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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Semi Conductors Page 2

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5.

Junction Diode as a Rectifier

 

Rectification: The process of conversion of an alternating current (A.C.)into a direct current (D.C.) is called rectification.

Rectifier: A rectifier is a device which is used to convert alternating current (A.C.) to direct current when they are forward-biased and not when they are reverse-biased. Hence they can be used as rectifiers.

Half Wave Rectifier
In a half-way rectifier only one half of each cycle of the A.C. supply is rectified.

Circuit

                                                      Half Wave Rectifier


A.C.I/P
- A.C. Input
P1P2
- Primary of transformer
S1S2
- Secondary of transformer
D
- Junction diode
RL
- Load resistance
O/P
- output

  • Transformer: The a.c. input is applied across the primary coil of the transformer (P1P2).
  • The secondary coil (S1S2) is connected to the junction diode and the load resistance RL.
  • Load Resistance: The d.c. voltage obtained across the load resistance RL is the output.

Working

  • During the positive half cycle of the a.c. input, S1 is positive and S2 is negative. The diode is now forward-biased and hence conducting. A current IL flows through RL.
  • During the negative half cycle, the polarity gets reversed. The diode now gets reverse-biased and stops conducting.
  • No current will flow during this half cycle through the load resistance and hence no output voltage is developed across it.
  • The nature of the input and output voltages is shown in the diagrams.
  • The output is intermittent and unidirectional.

 

Input Wave Form

 

Output Wave Form

Full Wave Rectifier

A rectifier which rectifies both half cycles of the a.c. supply is called a full wave rectifier. Two
p-n junction diodes and a transformer whose secondary coil has a centre-tap are used in a full wave rectifier.

A.C.I/P - A.C. Input
P1P2
- Primary coil of transformer
S1S2
- Secondary coil of transformer
C
- Centre tap of transformer
D1, D2
- Junction diodes
RL
- load resistance
O/P
- output

Transformer: The a.c. unit is applied across the primary coil (P1P2) of a transformer whose secondary coil (S1S2) has a centre tap C. The ends of the secondary coil are connected to the
p-side of two p-n junction diodes.

Load Resistance: The load resistance RL is connected between the common points of the
n-side of two p-n junction diodes and the centre tap C of the transformer. The d.c. output is measured across the load resistance RL.

Working

  • Due to the alternating input, the polarities of the terminals S1 and S2 of the secondary coil keep changing alternately with respect to the polarity of the centre tap C.
  • During the first half cycle, let S1 positive with respect to C while S2 negative. Thus diode D1 is forward-biased and conducting while diode D2 will be reverse-biased and non-conducting.
  • The current IL through the load resistance RL is from A to B.
  • During the next half cycle, S2 is positive with respect to the centre tap while S1 is negative.
    Diode D1 is now reverse-biased while diode D2 is forward-biased.
  • Diode D2 now conducts current. The current through RL flows again from A to B.
  • Thus, though the input current is alternating, the output current IL flowing through RL is always in the same direction, i.e. from A to B.
  • The output voltage is hence direct and full wave rectification is obtained.
  • The nature of the input and output voltage is shown in the diagram.

 

Input Wave Form

 

Output Wave Form

 

 

6.

Transistor

 

A transistor is a semiconductor device which functions like a vacuum triode, but does not require a filament or a vacuum envelope.

There are two types of transistors: a)
p-n-p type and b) n-p-n type.

 

p-n-p Transistor

 

n-p-n Transistor


p-n-p Transistor: Here a layer of n-type semiconductor is sandwiched between two layers of p-type semiconductor.

n-p-n Transistor : Here a layer of p-type semiconductor is sandwiched between two layers of n-type.

  • In both types, the central layer (which is very thin) and lightly doped is called the base.
  • The layer at one end is called the emitter which the layer at the other end is called the collector.
  • The emitter corresponds to the cathode of a triode valve while the collector corresponds to the plate.
  • A transistor can be considered as consisting of two p-n junctions connected back to back.
  • Normally, the emitter-base junction is forward-biased, while the collector-base junction is reverse-biased.

Symbols Used to Represent Transistors:

 

n-p-n Type

 

p-n-p Type

 

 

7.

Working of a Transistor (n-p-n Common Base-type Transistor)

 

  • The base-emitter (B-E) junction is forward-biased while the collector-base (C-B) junction is reverse-biased.
  • Since the B-E junction is forward-biased, electrons from the emitter flow towards the base, producing the emitter current, IE.
  • Since the base layer (p-type) is very thin and lightly doped, most of the electrons flow into the collector region constituting the collector current, IC.
  • A few electrons recombine with holes in the base giving rise to a small base current IB.
  • Thus IE = IC + IB IC since IB << IC.
  • The E-B junction forms the input circuit. This offers very low resistance since it is forward-biased.
  • The C-B junction forms the output circuit. This offers resistance since it is reverse-biased.
  • A small change in the input signal fed at the E-B junction causes a large change in the output signal.

Diagram

 

 

8.

Transistor as an Amplifier

 

A transistor which is used as an amplifier can be connected in any one of three configurations.

a. Common base (C-B)
b. Common emitter (C-E)
c. Common collector (C-C)

The common emitter amplifier is widely used because it provides greater power gain than other two configurations.
Further, for the CE configuration all three gains (current gain, voltage gain and power gain) are greater than unity.

Common-Emitter Amplifier: (
n-p-n Transistor)
Principle:
A small change DIB in base current produces a comparatively larger change DIC in the collector current.

n-p-n Transistor Used in CE Configuration

  • VBB is the voltage applied across the emitter-base junction, keeping it forward-biased.
  • The a.c. input signal (VS) is connected in series with VBB.
  • The signal voltage VS is superimposed on VBB.
  • VCC is the voltage across the base collector junction, keeping it reverse-biased.
  • The load resistance RL is connected across the collector and emitter in series with VCC.
  • The output is obtained across RL.

Working

  • During the positive half cycle of the input a.c. signal, more electrons flow from the emitter to the collector through the base since the forward bias across the emitter base junction is greater.
  • Thus the base current and hence the collector current increases.
  • This causes a larger voltage drop across RL
  • Similarly, voltage drop across RL decreases during the negative half cycle of the input signal.
  • Thus, small changes in the applied signal cause changes in the base current which produces corresponding changes in the collector current.
  • As the collector current is comparatively large, the changes in the voltage across RL are also large.
  • Thus an amplified voltage of the input signal is obtained as the output.

Remember!

  1. The emitter to base circuit is always forward-biased.
  2. The base to collector circuit is always reverse-biased.
  3. For n-p-n transistor, emitter must be negative w.r. to base and collector must be positive w.r.t. base.
  4. For p-n-p transistor, the emitter must be positive w.r. to base and collector must be negative w.r.t. base.
  5.  

Current gain Ai =  

DIC

DIB

  1.  

Current gain Av =  

DVC

DVB

  1. Power gain = Ap = Av.Ai

 

 

9.

Advantages of Semiconductor Devices Over Vacuum Tubes

 

  1. A vacuum tube device needs a high degree of vacuum and there is a danger of leakage of air. No vacuum is needed in a semiconductor device.
  2. A vacuum tube device has a filament which has to be heated to a high temperature. In the case of a semiconductor there is no filament and hence no loss of energy in the form of heat.
  3. A vacuum tube device requires a high voltage for its operation while a semiconductor device operates at a low voltage.
  4. A vacuum tube takes some time to start functioning as its filament requires time to heat up to a high temperature. A semiconductor device starts functioning as soon as its circuit is closed.
  5. Vacuum tubes are more expensive than semiconductor devices.
  6. Semiconductors devices are lighter, smaller and sturdier than vacuum devices.
  7. A semiconductor device has a much longer life than a vacuum device.
  8. A semiconductor device is more efficient than a vacuum tube device and its power consumption is much less.

 

 

 

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