Bipolar Junction Transistors
Online Resource for ETCH 213
Faculty: B. Allen
Transistor types
NPN Transistor – A thin, highly doped
p-type region (base) is sandwiched between
two n-type regions (emitter and collector).
PNP Transistor – A thin, highly doped
n-type region (base) is sandwiched between
two p-type regions (emitter and collector).
Online Resource for ETCH 213
Faculty: B. Allen
Base, emitter, collector
Base – The region that lies between an emitter
and a collector of a transistor and into which
minority carriers are injected.
Emitter – A transistor region from which
charge carriers are injected into the base.
Collector – A semiconductor region through
which a flow of charge carriers leaves the base
of the transistor.
Online Resource for ETCH 213
Faculty: B. Allen
Bipolar junction transistor (BJT)
types.
Online Resource for ETCH 213
Faculty: B. Allen
Transistor construction and
packaging
Online Resource for ETCH 213
Faculty: B. Allen
Transistor operation
Transistors are basically controlled to operate
as a switch, or are controlled to operate as a
variable resistor.
Online Resource for ETCH 213
Faculty: B. Allen
ON/OFF switching
Online Resource for ETCH 213
Faculty: B. Allen
Transistor’s variable resistor
Transistance – The effect of transferring resistance
Online Resource for ETCH 213
Faculty: B. Allen
Transistor applications
Digital Logic Gate Circuit
Analog Amplifier Circuit
Online Resource for ETCH 213
Faculty: B. Allen
Transistor as a digital logic gate
Online Resource for ETCH 213
Faculty: B. Allen
Transistor as an analog
amplifier
Amplification – Boosting in strength, or increasing
amplitude, of an electronic signal.
Online Resource for ETCH 213
Faculty: B. Allen
Input-output voltage waveforms
Online Resource for ETCH 213
Faculty: B. Allen
Voltage Gain (AV) – The ratio of the output signal
voltage change to the input signal voltage change.
Output Signal Voltage Change – Change in the output
signal voltage in response to a change in the input
signal voltage.
Input Signal Voltage Change – The input voltage
change that causes a corresponding change in the
output voltage.
Online Resource for ETCH 213
Faculty: B. Allen
Correctly biased NPN transistor
circuit
For normal operation, the NPN transistor’s
emitter diode junction is forward biased while
the collector diode or junction is reverse
biased.
Online Resource for ETCH 213
Faculty: B. Allen
Basic switching regulator action
Online Resource for ETCH 213
Faculty: B. Allen
Emitter Current (IE) – The current at the transistor’s
emitter terminal.
Base Current (IB) – The relatively small current at
the transistor’s base terminal.
Collector Current (IC) – The current emerging out
of the transistor’s collector.
IE = IB + IC
IC ≅ I E
Online Resource for ETCH 213
Faculty: B. Allen
Current-controlled transistor
As IB
IC and IE
Online Resource for ETCH 213
Faculty: B. Allen
Operating a transistor in the
active region
Active Operation – When the base-emitter is
forward biased and the base-collector junction
is reverse biased. In this mode, the transistor
is equivalent to a variable resistor between the
collector and the emitter.
Online Resource for ETCH 213
Faculty: B. Allen
Operation in cutoff and
saturation
Cutoff – A transistor is in cutoff when the
bias voltage is reduced to a point that it stops
current in the transistor.
Saturation – A transistor is in saturation when
the bias voltage is increased to such a point
that further increase will not cause any
increase in the current through the transistor.
Online Resource for ETCH 213
Faculty: B. Allen
Biasing PNP bipolar transistors
Use negative base voltage.
Use negative collector voltage.
Online Resource for ETCH 213
Faculty: B. Allen
Bipolar transistor configurations
Configurations – Different circuit interconnections.
Common – Shared by two or more services, circuits,
or devices. Although the term “common ground” is
frequently used to describe two or more connections
sharing a common ground, the term common alone
does not indicate a ground connection, only a share
connection.
Online Resource for ETCH 213
Faculty: B. Allen
A correctly biased PNP
transistor circuit
Online Resource for ETCH 213
Faculty: B. Allen
Common-emitter circuits
Common-Emitter (C-E) Circuits – A configuration in which
the input signal is applied between the base and the emitter
while the output signal appears between the transistor’s
collector and emitter.
Current Gain – The increase in current produced by the
transistor circuit.
Beta (β) – The transistor’s current gain in a common-emitter
configuration.
DC Beta (βDC) – The ratio of a transistor’s dc output current to
its input current.
Online Resource for ETCH 213
Faculty: B. Allen
Common-emitter circuits (cont.)
AC Beta (βAC) – The ratio of a transistor’s ac output current
to its input current
Voltage Gain:
∆Vout
AV =
∆Vin
Power Gain:
Pout
AP =
Pin
AP = AV × β AC
Online Resource for ETCH 213
Faculty: B. Allen
Online Resource for ETCH 213
Faculty: B. Allen
Common-emitter circuits (cont.)
Active Region – Flat part of the collector’s
characteristic curve. A transistor is normally operated
in this region, where it is equivalent to a variable
resistor between the collector and emitter.
Quiescent Operating Point (Q-point) – The voltage or
current values that set up the no input signal or
operating point bias voltage.
Online Resource for ETCH 213
Faculty: B. Allen
Common-emitter circuits (cont.)
Breakdown Region – The point at which the
collector supply voltage will cause a damaging
value of current through the transistor.
Saturation region – The point at which the collector
supply voltage has the transistor operating in
saturation.
Cutoff region – The point at which the collector
supply voltage has the transistor operating in cutoff.
Online Resource for ETCH 213
Faculty: B. Allen
Common-emitter circuits (cont.)
Input Resistance (Rin) – The amount of opposition offered to
an input signal by the input base-emitter junction (emitter
diode).
Input Impedance (Zin) – The total opposition offered by the
transistor to an input signal.
Output Resistance (Rout) – The amount of opposition offered to
an output signal by the output base-collector junction
(collector diode).
Output Impedance(Zout) – The total opposition offered by the
transistor to the output signal.
Online Resource for ETCH 213
Faculty: B. Allen
Common-Base (C-B)
configuration
Common-Base Configuration – A configuration in which the
input signal is applied between the transistor’s emitter and
base, while the output is developed across the transistor’s
collector and base.
Online Resource for ETCH 213
Faculty: B. Allen
Common-Base configuration
(continued)
DC Alpha (αDC) – The ratio for calculation a C-B transistor’s
dc current gain.
I
α DC =
C
IE
AC Alpha (αAC) – The ratio of the input emitter current
change to the output collector current change in a C-B
transistor
∆I
α AC =
C
∆I E
Power gain in a C-B transistor configuration
AP = AV × αAC
Online Resource for ETCH 213
Faculty: B. Allen
Common-Collector (C-C)
configuration
A configuration in which the input signal is applied between the
transistor’s base and collector, while the output is developed
across the transistor’s collector and emitter.
Online Resource for ETCH 213
Faculty: B. Allen
C-C (continued)
DC current gain = IE/IB
DC current gain = 1 + βDC
Since βDC = IC/IB
Online Resource for ETCH 213
Faculty: B. Allen
C-C (continued)
C-C dc current gain
≅
C-E dc current gain (βDC)
AC current gain = ∆I E
∆I B
C-C ac current gain
≅
C-E ac current gain (βAC)
Online Resource for ETCH 213
Faculty: B. Allen
Degenerative effect
An effect that causes a reduction in the amplification due to
negative feedback.
Emitter-Follower or Voltage-Follower – The C-C circuit in
which the emitter output voltage seems to track or follow the
phase and amplitude of the input voltage.
Online Resource for ETCH 213
Faculty: B. Allen
C-C as an impedance matching
circuit
Impedance Matching Circuit – A circuit that can
match, or isolate, a high resistance (low current)
source.
Buffer Current Amplifier – The C-C circuit that can
ensure that power is efficiently transferred from the
source to the load.
Online Resource for ETCH 213
Faculty: B. Allen
Bipolar data sheet
Online Resource for ETCH 213
Faculty: B. Allen
Testing bipolar junction
transistors
Transistor Tester – A special test instrument
that can be used to test both NPN and PNP
bipolar transistors.
Online Resource for ETCH 213
Faculty: B. Allen
Using an Ohmmeter to test a
BJT
Online Resource for ETCH 213
Faculty: B. Allen
Bipolar transistor biasing circuits
Base biasing
Voltage divider biasing
Online Resource for ETCH 213
Faculty: B. Allen
Base biasing
Base Biasing – A transistor biasing method in which the dc
supply voltage is applied to the base of the transistor via a base
bias resistor
VBE = 0.7V
Online Resource for ETCH 213
Faculty: B. Allen
DC load line
DC Load Line – A line representing
all the DC operating points of a
transistor for a given load resistance.
Online Resource for ETCH 213
Faculty: B. Allen
Typical dc load line with
Q point
Online Resource for ETCH 213
Faculty: B. Allen
Voltage divider biasing
A biasing method used with amplifiers
in which a series arrangement of two
fixed-value resistors is connected
across the voltage source. The result is
that a desired fraction of the total
voltage is obtained at the center of the
two resistors and is used to bias the
amplifier.
Online Resource for ETCH 213
Faculty: B. Allen
End of Bipolar Junction Transistors
Online Resource for ETCH 213
Faculty: B. Allen