EELE 2321 – Electronics
Spring, 2013
Bipolar Junction Transistor (BJT) Structure
Eng. Wazen M. Shbair
Content


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Bipolar Junction Transistor (BJT)
BJT parameters and characteristics
Collector Characteristic Curves.
The BJT as Amplifier.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Bipolar Junction Transistor
 The BJT is constructed with three doped
semiconductor regions separated by two pn
junctions
 Three regions are called emitter, base, and
collector.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Operation
 BJT to operate properly as an amplifier, the two
pn junctions must be correctly biased with
external dc voltages.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Transistor Currents
 The directions of the currents in an npn transistor and its
schematic symbol.
 These diagrams show that the emitter current (IE) is the
sum of the collector current (IC) and the base current
(IB)
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT parameters and characteristics
 The dc current gain of a transistor is the ratio of the dc
collector current (IC) to the dc base current (IB) and is
designated dc βDC.
 The ratio of the dc collector current (IC) to the dc emitter
current (IE) is the dc alpha αDC
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Circuit Analysis
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Circuit Analysis
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Example
VBE= 0.7 v
IB=430 MA
IC=64.5 mA
IE=64.9
VCE=3.55v
VCB=2.85 v
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
BJT Characteristics
The collector characteristic curves show the relationship
of the three transistor currents.
IC
The curve shown is for a fixed
based current. The first region is
the saturation region.
As VCE is increased, IC increases
until B. Then it flattens in region
between points B and C, which
is the active region.
After C, is the breakdown
region.
Electronic Devices, 9th edition
Thomas L. Floyd
Breakdown
region
B
Active region
C
Saturation
region
A
0
0.7 V
VCE(max)
VCE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
BJT Characteristics
The collector characteristic curves illustrate the relationship of the
I
three transistor currents.
By setting up other values of
I
base current, a family of
I
collector curves is developed.
I
C
B6
B5
B4
bDC is the ratio of collector
current to base current.
I
b DC  C
IB
It can be read from the curves.
The value of bDC is nearly the
same wherever it is read.
Electronic Devices, 9th edition
Thomas L. Floyd
I B3
I B2
I B1
Cutoff region
0
IB = 0
VCE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
BJT Characteristics
What is the bDC for the transistor shown?
I C (mA)
Choose a base current near the
center of the range – in this
case IB3 which is 30 mA.
Read the corresponding
collector current – in this case,
5.0 mA. Calculate the ratio:
b DC
I
5.0 mA
 C 
 167
I B 30 m A
Electronic Devices, 9th edition
Thomas L. Floyd
10.0
IB6 = 60 mA
IB5 = 50 mA
8.0
IB4 = 40 mA
6.0
I B3 = 30 mA
4.0
IB2 = 20 mA
2.0
IB1 = 10 mA
IB = 0
0
VCE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
Cutoff
In a BJT, cutoff is the condition in which there is no base
current, which results in only an extremely small leakage
current (ICEO) in the collector circuit. For practical work, this
current is assumed to be zero.
RC
In cutoff, neither the base-emitter
junction, nor the base-collector
junction are forward-biased.
Electronic Devices, 9th edition
Thomas L. Floyd
RB
+
ICEO
VCE ≅ VCC
IB = 0
–
+
–
VCC
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
Saturation
In a BJT, saturation is the condition in which there is
maximum collector current. The saturation current is
determined by the external circuit (VCC and RC in this case)
because the collector-emitter voltage is minimum (≈ 0.2 V)
–
In saturation, an increase of base
current has no effect on the
collector circuit and the relation
IC = bDCIB is no longer valid.
Electronic Devices, 9th edition
Thomas L. Floyd
RC
+
IC
RB
+
VCE = VCC – IC RC
+
VBB
IB
–
+
–
VCC
–
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Collector Characteristic Curves
DC Load Line
RC
What is the saturation current and
the cutoff voltage for the circuit?
Assume VCE = 0.2 V in saturation.
ISAT 
βDC = 200
+
V BB
3V –
220 kW
–
VCC
15 V
VCO  VCC  15 V
3.0 V  0.7 V
 10.45 m A
220 kW
IC = b IB = 200 (10.45 mA) = 2.09 mA
Electronic Devices, 9th edition
Thomas L. Floyd
+
RB
VCC  0.2 V 15 V  0.2 V

 4.48 mA
RC
3.3 kW
Is the transistor saturated? I B 
3.3 kW
Since IC < ISAT, it is not saturated.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT as a voltage amplifier
 Amplification is the process of linearly increasing
the amplitude of an electrical signal.
 a BJT exhibits current gain (called b). When a
BJT is biased in the active (or linear) region, the
BE junction has a low resistance due to forward
bias and the BC junction has a high resistance
due to reverse bias.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT as a voltage amplifier
DC and AC Quantities
The text uses capital letters for both AC and DC currents and voltages
with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE.
(The second letter in the subscript indicates the reference point.)
AC Quantities and time varying signals use lower case italic
subscripts. Example: Vce.
Internal transistor resistances are indicated as lower case
quantities with a prime and an appropriate subscript. Example: re’.
External resistances are indicated as capital R with either a
capital or lower case subscript depending on if it is a DC or ac
resistance. Examples: RC and Rc.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Voltage Amplification
 A transistor amplifies current because the
collector current is equal to the base current
multiplied by the current gain, β.
 The base current in a transistor is very small
compared to the collector and emitter currents.
 The collector current is approximately equal to
the emitter current.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Voltage Amplification
A BJT amplifies AC signals by converting some of the DC power from
the power supplies to AC signal power. An ac signal at the input is
superimposed in the dc bias by the capacitive coupling. The output ac
signal is inverted and rides on a dc level of VCE.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Voltage Amplification
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Voltage Amplification
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Switch
A BJT can be used as a switching device in logic circuits to turn on or
off current to a load. As a switch, the transistor is normally in either
cutoff (load is OFF) or saturation (load is ON).
+ VCC
RC
RB
IC = 0
RC
RC
RB
C
+VBB
0V
IB = 0
E
In cutoff, the transistor
looks like an open switch.
Electronic Devices, 9th edition
Thomas L. Floyd
+VCC
+VCC
IB
+VCC
IC(sat)
IC(sat)
RC
C
+
–
E
In saturation, the transistor
looks like a closed switch.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
 Example 4.10
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
 End Of Chapter 4.
 Home work for Charter 3&4 will be
announced on MOODEL.
 Next Monday(4/3/2013)
we will have Quiz on Chapter 4.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.