Collector-Emitter vol***e of 0v resulting in
maximum Collector current flowing, the device is switched fully "ON".
An example of an NPN Transistor as a switch being used to operate a
relay is given below.
With inductive loads such as relays or solenoids a flywheel diode is
placed across the load to dissipate the back EMF generated by the
inductive load when the transistor switches "OFF" and so protect the
transistor from damage.
If the load is of a very high current or vol***e nature, such as
motors, heaters etc, then the load current can be controlled via a
suitable relay as shown.
Transistor Switching Circuit
The circuit resembles that of the Common Emitter circuit we looked at in
the previous tutorials. The difference this time is that to operate the
transistor as a switch the transistor needs to be turned either fully
"OFF" (Cut-off) or fully "ON" (Saturated).
An ideal transistor switch would have an infinite resistance when
turned "OFF" resulting in zero current flow and zero resistance when
turned "ON", resulting in maximum current flow.
In practice when turned "OFF", small leakage currents flow through the
transistor and when fully "ON" the device has a low resistance value
causing a small saturation vol***e (Vce) across it.
In both the Cut-off and Saturation regions the power dissipated by the
transistor is at its minimum.
To make the **** current flow, the **** input terminal must be made more
positive than the Emitter by increasing it above the 0.7 volts needed
for a silicon device.
By varying the ****-Emitter vol***e Vbe, the **** current is altered
and which in turn controls the amount of Collector current flowing
through the transistor as previously discussed.
When maximum Collector current flows the transistor is said to be
Saturated.
The value of the **** resistor determines how much input vol***e is
required and corresponding **** current to switch the transistor fully
"ON".
Example No1.
For example, using the transistor values from the previous tutorials of:
β = 200, Ic = 4mA and Ib = 20uA, find the value of the **** resistor
(Rb) required to switch the load "ON" when the input terminal vol***e
exceeds 2.5v.
Example No2.
Again using the same values, find the minimum **** current required to
turn the transistor fully "ON" (Saturated) for a load that requires
200mA of current.
Transistor switches are used for a wide variety of applications such as
interfacing large current or high vol***e devices like motors, relays or
lamps to low vol***e digital logic IC's or gates like AND Gates or OR
Gates.
Here, the output from a digital logic gate is only +5v but the device
to be controlled may require a 12 or even 24 volts supply.
Or the load such as a DC Motor may need to have its speed controlled
using a series of pulses (Pulse Width Modulation) and transistor
switches will allow us to do this faster and more easily than with
conventional mechanical switches.
Digital Logic Transistor Switch
The **** resistor, Rb is required to limit the output current of the
logic gate.
Darlington Transistors
Sometimes the DC current gain of the bipolar transistor is too low to
directly switch the load current or vol***e, so multiple switching
transistors are used.
Here, one small input transistor is used to switch "ON" or "OFF" a much
larger current handling output transistor.
To maximise the signal gain the two transistors are connected in a
"Complementary Gain Compounding Configuration" or what is generally
called a "Darlington Configuration" where the amplification factor is
the product of the two individual transistors.
Darlington Transistors simply contain two individual bipolar NPN or PNP
type transistors connected together so that the current gain of the
first transistor is multiplied with that of the current gain of the
second transistor to produce a device which acts like a single
transistor with a very high current gain. The overall current gain Beta
(β) or Hfe value of a Darlington device is the product of the two
individual gains of the transistors and is given as:
So Darlington Transistors with very high β values and high Collector
currents are possible compared to a single transistor. An example of the
two basic types of Darlington transistor are given below.
Darlington Transistor Configurations
The above NPN Darlington transistor configuration shows the Collectors
of the two transistors connected together with the Emitter of the first
transistor connected to the **** of the second transistor therefore, the
Emitter current of the first transistor becomes the **** current of the
second transistor.
The first or "input" transistor receives an input signal, amplifies it
and uses it to drive the second or "output" transistors which amplifies
it again resulting in a very high current gain.
As well as its high increased current and vol***e switching
capabilities, another advan***e of a Darlington transistor is in its
high switching speeds making them ideal for use in Inverter circuits and
DC motor or stepper motor control applications.
One difference to consider when using Darlington transistors over the
conventional single bipolar transistor type is that the ****-Emitter
input vol***e Vbe needs to be higher at approx 1.4v for silicon devices,
due to the series connection of the two PN junctions.
Then to summarise when using a Transistor as a Switch
• Transistor switches can be used to switch and
control lamps, relays or even motors.
• When using bipolar transistors as switches
they must be fully "OFF" or fully "ON".
• Transistors that are fully "ON" are said to
be in their Saturation region.
• Transistors that are fully "OFF" are said to
be in their Cut-off region.
• In a transistor switch a small **** current
controls a much larger Collector current.
• When using transistors to switch inductive
relay loads a "Flywheel Diode" is required.
• When large currents or vol***es need to be
controlled, Darlington Transistors are used
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