BJT - Globarena

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BIJUNCTION TRANSISTOR
C
B
E
Transistor
 A transistor is a semiconductor device used to
amplify and switch electronic signals.
 A transistor is made up of three layers – an ‘n’
layer sandwiched between two ‘p’ layers or a ‘p’
layer between two ‘n’ layers.
 Doping of each layer is different and that is what
is responsible for the operation(amplification).
The BJT – Bipolar Junction Transistor
The Two Types of BJT Transistors:
P N P
N P N
E
n
p
n
C
C
Cross Section
B
E
p
n
C
Cross Section
B
B
Schematic
Symbol
Click to view NPN
Transistor
C
p
B
E
Schematic
Symbol
 Collector is moderately doped
 Base is lightly doped
 Emitter is heavily doped
E
Click to view PNP
Transistor
BJT Relationships - Equations
IE
-
E
VCE
IE
IC
+
C
-
VBE
VBC
IB
+
+
E
+
-
C
+
+
VEB
B
pnp
IE = I B + I C
VEC = VEB - VCB
VCB
IB
-
B
npn
IE = I B + I C
VCE = -VBC + VBE
VEC
IC
DC
and DC 
= Common-emitter current gain
 = Common-base current gain
= IC
 = IC
IB
IE
The relationships between the two parameters are:
=
=

+1
1-
Note:  and are sometimes referred to as dc
and dc because the relationships being dealt with
in the BJT are DC.
Modes of Operation
Active:
Most important mode of operation
Central to amplifier operation
Emitter –Base junction Forward biased
and Collector –base Reverse Biased
Saturation: Barrier potential of the junctions cancel
each other out causing a virtual short
Ideal transistor behaves like an closed
switch
Both junction are Forward biased
Cutoff:
Current reduced to zero
Ideal transistor behaves like an open
switch
Both junction are Reverse biased
Three Types of BJT Configurations
Biasing the transistor refers to applying
voltage to get the transistor to achieve
certain operating conditions.
Common-Base Biasing (CB)
input = VEB & IE
output = VCB & IC
Common-Emitter Biasing (CE)
input = VBE & IB
output = VCE & IC
Common-Collector Biasing (CC)
input = VBC & IB
output = VEC & IE
BJT Transconductance Curve
Typical NPN Transistor
Collector Current:
IC
I C   T I ES e
8 mA
6 mA
4 mA
2 mA
0.7 V
VBE
V BE
 VT
Transconductance:
(slope of the curve)
gm = IC /  VBE
IES = The reverse saturation
current of the B-E Junction.
VT = kT/q = 26 mV (@
T=300K)
 = the emission coefficient
and is usually
1
Common-Base
Circuit Diagram: NPN Transistor
The Table Below lists assumptions
that can be made for the attributes
of the common-base biased circuit in
the different regions of operation.
Given for a Silicon NPN transistor.
Region of
Operation
IC
VCE
VBE
VCB
C-B
Bias
E-B
Bias
Active
βIB
=VBE+VCE
0.7V
0V
Rev.
Fwd.
Saturation
Max
0V
0.7V
-0.7V<VCE<0
Fwd.
Fwd.
Cutoff
0
=VBE+VCE
0V
Rev.
None/
Rev.
0V
Common-Base input characteristics
Input characteristics for the CB configuration gives
relation between the input quantities, input voltage VEB
and input current IE for fixed VCB values
The input circuit in CB configuration involves the
emitter-base diode, which is forward biased in active
region. Therefore, the relationship between VEB and IE
is nothing but the forward characteristics of a diode
Common-Base input characteristics
In the above characteristics, VCB = Open represents
the characteristics of the forward biased emitter
With increase of VCB, the curves shift downwards i.e.,
we get the same IE with less VEB. This is because, from
the early effect increases the IE increases with VEB held
constant
Common-Base out put characteristics
Although the Common-Base configuration is not the most common
biasing type, it is often helpful in the understanding of how the
BJT works.
Emitter-Current Curves
Circuit Diagram
IC
VCC
VCE
Common-Emitter
Collector-Current Curves
IC
+
_
Active
Region
IB
Region of
Operation
Description
Active
Small base current
controls a large
collector current
Saturation
VCE(sat) 0.2V, VCE
increases with IC
Cutoff
Achieved by reducing IB to
0, Ideally, IC will also equal
0.
IB
VCE
Saturation Region
Cutoff Region
IB = 0
Common Emitter Input Characteristics
The input quantities for C.E. configuration are base
current IB and base emitter voltage VBE
The input characteristics curves are in between IB and
VBE for various values of collector to emitter voltage VCE
If VCE = 0 and if the base-emitter junction is forward
biased, the input characteristics is the same as the
characteristics of forward biased diode
If VCE is increased then VCB increases
By applying KVL around the transistor

VB E - VC E + V C B = 0
 V C E = V C B + VB E
If VCE is increased then VCB increases
Common Emitter Input Characteristics
Increase in VCB leads to decrease in effective base
width WB| due to early effect, resulting in decrease
of recombination and consequently, decrease in
base current due to recombination.
VC E   VC B

 IB

 T h e cu rv e s m o v e rig h t sid e as V C E in cre as e s
Common Emitter output Characteristics
The output quantities in C.E. configuration are IC and
VCE the o/p characteristics gives a relationship between
IC and VCE with base current IB as a parameter.
This family of curves may be divided into three regions
those are active region, saturation region and cutoff
region.
Common-Collector
It is often called an
emitter follower since its
output is taken from the
emitter resistor.
Is useful as an
impedance matching
device since its input
impedance is much
higher than its output
impedance.
It is also termed a
"buffer" for this reason
and is used in digital
circuits with basic gates.
Common-Collector
The CommonEmitter-Current Curves
collector biasing circuit
IE
is basically equivalent
to the common-emitter
Active
biased circuit except
Region
instead of looking at IC
IB
as a function of VCE
and IB we are looking
at IE.
VCE
Also, since 
1,
and
Saturation
Cutoff Region
  = IC/IE that means Region
IB = 0
IC
IE
Common collector input Characteristics
Input Characteristics
As VCB increases according to early effect base width
decreases and IB decreases.
Common collector output Characteristics
Output Characteristics:
The common-collector circuit is basically same as
the common-emitter, with the exception that the
load resistor is in the emitter circuit, the output
characteristics are similar to that of CE
configuration. It is because IE IC
Transistor as amplifier
Transistor amplifies
current as well as
voltage and is a current
operated device.
The CE configuration
is widely used as it
amplifies current and
voltage unlike the other
configurations.
Click to view Image
COMPARISION of CB, CE AND CC PARAMETERS
PARAMETER
Input impedance
(Ri)
Output Impedance
(Ro)
D.C current gain
CB
CE
CC
VC B
VEB
IE
VC B
IC
(Lo w )
(H ig h )
10
VB E
IB
VC B
100k
IC
 
IC
IE
1
 =
IC
IB
(M o d erate )
(M od erate )
1K
IB
(H ig h )
VEC
10K
IE
H ig h valu e (5 0 -5 0 0 )
Moderate
 =
IE
IB
100K
(Low )
10 
(5 1 -5 0 1 )
very low
Voltage gain
Very Large
Applications
For High Frequency
For Audio frequency
For impedance
matching
Phase relationship
between i/p and o/p
In- phase
Out-of phase
In- phase
1
Transistor summary

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