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Unit II : BJT Biasing
Introduction, Transistor, construction, transistor
Operations, BJT characteristics, load line, operating
point, Necessity of BJT biasing, Transistor biasing
methods, Stability factor, Thermal stabilization,
Thermal runaway and Compensation circuits,
Transistor as an Amplifier
INTRODUCTION
A bipolar junction transistor (BJT) is a three terminal device in which operation depends
on the interaction of both majority and minority carriers and hence the name bipolar.
The BJT is analogues to vacuum triode and is comparatively smaller in size. It is used in
amplifier and oscillator circuits, and as a switch in digital circuits. It has wide
applications in computers, satellites and other modern communication systems.
CONSTRUCTION OF BJT AND ITS SYMBOLS
The Bipolar Transistor basic construction consists of two PN-junctions producing three
connecting terminals with each terminal being given a name to identify it from the other
two. These three terminals are known and labeled as the Emitter ( E ), the Base ( B ) and
the Collector ( C ) respectively. There are two basic types of bipolar transistor
construction, PNP and NPN, which basically describes the physical arrangement of the
P-type and N-type semiconductor materials from which they are made.
Transistors are three terminal active devices made from different semiconductor
materials that can act as either an insulator or a conductor by the application of a small
signal voltage. The transistor's ability to change between these two states enables it to
have two basic functions: "switching" (digital electronics) or "amplification" (analogue
electronics). Then bipolar transistors have the ability to operate within three different
regions:
1. Active Region - the transistor operates as an amplifier and Ic = β.Ib
2. Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
3. Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0
Bipolar Transistors are current regulating devices that control the amount of current
flowing through them in proportion to the amount of biasing voltage applied to their base
terminal acting like a current-controlled switch. The principle of operation of the two
transistor types PNP and NPN, is exactly the same the only difference being in their
biasing and the polarity of the power supply for each type(fig 1).
TRANSISTOR CURRENT COMPONENTS:
FIG 2
The above fig 2 shows the various current components, which flow across the forward biased emitter junction and reverse- biased collector junction. The emitter current IE consists of hole current IPE (holes crossing from emitter into base) and electron current InE (electrons crossing from base into emitter).The ratio of hole to electron currents, IpE / InE , crossing the emitter junction is proportional to the ratio of the conductivity of the p material to that of the n material. In a transistor, the doping of that of the emitter is made much larger than the doping of the base. This feature ensures (in p-n-p transistor) that the emitter current consists an almost entirely of holes. Such a situation is desired since the current which results from electrons crossing the emitter junction from base to emitter does not contribute carriers, which can reach the collector.
Not all the holes crossing the emitter junction JE reach the the collector junction JC Because some of them combine with the electrons in n-type base. If IpC is hole current at junction JC there must be a bulk recombination current ( IPE- IpC ) leaving the base.
Actually, electrons enter the base region through the base lead to supply those charges, which have been lost by recombination with the holes injected in to the base across JE. If the emitter were open circuited so that IE=0 then IpC would be zero. Under these circumstances, the base and collector current IC would equal the reverse saturation current ICO. If IE≠0 then IC= ICO- IpC
For a p-n-p transistor, ICO consists of holes moving across JC from left to right (base to collector) and electrons crossing JC in opposite direction. Assumed referenced direction for ICO i.e. from right to left, then for a p-n-p transistor, ICO is negative. For an n-p-n transistor, ICO is positive.The basic operation will be described using the pnp transistor. The operation of the pnp transistor is exactly the same if the roles played by the electron and hole are interchanged.
One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.
Forward-biased junction of a pnp transistor
Reverse-biased junction of a pnp transistor
Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated.
E
C CO I
( I I )
Since IC and IE have opposite signs, then α, as defined, is always positive. Typically numerical values of α lies in the range of 0.90 to 0.
E
pE nE
pC E
pC
I
I
I
I
I
I *
The transistor alpha is the product of the transport factor and the emitter efficiency. This statement assumes that the collector multiplication ratio *is unity. *is the ratio of total current crossing JC to hole arriving at the junction.
Bipolar Transistor Configurations
As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.
- Common Base Configuration - has Voltage Gain but no Current Gain.
2 Common Emitter Configuration - has both Current and Voltage Gain.
- Common Collector Configuration - has Current Gain but no Voltage Gain.
COMMON-BASE CONFIGURATION
Common-base terminology is derived from the fact that the : base is common to both input and output of t configuration. base is usually the terminal closest to or at ground potential. Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material. All current directions will refer to conventional (hole) flow and the arrows in all electronic symbols have a direction defined by this convention.
Note that the applied biasing (voltage sources) are such as to establish current in the direction indicated for each branch.
To describe the behavior of common-base amplifiers requires two set of characteristics:
- Input or driving point characteristics.
- Output or collector characteristics
The output characteristics has 3 basic regions:
Active region – defined by the biasing arrangements Cutoff region – region where the collector current is 0A Saturation region- region of the characteristics to the left of VCB = 0V
For ac situations where the point of operation moves on the characteristics curve, an ac alpha
defined by αac
Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector. The value of is typical from 0.9 ~ 0.998.
Biasing: Proper biasing CB configuration in active region by approximation IC IE (IB 0 uA)
TRANSISTOR AS AN AMPLIFIER
Common-Emitter Configuration
It is called common-emitter configuration since : emitter is common or reference to both input and output terminals. emitter is usually the terminal closest to or at ground potential. Almost amplifier design is using connection of CE due to the high gain for current and voltage.
Two set of characteristics are necessary to describe the behavior for CE ;input (base terminal) and output (collector terminal) parameters.
Proper Biasing common-emitter configuration in active region
IB is microamperes compared to miliamperes of IC.
IB will flow when VBE > 0.7V for silicon and 0.3V for germanium
Before this value IB is very small and no IB.
Base-emitter junction is forward bias Increasing VCE will reduce IB for different values.
The ratio of dc collector current (IC) to the dc base current (IB) is dc beta ( dc ) which is dc current gain where IC and IB are determined at a particular operating point, Q-point (quiescent point). It’s define by the following equation:
30 < dc < 300 2N
On data sheet, (^) dc= hfe with h is derived from ac hybrid equivalent cct. FE are derived from forward-current amplification and common-emitter configuration respectively.
For ac conditions, an ac beta has been defined as the changes of collector current (IC) compared to the changes of base current (IB) where IC and IB are determined at operating point. On data sheet, (^) ac= hfe It can defined by the following equation:
From output characteristics of commonemitter configuration, find ac and dc with an
Operating point at IB=25 A and VCE =7.5V
For the common-collector configuration, the output characteristics are a plot of IE vs VCE for a range of values of IB.
Limits of opearation
Many BJT transistor used as an amplifier. Thus it is important to notice the limits of operations.At least 3 maximum values is mentioned in data sheet.
There are:
a) Maximum power dissipation at collector: PCmax or PD
b) Maximum collector-emitter voltage: VCEmax sometimes named as VBR(CEO) or VCEO.
c) Maximum collector current: ICmax
There are few rules that need to be followed for BJT transistor used as an amplifier. The rules are: transistor need to be operate in active region!
IC < ICmax
PC < PCmax
Note: VCE is at maximum and IC is at minimum (ICMAX=ICEO) in the cutoff region. IC is at maximum and VCE is at minimum (VCE max = Vcesat = VCEO) in the saturation region. The transistor operates in the active region between saturation and cutoff.
TRANSISTOR BIASING AND
STABILIZATION
NEED FOR TRANSISTOR BIASING:
If the o/p signal must be a faithful reproduction of the i/p signal, the transistor
must be operated in active region. That means an operating point has to be established in
this region. To establish an operating point (proper values of collector current Ic and
collector to emitter voltage VCE) appropriate supply voltages and resistances must be
suitably chosen in the ckt. This process of selecting proper supply voltages and
resistance for obtaining desired operating point or Q point is called as biasing and the ckt
used for transistor biasing is called as biasing ckt.
There are four conditions to be met by a transistor so that it acts as a faithful
ampr:
1) Emitter base junction must be forward biased (VBE=0.7Vfor Si, 0.2V for Ge) and
collector base junction must be reverse biased for all levels of i/p signal.
2) Vce voltage should not fall below VCE (sat) (0.3V for Si, 0.1V for Ge) for any part
of the i/p signal. For VCE less than VCE (sat) the collector base junction is not
probably reverse biased.
3) The value of the signal Ic when no signal is applied should be at least equal to the
max. collector current t due to signal alone.
4) Max. rating of the transistor Ic(max), VCE (max) and PD(max) should not be exceeded at
any value of i/p signal.
Consider the fig shown in fig1. If operating point is selected at A, A represents a
condition when no bias is applied to the transistor i.e, Ic=0, VCE =0. It does not satisfy the
above said conditions necessary for faithful amplification.
Point C is too close to PD(max) curve of the transistor. Therefore the o/p voltage swing
in the positive direction is limited.
Point B is located in the middle of active region .It will allow both positive and
negative half cycles in the o/p signal. It also provides linear gain and larger possible o/p
voltages and currents
Hence operating point for a transistor amplifier is selected to be in the middle of
active region.
fig
DC LOAD LINE:
Referring to the biasing circuit of fig 4.2a, the values of VCC and RC are fixed and Ic
and VCE are dependent on RB.
Applying Kirchhoff’s voltage law to the collector circuit in fig. 4.2a, we get
PD(max)
PD(max)
Vce(sat)
PD(max)
IC(max)
PD(max)
