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Bipolar Transistor Review
•Transistors are three terminal devices that
acts like electrically controlled switches or
amplifier controls.
•It has either NPN or PNP 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
•Conversely, a pnp transistor uses a small output base current and negative
base voltage (relative to its emitter) to control a larger emitter-to-collector
current.
Bipolar Transistor Review
Types of Bipolar Transistor
Used to amplify low
level signals. Gain is
10 to 500. IC max 80
to 600ma, operating
frequency 1 to
300Mhz
Used to amplify high
frequency signals.
Gain is 10 to 500 IC
max 10 to 600mA,
max operating
frequency 2000Mhz
Two transistors in one
which has more
stability at high current
levels. Gain is much
larger than a single
transistor
Used primarily as
switches. Gain is 10 to
200. Max IC rating 10 to
1000mA. Max switching
rates between 10 to
2000MhZ
Used in high power
amplifiers or power
supplies. They have heat
sinks. Power rating 10 to
300W. Gain is 10 to 500 IC
max 1 to 100 A, operating
frequency 1 to 100Mhz
Light sensitive
transistors. When
base exposed to light
it flows a base current
which operates the
transistor.
Bipolar Transistor Review
•An npn bipolar transistor is made by sandwiching
a thin slice of p-type semiconductor between two
n-type semiconductors.
•When no voltage is applied at the transistor base
electrons in the emitter are prevented from passing
to the collector side because of the depletion region
at the p-n junction.
Bipolar Transistor Review
•When a positive voltage is applied to the base of an
npn transistor, the pn junction between the base and
emitter is forward biased.
•Electron enters into the very thin base region from
the emitter.
•This process is a trick by which most of the
electrons are driven into the collector region and
hence heavy current flows from emitter to collector
region.
•The amount of emitter-to-collector current can be
varied by varying the base current.
Bipolar Transistor Review
•The figure shows a typical characteristic curve for a bipolar transistor.
•It describes the effects the base current IB and the emitter-to-collector
voltage VEC have on the emitter/collect currents IE and IC.
•IE is practically equal to IC
Bipolar Transistor Review
•Saturation Region: It is the region of operation where the maximum collector
current flows and the transistor works like a closed switch from collector to
emitter.
•Cutoff Region: It refers to the region of operation
near the voltages axis of the collector characteristic
curve where the transistor acts like an open switch.
Only a very small leakage current flows in this mode
of operation.
•Active mode/region: It describes transistor operation
in the region to the right of saturation and above cutoff,
where a near-linear relationship exists between terminal
currents (IB, IC, IE).
•Bias: Bias refers to the specific dc terminal voltages and current of the
transistor to set a desired point of active-mode operation or quiescent point (Qpoint)
Bipolar Transistor Review
• Formulae
Bipolar Transistor Review
RC
RB
Relay Driver
+12V
RB
120Vac