The action of this filter can best be understood by considering the action of L-section filter, formed by L

and C2, upon the triangular output voltage wave from the input capacitor C 1 The charging and
discharging action of input capacitor C1 has already been discussed. The output voltage is roughly the
same as across input capacitor C1 less the dc voltage drop in inductor. The ripples contained in this
output are reduced further by L-section filter. The output voltage of pi-filter falls off rapidly with the
increase in load-current and, therefore, the voltage regulation with this filter is very poor.

SALIENT FEATURES OF L-SECTION AND PI-FILTERS.

1. In pi-filter the dc output voltage is much larger than that can be had from an L-section filter with the
same input voltage.

2.In pi-filter ripples are less in comparison to those in shunt capacitor or L-section filter. So smaller
valued choke is required in a pi-filter in comparison to that required in L-section filter.

3.In pi-filter, the capacitor is to be charged to the peak value hence the rms current in supply
transformer is larger as compared in case of L-section filter.

4.Voltage regulation in case of pi-filter is very poor, as already mentioned. So n-filters are suitable for
fixed loads whereas L-section filters can work satisfactorily with varying loads provided a minimum
current is maintained.

5.In case of a pi-filter PIV is larger than that in case of an L-section filter.

COMPARISON OF FILTERS
1) A capacitor filter provides Vm volts at less load current. But regulation is poor.
2) An Inductor filter gives high ripple voltage for low load currents. It is used for
high load currents
3) L – Section filter gives a ripple factor independent of load current. Voltage
Regulation can be improved by use of bleeder resistance
4) Multiple L – Section filter or π filters give much less ripple than the single L –
Section Filter.
 UNIT III

BIPOLAR JUNCTION TRANSISTOR

3.1 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 as amplifier and oscillator circuits, and as
a switch in digital circuits. It has wide applications in computers, satellites and other modern
communication systems.

3.2 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 labelled 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).
Bipolar Transistor Construction

Fig 3.1 Bipolar Junction Transistor Symbol

The construction and circuit symbols for both the PNP and NPN bipolar transistor are given
above with the arrow in the circuit symbol always showing the direction of "conventional
current flow" between the base terminal and its emitter terminal. The direction of the arrow
always points from the positive P-type region to the negative N-type region for both transistor
types, exactly the same as for the standard diode symbol.
3.3 TRANSISTOR CURRENT COMPONENTS:

Fig 3.2 Bipolar Junction Transistor Current Components

The above fig 3.2 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 do 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 J E. 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.

3.3a Forward-biased junction of a pnp transistor

3.3b Reverse-biased junction of a pnp transistor

 3.3c Both biasing potentials have been applied to a pnp transistor and resulting majority and
minority carrier flows indicated.

Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.

A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is
typically in order of microamperes.

The large number of majority carriers will diffuse across the reverse-biased junction into the p-type
material connected to the collector terminal

Applying KCL to the transistor :

IE = IC + IB

The comprises of two components – the majority and minority carriers

IC = ICmajority + ICOminority

ICO – IC current with emitter terminal open and is called leakage current

Various parameters which relate the current components is given below

Emitter efficiency:

currentofi njectedcar riersatJ E


totalemitt ercurrent

I PE I pE
  
I pE  I nE I nE
 Transport Factor:

injectedca rriercurrentreaching J C
* 
injectedca rrierncurrentatJ E
I pC
* 
I nE

Large signal current gain:

The ratio of the negative of collector current increment to the emitter current change from zero (cut-
off)to IE the large signal current gain of a common base transistor.

 ( I C  I CO )

IE

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 0.995

I pC I pC I pE
  *
IE I nE I E    *

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.

3.4 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.

 1. Common Base Configuration - has Voltage Gain but no Current Gain.

 2 Common Emitter Configuration - has both Current and Voltage Gain.
 3. Common Collector Configuration - has Current Gain but no Voltage Gain.
3.5 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.

Fig 3.4 CB Configuration

To describe the behavior of common-base amplifiers requires two set of characteristics:

1. Input or driving point characteristics.

2. 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

Fig 3.5 CB Input-Output Characteristics

The curves (output characteristics) clearly indicate that a first approximation to the relationship
between IE and IC in the active region is given by

IC ≈IE

Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to beVBE = 0.7V
In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha
= αdc

IC = IE + ICBO

It can then be summarize to IC = IE (ignore ICBO due to small value)

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)

Fig 3.6 CE Configuration

3.6 TRANSISTOR AS AN AMPLIFIER

Fig 3.7 Basic Transistor Amplifier Circuit

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

Fig 3.8 CE Configuration

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.

Fig 3.9a Input characteristics for common-emitter npn transistor

Fig 3.9b Output characteristics for common-emitter npn transistor

For small VCE (VCE < VCESAT, IC increase linearly with increasing of VCE

VCE > VCESAT IC not totally depends on VCE  constant IC

IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC

IB=0 A  ICEO occur.

Noticing the value when IC=0A. There is still some value of current flows.

Beta () or amplification factor

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  2N3904

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 (I C) 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

Relationship analysis between α and β

3.7 COMMON – COLLECTOR CONFIGURATION

Also called emitter-follower (EF). It is called common-emitter configuration since both the signal
source and the load share the collector terminal as a common connection point.The output voltage
is obtained at emitter terminal. The input characteristic of common-collector configuration is
similar with common-emitter. configuration.Common-collector circuit configuration is provided
with the load resistor connected from emitter to ground. It is used primarily for impedance-
matching purpose since it has high input impedance and low output impedance.

Fig 3.10 CC Configuration

For the common-collector configuration, the output characteristics are a plot of IE vs VCE for a range

of values of IB.

Fig 3.11 Output Characteristics of CC Configuration for npn Transistor

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.
Refer to the fig. Example; A derating factor of 2mW/°C indicates the power dissipation is reduced
2mW each degree centigrade increase of temperature.

Step1:

The maximum collector power dissipation,

PD=ICMAX x VCEmax= 18m x 20 = 360 mW

Step 2:

At any point on the characteristics the product of and must be equal to 360 mW.

Ex. 1. If choose ICmax= 5 mA, substitute into the (1), we get

VCEmaxICmax= 360 mW

VCEmax(5 m)=360/5=7.2 V
 Ex.2. If choose VCEmax=18 V, substitute into (1), we get

VCEmaxICmax= 360 mW

(10) ICMAX=360m/18=20 mA

Derating PDmax

PDMAX is usually specified at 25°C.

The higher temperature goes, the less is PDMAX

Example;A derating factor of 2mW/°C indicates the power dissipation is reduced 2mW each degree
centigrade increase of temperature.

BJT HYBRID MODEL

Small signal low frequency transistor Models:

All the transistor amplifiers are two port networks having two voltages and two currents. The positive
directions of voltages and currents are shown in fig. 1.

Fig. 1

A two-port network is represented by four external variables: voltage V1 and current I1 at the input port,
and voltage V2 and current I2 at the output port, so that the two-port network can be treated as a black
box modeled by the relationships between the four variables,V 1,V2, I1,I2 . Out of four variables two can
be selected as are independent variables and two are dependent variables.The dependent variables can
be expressed interns of independent variables. This leads to various two port parameters out of which
the following three are important:

1. Impedance parameters (z-parameters)

2. Admittance parameters (y-parameters)
3. Hybrid parameters (h-parameters)