Test: The Transistor As An Amplifier & Transistor Construction


20 Questions MCQ Test Electronic Devices | Test: The Transistor As An Amplifier & Transistor Construction


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QUESTION: 1

The emitter current consist of_________

Solution:

The emitter current consists of two parts. It consists of hole current IpE constituted by holes. The other part is that it consists the electron current InE constituted by electrons.

QUESTION: 2

The total emitter current (IE) is given by_________

Solution:

The total emitter current is the sum of InE and IpE. In commercial transistors, the doping of emitter region is made much heavier than base. Hence current by majority charge carriers InE is negligible when compared to current by minority charge carriers IpE.

QUESTION: 3

A common base transistor amplifier has an input resistance of 20Ω and output resistance of 100kΩ. If a signal of 400mV is applied between emitter and base, find the voltage amplification. Assume αac to be one.

Solution:

IE = V/R=400M/20=20mA
IC=αIE= 1*20mA=20mA. VO=IC*RL=20m*1k=20V
Amplification, A= VO/signal voltage=20V/400m=50.

QUESTION: 4

The amplification factor for a transistor is given by_________

Solution:

One of the most important application of a transistor is an amplifier. A small change in signal voltage produces an appreciable change in emitter current because the input circuit has low resistance (α=∆IC/IE).

QUESTION: 5

Why is the silicon mostly chosen when compared to germanium?

Solution:

The normal working temperature of germanium is approximately 70°C .The normal working temperature of silicon is approximately 150°C. The other advantages of using a silicon material are, it has a smaller ICBO and its variations are smaller with temperature.

QUESTION: 6

The change in output voltage across the load resistor for a transistor during amplification is_________

Solution:

A small change of voltage ∆Vi between emitter and base causes a relatively large emitter current change ∆IE. We define by the symbol α that fraction of this current change which is collected and passes through RL.

QUESTION: 7

A transistor has an IC of 100mA and IB of 0.5mA. What is the value of αdc?

Solution:

Emitter current IE=IC+IB=100+0.5=100.5mA.
αdc=IC/IE=100/100.5=0.995.

QUESTION: 8

 A germanium transistor used as an amplifier has a collector cut off current ICBO=10µA at a temperature 27°C and β=50. What is the collector current when the base current is 0.25mA?

Solution:

 IC=βIB+(1+β)ICBO
IC=50*0.25/1000+51/100000=13.01mA.

QUESTION: 9

In a PNP germanium transistor, the cut in voltage is about_________

Solution:

The cut in voltage of germanium is lower than that of silicon. If both germanium and silicon are in parallel, Ge starts conducting earlier and stops silicon from conducting.

QUESTION: 10

 In a PNP transistor operating in active region, the main stream of current is_________

Solution:

The emitter-base junction is forward biased while collector-base junction is reversed biased. The transistor now operates in active region. Here, it can be used for amplification purpose.

QUESTION: 11

Which gas is used to fill the chamber in the grown junction type transistor construction?

Solution:

In the process of transistor construction, a crucible is placed in the chamber. This chamber consists of hydrogen or nitrogen. These gases help in the prevention of oxidation. It also contains purified Ge or Si at a temperature few degrees above its melting point.

QUESTION: 12

In a grown junction type construction, the method used form a junction transistor is_________

Solution:

The grown junction may be formed by suddenly varying the rate of pulling the seed crystal from the melt. This method is based on the fact that proportion in which N and P type impurities crystallise i.e.., enter the grown crystal depends on the rate of pulling.

QUESTION: 13

Which of the following methods take impurity variation method for transistor construction?

Solution:

 In impurity variation method, the impurity content of the semiconductor is altered in its type as well as the quantity. For example, in making NPN germanium grown junction transistor, a small type of N type impurity is added to molten germanium and the crystal growth is started.

QUESTION: 14

Which of the following is true about grown junction type construction?

Solution:

This method is based on the fact that proportion in which N and P type impurities crystallise i.e.., enter the grown crystal depends on the rate of pulling. If the pulling rate is small, a P type crystal is grown. If the pulling rate is fast, an N type crystal is grown.

QUESTION: 15

What is the melting point of indium in alloy type transistors?

Solution:

This is similar to soldering and PNP transistor is generally is made by this process. In this method, first of all N type germanium is obtained. The N type wafer and indium dots are placed in a furnace and heated to about 500°C.

QUESTION: 16

The non rectifying base contact is made from_________

Solution:

Leads for emitter and collector are soldered to the dots making non rectifying contacts. Further, non rectifying base contact is usually made from a welding a strip or loop of gold plated wire to the base plate.

QUESTION: 17

What is the thickness of wafer in the alloy type transistors?

Solution:

The wafer of crystal has a 3-5m inch thickness and 80m inch square. This is placed in a graphite jig with a dot of prepared indium. One dot of an indium is 3 times larger than the other.

QUESTION: 18

The larger dot of the indium is used as_________

Solution:

The wafer is placed in a graphite jig with a dot of prepared indium. One dot of an indium is 3 times larger than the other. Finally the larger dot is used as collector. The smaller dot is used as emitter.

QUESTION: 19

The electrical properties of a transistor in alloy type construction is determined by_________

Solution:

Large area collector junction helps in collecting most of the holes emitted from the emitter ensuring that the collector current almost equals the emitter current. The spacing between two junctions inside germanium wafer is very small and determines the electrical properties.

QUESTION: 20

The grown junction type transistors is generally used for_________

Solution:

Grown junction type transistors are manufactured through growing single large crystal which is slowly pulled from the melt in crystal growing furnace. This is generally used for NPN transistors.

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