BJT Key Point

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BJT Most Important Notes


A BJT is abbreviated as bipolar Junction Transistor. The term bipolar reflects the fact that both electrons and holes participate in the injection process into the oppositely polarized material.
A transistor is a three layer semiconductor device consisting of either two N and one P layer of material or two P and one N type layer of material, the former is called an NPN transistor while the later is called PNP transistor. A transistor is similar to two diodes connected back to back.
Applications :
Presently the transistors are used in:
(1) High speed computers
(2) Vehicles
(3) Satellites
(iv) Communication systems
(v) Power systems
(vi) Switching
(vii) EH


Transistor has basically three section of doped semiconductor. These sections are known as emitter, base and

(1) Emitter

It is a region on one side of transistor which supplies the charge carriers (electrons or holes) to
the other region. The emitter is always forward biased w.rit. base, so that it can supply a larger number of majority
carriers. The width of emitter is moderate i.e. wider than base but smaller than collector. It is heavily doped among
the three regions.

(ii) Base

It is middle region that forms two junctions. The base emitter junction is forward biased allowing
low resistance for the emitter circuit. The base collector junction is reverse biased and provides high resistance to the
collector circuit. It is the thinnest layer and is most lightly doped among the three regions.

(iii) Collector

It is a region situated in the other side of transistor which collects charge carriers. Collector
junction is always reverse biased. Its function is to remove charges from its junction with the base. Collector is larger
than both emitter and base. Doping of collector is less than doping of emitter but higher than doping of base.
Following points about the transistor should be noted

BJT Most Important Key point

  • Common base configuration has very good high frequency response.
  • The base of a transistor is thinnest and most lightly doped.
  • The doping of emitter is highest followed by collector and then base.
  • The width of the layer is highest in collector followed by emitter and then base.
  • Transistor is operated in active region by making emitter-base junction forward biased and collector-
  • base junction reverse biased.
  • Transistor is operated in saturation region by making both the junctions forward biased.
  • Transistor is operated in cut-off region by making both the junctions reverse biased.
  • Voltage divider bias circuit is the most stable biasing circuit.
  • Common emitter configuration yields the highest power gain.
  • The inverted operation of a transistor is achieved by making the emitter junction reverse biased and
  • collector junction forward biased.
  • The input impedance of an amplifier cannot be measured with an ohmmeter.
  • The output impedance of an amplifier is measured with the input signal set to zero. It cannot be
  • measured with an ohmmeter.
  • For all transistor amplifiers, the no load gain is always greater than the loaded gain.
  • The gain from source to load is always reduced by the internal resistance of the source
  • CE with emitter bias is equivalent to scries feedback configuration.
  • CB configuration is useful at high frequencies. It is also used as a constant current source
  • CC configuration finds wide application as a buffer stage between a high impedance source and a low
  • impedance load
  • The CE fixed bias configuration have significant voltage gain, its input impedance is relatively low.
  • The voltage divider bias configuration has a higher stability than the fixed bias configuration. It has
  • about the same voltage gain, current gain and output impedance. Due to biasing resistors, its input
  • impedance is lower than fixed bias configuration
  • The CE emitter bias configuration with an unbypassed emitter resistor has larger input resistance than
  • the bypassed configuration. It has much smaller voltage gain than the bypassed configuration.
  • The emitter follower configuration will always have an output voltage slightly less than the input signal,
  • Its input impedance is very large. Its output impedance is extremely low.
  • The collector feedback configuration has an input impedance that is sensitive to beta and is quite low.
  • The collector DC feedback configuration uses the DC feedback to increase the stability.
  • RC coupling of multistage amplifier is most widely used as compared to transformer coupled or direct
  • coupled amplifiers
  • RC coupled amplifier has wide frequency response.
  • RC coupled amplifier provides less frequency distortion.
  • Transformer coupled amplifier has higher voltage gain than RC coupled amplifier.
  • Transformer coupled amplifier provides excellent impedance matching.
  • Direct coupled amplifier is used for low frequency signals and DC signals.
  • The frequencies at which the gain drops to 0.707 of the mid band value are called the cutoff, corner,
  • band, break or half power frequencies.
  • Change in frequency by a factor of 2 is equivalent to 1 octave, results in a 6 dB change in gain.
  • Change in frequency by a factor of 10 is equivalent to 1 decade, results in a 20 dB change in gain.
  • A cascade connection is a series connection.
  • For cascade connection, voltage amplification is the product of the stage voltage gains.
  • Cascade connection provides a high input impedance and a low output impedance.
  • Darlington connection provides two transistors connected as one super transistor.
  • Maximum efficiency of Class A amplifier is 25% (without transformer) and 50% (with transformer).
  • Maximum efficiency of Class B amplifier is 78.5%.
  • Maximum efficiency of Class A amplifier is about 100%.
  • Class A conducts for full 360 cycle.
  • In Class B each stage conducts for 180 of the full cycle.
  • In Class AB each stage conducts between 180 and 360.
  • Class C conducts for less than 180 of the full cycle.
  • Class D has operation using digital or pulsed signals.
  • Power amplifiers are non-linear amplifiers.
  • Class A amplifier has least distortion,
  • Class C amplifier consists of tuned circuit.
  • Class B amplifier posses maximum distortion.
  • Compared to single ended amplifier, a push pull amplifier offers more distortion and more output power.
  • Cross over distortion exists in Class B push pull amplifier.
  • Efficiency of Class D amplifier is above 90%.
  • Cross over distortion is due to non-linearity in the input characteristics.

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