TRANSISTORS AND BIASING
What is a transistor
• Transistors are semiconductor devices that act as either
electrically controlled switches or amplifier controls.
• The beauty of transistors is the way they can control
electric current flow in a manner similar to the way a
faucet controls the flow of water.
• With a faucet, the flow of water is controlled by a control
knob. With a transistor, a small voltage and/or current
applied to a control lead acts to control a larger electric
flow through its other two leads.
Application
• Transistors are used in almost every electric circuit you
can imagine.
• switching circuits,
• amplifier circuits,
• oscillator circuits,
• current source circuits,
• voltage-regulator circuits,
• power-supply circuits,
• digital logic ICs, and
• Any circuit that uses small control signals to control larger currents.
Design and functionality
• Transistors come in a variety of designs
• come with unique control and current flow features.
• Most transistors have a variable current-control feature,
but a few do not.
• Some transistors are normally off until a voltage is applied
to the base or gate, whereas others are normally on until
a voltage is applied.
• Also, on can represent a variable amount of current flow.
• Some transistors require both a small current and a small
voltage applied to their control lead to function, whereas
others only require a voltage.
• Some transistors require a negative voltage and/or output
current at their base lead to function, whereas others
require a
• positive voltage and/or input current at their base.
Families
• The two major families of transistors exist
• bipolar transistors and
• field-effect transistors (FETs).
• The major difference between these two families is that:
• bipolar transistors require a biasing input (or output) current at their
control leads,
• FETs require only a voltage practically no current.
• Bipolar transistors require both positive (holes) and negative
(electrons) carriers to operate, FETs only require one charge carrier
• Because FETs draw little or no current, they have high
input impedances
• FETs are definitely more popular in circuit design today
than bipolar transistors due to
• They draw essentially zero input-output current at their control
leads
• they are easier to manufacture
• cheaper to make (require less silicon)
• and can be made extremely small—making them useful elements
in integrated circuits.
Transistor types
BJT TRANSISTORS AND BIASING
BJTs
• Bipolar junction transistors(BJT) are three-terminal
devices that act as electrically controlled switches or as
amplifier controls
• BJTs come in two configurations
BJT Configuration
NPN configuration
An npn bipolar transistor
uses a small input current
and positive voltage at its
base (relative to its emitter)
to control a much larger
collector-to-emitter current.
PNP Configuration
pnp transistor uses a small
output base current and
negative base voltage
(relative its emitter) to
control a larger emittertocollector current.
Applications of BJTs
Their ability to control current flow by means of applied
control signals makes them essential elements in
• Electrically controlled switching circuits, current-regulator
circuits,
• voltage-regulator circuits,
• amplifier circuits,
• oscillator circuits,
• and memory circuits
How BJTs Work
I-V characteristic curve for a BJTt.
• The effects the base current IB and the emitter-to-collector
voltage VEC have on the emitter/collector currents IE and
IC.
Regions of operation
• Saturation : maximum collector current flows and the
transistor acts much like a closed switch from collector to
emitter.
• Cutoff region: the transistor acts like an open switch—
only a very small leakage current flows in this mode of
operation.
• Active région: a near-linear relationship exists between
terminal currents (IB, IC, IE).
• Bias and Q-point
Key Notes