1st PUC Electronics Question Bank Chapter 7 Bipolar Junction Transistor

Karnataka 1st PUC Electronics Question Bank Chapter 7 Bipolar Junction Transistor

One Mark Questions and Answers

Question 1.
What is a transistor?
Answer:
The transistor is a three-terminal semiconductor device with two p-n junctions which can amplify weak electrical signals and it can also be used as a switch.

Question 2.
Write one important application of a transistor.
Answer:
Amplification of weak ac signal.

Question 3.
What is an npn transistor?
Answer:
npn transistor is a semiconductor device that has a layer of p type material sandwiched between two layers of n-type materials.

Question 4.
Draw the symbols of (i) npn transistor and (ii) pnp transistor
Answer:
(i) npn transistor

(ii) pnp transistor

Question 5.
Why is a transistor called a transfer resistor device?
Answer:
A transistor is called a transfer resistor device because it transfers charge carriers from a low resistance region to a high resistance region.

Question 6.
How many p-n junctions a transistor has?
Answer:
Two

Question 7.
What is an unbiased transistor?
Answer:
A transistor without an external voltage (or biasing) applied is called an unbiased transistor.

Question 8.
What is a biased transistor?
Answer:
A transistor with an external voltage applied is called a biased transistor.

Question 9.
Mention (i) heavily doped region (ii) lightly doped regions of a transistor.
Answer:

1. Emitter
2. Base.

Question 10.
Write the relation between the current components of a transistor.
Answer:
I_E = I_B + I_C

Question 11.
What does the arrow in the circuit symbol of transistor indicate?
Answer:
The arrow is the symbol of the transistor indicates the direction of flow of conventional current.

Question 12.
In which mode of operation the transistor can be used as an amplifier?
Answer:
Active mode.

Question 13.
In which modes of operation the transistor can be used as a switch?
Answer:
Saturation (closed switch) and cutoff (open switch)

Question 14.
What is an optocoupler?
Answer:
It is a device that connects two electrical circuits by means of an optical interface.

Question 15.
Write the regions of the transistor in the decreasing size of their area.
Answer:
Collector, emitter, base.

Question 16.
Mention the regions of the transistor in the increasing doping levels.
Answer:
Base, collector, emitter.

Question 17.
Which are the (i) largest and (ii) smallest currents of a transistor?
Answer:

1. Emitter current
2. Base current.

Question 18.
How are emitter-base and collector-base junctions biased in forwarding active mode?
Answer:
Emitter base junction is forward biased and collector-base junction is reverse biased.

Question 19.
Why npn transistors are often used?
Answer:
The majority of charge carriers in npn transistors are free electrons which have more mobility compared to the holes. Hence npn transistors are often used.

Two Mark Questions and Answers

Question 1.
Why is the collector of a transistor made larger?
Answer:
The collector of a transistor is made larger to dissipate the heat produced due to the flow of current.

Question 2.
Distinguish between αdc and βdc.
Answer:
β_dc is the DC current gain which is the ratio collector current to emitter current in CB mode. β_dc is the decurrent gain in CE mode which is the ratio of collection current to base current.
α_dc = \(\frac{\mathrm{I}_{\mathrm{C}}}{\mathrm{I}_{\mathrm{E}}}\) and β_dc = \(\frac{\mathrm{I}_{\mathrm{C}}}{\mathrm{I}_{\mathrm{B}}}\)

Question 3.
Distinguish between cutoff region and saturation region.
Answer:
The region of the output characteristics below I_B = OA is known as cutoff region . In cut off region, both emitter base and collector base junctions are reverse biased.

The region of output characteristics between V_CE = O and knee voltage is called saturation region. In this region, both emitter base and collector base junctions are forward biased.

Question 4.
What is a phototransistor? Draw its symbol.
Answer:
A photo transistor is a three terminal optoelectronics device.

Question 5.
Write any two application of phototransistors.
Answer:

1. optocouplers
2. fire alarms
3. Counting systems
4. Lighting controls in high ways.

Question 6.
Mention lightly doped and moderately doped regions of a transistor.
Answer:
Lighty doped region of transistor is base and moderately doped region is collector.

Question 7.
What is an IR transistor?
Answer:
IR transistor is a phototransistor which is sensitive to the infrared radiation.

Question 8.
Write any two applications of an IR transistor.
Answer:
IR transistors are used in

1. Remote controls.
2. Counting machines.

Question 9.
Which is the smallest of all the three currents in a transistor?
Answer:
Base current (I_B).

Question 10.
How are emitter base junction and collector base junctions biased under normal operation?
Answer:
Emitter base junction is forward biased and collector base junction is reverse biased.

Question 11.
What are saturation and cutoff regions?
Answer:
Saturation region is the region of output characteristics between V_CE = O and knee voltage. Cut off region is the region of the output characteristics below I_B = O.

Five Mark Questions and Answers

Question 1.
Explain the working of an npn transistor.

Answer:
The emitter base junction is forward biased by power supply V_EE and the collector base junction is reverse biased by the power supply V_CC.

The negative terminal of V_EE repels the majority charge carriers electrons in the n type emitter towards the base region. When V_EE is greater than the barrier potential of emitter base junction the electrons from emitter region move to thep type base region. This forms the emitter current L.

The base is thin and has only few electrons. Hence only few electrons and holes recombine and this forms the base current I_E. The remaining free electrons in the base region move to collector region attracted by the +V_CC. They move through the collector region into +V_CC, This forms the collector current I_C.

The relation connecting the currents in the transistor is I_E = I_B + I_C

Question 2.
Explain the different types of transistor configurations.
Answer:
The different types of transistor configurations are.
1. Common base configuration:
In this mode, base terminal is made common to both the input and output circuits. The input is applied between emitter and base terminals and the output is taken across collector and base terminals. CB configuration is used in high frequency amplifiers.

The current gain in CB mode is α = \(\frac{\text { output current }}{\text { input current }}\)
α = \(\frac{\mathrm{I}_{\mathrm{C}}}{\mathrm{I}_{\mathrm{E}}}\)

2. Common emitter configuration:
In this mode, emitter is made common to both input and output circuits. The input is applied between base and emitter terminals and the output is taken between collector and emitter terminals. CE configuration of the transistor is used in amplifier circuits.

Current gain in CE configuration is β = \(\frac{\mathrm{I}_{\mathrm{C}}}{\mathrm{I}_{\mathrm{E}}}\)

3. Common collector Configuration:
In this mode, collector is mode common to both the input and output circuits. The input is applied between base and colleqtor terminals and the output is taken from emitter and collector terminals.

CC configuration is used buffer amplifier and for impedance matching circuits.

Question 3.
Explain CE mode input and output characteristics of a npn transistor. Distinguish between the cutoff, active and saturation regions of a transistor.
Answer:

The circuit to plot the CE characteristics of the transistor is as shown in the diagram.

The two types of characteristics are :

1. Input characteristics.
2. Output characteristics.

Input Characteristics:
It gives the relationship between input voltage VBE and the input current IB, when the output voltage V_CE is kept constant. V_BE is varied and corresponding IB values are noted keeping V_CE constant. The input characteristics curve is similar to that of a forward biased semiconductor diode as emitter base junctioni s a pn junction.

Input resistance R_m = \(\frac{\Delta \mathrm{V}_{\mathrm{BE}}}{\Delta \mathrm{I}_{\mathrm{B}}}\)

CE output characteristics
It is a graph of output collector current Ic against output collector to emitter voltage V_CE for a constant input base current I_B.

From the graph, it is clear that the output current Ic depends upon input current I_B and V_CE has no control over I_C.
Output resistance, R_out = \(\frac{\Delta V_{\mathrm{CE}}}{\Delta \mathrm{I}_{\mathrm{C}}}\)

Question 5.
Draw the output characteristics of a transistor in CE configuration. Show different regions of operation. What are biasing requirements for the transistor to operate in these regions..
Answer:

Saturation Region: The region of output characteristics between V_CE = O and knee voltage V_K is called saturation region. Both emitter base and collector base junctions are forward biased in this region . With the increase in V_CE, collection current Ic increases.

Active Region: Active region is the region of the output characteristics to the right of knee voltage and above I_B = O. The emitter base junction is forward biased and the collector base junction is reverse biased. In amplifiers, the transistor is operated in the active region.

Cut off Region : It is the region of the output characteristics below I_B = O. Both emitter base junction and collector base junction are reverse biased in this region.

1st PUC Electronics Question Bank with Answers