What is a BJT?
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BJT is a bipolar junction transistor. BJT have three terminal. Base, emitter and collector are the three terminals of BJT. BJT is a current controlled device. NPN and PNP are the two types of BJT.
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Bipolar Junction Transistor (BJT):

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BJT is a Bipolar device. (Uses both electrons and holes charge carrier for current conduction)
Two Junctions (Emitter junction & collector junction)
Transfer-Resistor (Transistor) (Amplify by transferring signal from lower resistance to higher resistance region)
BJT is minority carrier device.
Three terminal device
Emitter (More doped) (Smaller area)
Base (Thin & lightly doped)
Collector (Less doped)(Larger area)
Two type semiconductor
N-type and P-type
N-type (Electrons are carrier)
P-type (Holes are carrier)
NPN and PNP
Basic function is to amplify current
Use as amplifiers and switches
BJT is a current controlled device.
The input impedance of BJT is low.
Small base current.
Emitter (Emits electrons)
Collector (Collects electrons)
Current gain β = Ic/Ib
The drive circuit of BJT is complex.
Switching speed of BJT is in the range of microseconds means slow switching speed.
BJT is the negative temperature coefficient device.
Conduction losses are low.
Switching losses are high for high frequency.
Applications: computer, mobile phones, audio amplifiers, radio etc.
Active mode (Current amplification)
Cutoff mode (Open switch)
Saturation mode (Close switch)
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