Team Quanta gladly presents all possible short questions of Modern Physics & Electronics’ Chapter#06: Basic Electronics.
Q.1.1 Name various types of conductors.
Answer: Conductors are of four types:
- Super conductors
- Good conductors
- Semiconductors
- Bad conductors or insulators
Q.1.2 what is meant by ideal diode? Give one drawback of diode.
Answer: Ideal diode is like a voltage controlled switch. When forward biased it acts like an ON switch and when reverse biased it acts likes an OFF switch.
Drawback of diode is that it cannot as an amplifier.
Q.1.3 what are uses of a diode in physics?
Answer: A diode can be used as demodulator, voltage multiplier, clamping, rectifier, clipping and peak detector.
Q.1.4 what is difference between Fermi electron gas and ordinary gas?
Answer: The difference between Fermi gas and ordinary gas is that electron gas is charged gas constituted by electrons while ordinary gas is natural constituted by atoms or molecules. Particle concentration of electron gas is of order of while that of ordinary gas is atoms or molecules per .
Q.1.5 what is Fermi energy?
Answer: The energy of the highest filled level at 0K is called Fermi energy of a free electron gas,
$$E_F=\frac{\hbar}{2m}\left(3\pi^2\frac{N}{V}\right)^\frac{2}{3}$$
Q.1.6 why is Si preferred to Ge for making semiconductor devices?
Answer: Since barrier potential of Si is higher than that of Ge and it can operate at high temperature, so Si is preferred to Ge for various semiconductor devices.
Q.1.7 Do pure semiconductors obey Ohm’s law?
Answer: Semiconductors have a unique property that their conductivity increases with rise in temperature. In other words, graph between voltage and current is not a straight line. Hence Ohm’s law does not hold for semiconductors.
Q.1.8 Write down various types of diodes.
Answer: Some types of diodes in use are,
- Zener Diode
- Varactor Diode
- Light Emitting Diode
- Tunnel Diode
- Photo Diode
Q.1.9 what are ripples? Define ripple factor.
Answer: Ripples: The ac components contained in the output of a rectifier are known as ripples.
Ripple factor: The ratio of effective value of alternating component of current (or voltage) to the average dc component of output current (or voltage) is known as ripple factor. Ripple factor is given by,
$$\gamma=\frac{\left|\left(a.c\right)\right.}{\left|\left(d.c\right)\right.}$$
Q.1.10 what are advantages of a full wave rectifier over a half wave rectifier?
Answer:
- The dc value of current and voltage in case of a full wave rectifier is double the value of dc current and voltage for a half wave rectifier.
- The dc power delivered to the load resistance by a full wave rectifier is four times the dc power delivered by a half wave rectifier.
- The full wave rectifier has a better voltage regulation as compared to a half wave rectifier.
- The ripple factor of full wave rectifier is very low (0.48) as compared to ripple factor of a half wave rectifier.
- The maximum efficiency of a full wave rectifier (81.2%) is twice the maximum efficiency of a half wave rectifier (40.6%).
Q.1.11 Distinguish an ideal diode from a practical diode.
OR
Differentiate between ideal diode and a practical diode.
OR
In what aspects a practical diode differs from ideal diode.
Answer: An ideal diode is a diode which,
- Conducts with zero resistance when forward biased, and
- Appears as an infinite resistance when reverse biased.
Whereas a practical diode has always some resistance in forward bias and some current although having a small value is present in reverse bias.
Q.1.12 Is the output of half wave or full wave rectifier pure D.C? if not, can it be converted into pure D.C. voltage?
Answer: The output of a rectifier developed across the load resistance is not purely unidirectional voltage but it has some as ripples. These ac ripples can be removed by passing it through a filter or smoothing circuit before connecting it across the load resistance. The smoothing circuit consist of suitable combination of inductors and capacitors.
Q.1.13 Summarize the steps involved in the formation of a pn-junction.
Answer: The processes involved in the formation of a pn-junction are summarized below:
- Holes from p-side diffuse into n-side where they combine with free electrons and free electrons from n-side diffuse into p-side where they combine with holes.
- The diffusion current also known as recombination current decays exponentially both with time and distance from junction.
- Due to departure of free and mobile carriers from both sides of a junction, a depletion region is formed. This region contains only immobile or fixed ions of opposite polarity. These fixed ions set up a potential barrier across the junction.
- The potential barrier opposes the diffusion of majority charge carriers from one side of junction to the other till the process is completely sopped.
- The width of depletion region depends on doping level. for heavy doping, the depletion region is physically thin because a diffusing charge carrier has to travel far across the junction for recombination. Opposite is the case if light doping is used.
Q.1.14 How a p-type semiconductor is formed?
Answer: A p-type material is formed by adding trivalent impurity to intrinsic semiconductor Ge or Si.
Q.1.15 Compare the depletion regions in forward bias and reverse bias.
Answer: The depletion region for reverse bias is much widener than for forward bias.
Q.1.16 why does reverse breakdown occur in diodes?
Answer: Breakdown in reverse bias occurs when reverse voltage equals or exceeds the breakdown voltage of pn-junction of diode.
Q.1.17 what happens to barrier potential when temperature increases?
Answer: When temperature increases, then barrier potential decreases.
Q.1.18 which is greater, the barrier potential or breakdown voltage?
Answer: Breakdown voltage is greater than barrier potential.
Q.1.19 what are intrinsic and extrinsic semiconductors?
Answer: Semiconductors in the pure form having no impurity are called intrinsic semiconductors.
Semiconductors in which some impurity has been added are called extrinsic semiconductors.
Q.1.20 Give a comparison of conductors, insulators and semiconductors.
Answer: A comparison of conductors, insulators and semiconductors is given below:
Conductors:
- In conductors, current can flow easily.
- In conductors, there are free electrons moving randomly, called conduction electrons.
- Examples of conductors are copper, aluminum etc.
Insulators:
- In insulators current cannot flow.
- In insulators, electrons are tightly bound and there are no free electrons.
- Examples of insulators are glass and plastic etc.
Semiconductors:
- Semiconductor materials are intermediate between insulators and conductors.
- Semiconductors have both free electrons and hoes.
- Examples of semiconductors are Si and Ge.
Q.1.21 what is effect of doping and temperature on conductivity of intrinsic semiconductors?
Answer: Effect of doping on conductivity of intrinsic semiconductors: Si and Ge in pure from have very low conductivity. By doping i.e. adding small amount of impurity to Si or Ge, conductivity increases and there is large number of free charge carriers. By adding trivalent impurity, conductivity increases due to increase in holes and by adding pentavalent impurity, conductivity increases due to increases in free electrons.
Effect of temperature on conductivity of intrinsic semiconductors: At absolute zero of temperature electrons are bound in covalent bonds in semiconductors. As temperature increases, resistance decreases because semiconductors have negative temperature coefficient. Hence conductivity of semiconductors increases as temperature increase.
Review Questions
R.1.1 Define peak reverse voltage and average current.
Answer: A particular voltage when applied to junction we have zero current.
R.1.2 How a p-type semiconductor is formed?
Answer: A p-type material is formed by adding trivalent impurity to intrinsic semiconductor Ge or Si.
R.1.3 what is difference intrinsic and extrinsic semiconductors?
Answer: intrinsic semiconductors: An intrinsic semiconductor is one which is made of semiconductor material in its extremely pure form. Common examples of such materials are pure Ge and Si, which have forbidden gaps (energy) of 0.72ev and 1.1ev, respectively.
Extrinsic semiconductors: Those intrinsic semiconductors to which some suitable impurity or doping agent has been added in extremely small amount are called extrinsic semiconductors or impurity semiconductors.
Depending on type of doping material used, extrinsic semiconductors can be subdivided into two classes:
- n-type semiconductors
- p-type semiconductors
R.1.4 By what process are minority carriers produced?
Answer: Minority carriers are thermally produced when electron-hole pairs are generated.
R.1.5 Explain the term potential barrier and how it is created?
Answer: Potential barrier is potential difference of electric field in the depletion region and is amount of energy required to move electrons through the depletion region.
R.1.6 Compare the depletion regions in forward bias and reverse bias.
Answer: The depletion region for reverse bias is much widener than for forward bias.
R.1.7 why does reverse breakdown occur in diodes?
Answer: Breakdown in reverse bias occurs when reverse voltage equals or exceeds the breakdown voltage of pn-junction of diode.
R.1.8 what happens to barrier potential when temperature increases?
Answer: When temperature increases, then barrier potential decreases.
R.1.9 Which is greater, the barrier potential or breakdown voltage?
Answer: Breakdown voltage is greater than barrier potential.
R.1.10 Does an n-type semiconductor material carry a net negative charge? Does a p-type semiconductor carry a net positive charge?
Answer: No, both material n-type and p-type are neutral.
R.1.11 Which type of full wave rectifier has greater output for same input voltage and transformers ratio?
Answer: The bridge rectifier has greater output voltage than full wave rectifier for same input voltage and transformer turns ratio.
Q.2.1 what is meant by term transistor?
Answer: Transistor is made from word from resistor). Thus transistor is a device which gives transfer of resistance. It is designed to make amplifiers.
Q.2.2 Give names of various types of a transistor.
Answer: Transistor are basically of three types:
- Uni-junction transistor (UJT)
- 2-bipolar junction transistor (BJT)
- 3-field effect transistor (FET)
BJT is of two pnp and npn. FET is of three types: Junction field effect transistor (JFET), metal oxide semiconductor field effect transistor (MOSFET) and insulated gate field effect transistor (IGFET).
Q.2.3 Name various types of oscillator.
Answer: Oscillators may be of two types:
- Audio frequency oscillators (AFO)
- Radio frequency oscillators (RFO)
To design an AF oscillator, one requires an RC circuit. RC phase shift oscillator and Wein’s bridge oscillator are familiar AF oscillators.
RF oscillators are LC oscillators, Hartley, Colpitts and Clapp’s crystal oscillators are popular examples of RF oscillators.
Q.2.4 On what factor h – parameters depend?
Answer: h – parameters depend on following factors:
- Transistor type
- Transistor configuration
- Temperature
- Frequency
- Operating point
Q.2.5 what are uses of common base configuration?
Answer: Uses of common base circuit: One of the important use of common base amplifier is in matching a low impedance circuit to a high impedance circuit. It has also high stability of collector current with temperature changes.
Q.2.6 what are properties of common base circuit?
Answer: Characteristics of common base amplifier:
Common base amplifier has:
- A current gain
- Large voltage gain of about 1500
- Power gain up to 30dB
- Very low input resistance
- Very high output resistance
- No phase reversal between input and output voltages.
Q.2.7 Give characteristics of a common emitter amplifier. Also give its uses.
Answer: Characteristics of common emitter amplifier are:
- It produces phase reversal of input signal i.e. input and output signals are out of phase with each other.
- It has very high voltage gain 1500 or so.
- It has moderately low input resistance (I kilo-ohm to 2 kilo-ohm).
- Its output resistance is moderately large.
- Its current gain is high
- It produces very high power gain of order of 40 dB
Uses of common emitter amplifier: Most of transistor amplifiers are of common emitter type because of large gains in voltage, current and power. Moreover, their input and output impedance characteristics are suitable for many applications.
Q.2.8 what are major advantages of common emitter amplifier over other forms?
OR
Point out the merits of using a transistor in common emitter configuration.
Answer: The merits of common emitter amplifier are:
- High current gain
- High voltage and power gain
- Moderate output to input impedance ratio
Q.2.9 what are characteristics of common collector amplifier? Give its uses.
Answer: Characteristics of common collector amplifier:
- It has high output impedance
- It has low input impedance
- It produces no phase reversal between input and output signals.
- It has voltage gain less than one
- It has power gain 10 to 20 dB
- It has high current gain (50-300)
Uses of common collector amplifier:
- It is used for impedance matching i.e. for conducting a circuit having high output impedance to one having low input impedance.
- It is used for switching circuits.
- It is used as two way amplifier since it can pass a signal in either direction.
- It is used for circuit solution.
Q.2.10 Compare common base, common emitter and collector configuration of a transistor.
Answer: The comparison of various characteristics of different configuration is given below:
| characteristics | Common base | Common collector | Common emitter |
| Input resistance | Low | Very high | Low |
| Output resistance | Very high | Low | High |
| Current gain | Slightly less than one | High | High |
| Voltage gain | About 150 | Less than one | About 500 |
| Applications | For high frequency application | For impedance matching | For audio frequency applications |
Q.2.11 what is ac load line? How it can be drawn?
Answer: ac load line: It is line along which Q point shifts up and down when changes in output voltage and current of an amplifier are caused by an ac signal. This line is steeper than a dc line but the two intersect at Q point determined by biasing dc voltages and currents. Ac load line takes into account the ac load resistance and dc load line considers only the dc load resistance. The cut off point for this dc load line is where
$$V_{CE}=V_{CC}$$
It is also written as (cut –off).
Saturation point is given by, $I_C=\frac{V_{CC}}{R}$.
It is also written as, $I_C=\frac{V_{CC}}{R}$ . For ac load line the cut-off point is given by,
$$V_{CE}\ \left(cut-off\right)=V_{CEQ}+I_{CQ}\ R_{ac}$$
Where is load resistance.
Saturation point is given by,
$$I_C\left(sat\right)=I_{CQ}+\frac{V_{CER}}{R_{ac}}\ \rightarrow\left(2.10\right)$$
It is represented by straight line CQD in fig. The slope of ac load line is given by,
$$Y=-\frac{1}{R_{ac}}\ \ \ \ \ \ \rightarrow\left(2.11\right)$$
It is seen from fig 2.36, that maximum positive signal swing is $=I_{CQ}\ R_{ac}$ Similarly maximum possible negative signal swing is $=V_{CEQ}$ . In other words, peak signal handling capacity is limited to $I_{CQ}\ R_{ac}\ \ or\ V_{CEQ}$ whichever is smaller.
Q.2.12 Can two separate pn-junction diodes placed back to back be used to form pnp transistor?
Answer: For a transistor base must be very thin and it must be lightly doped. These two conditions will not be satisfied when we make a transistor by joining two pn-junctions. The thickness of base will be large and doping will be heavy. So pnp transistor cannot be formed by joining two pn-junctions.
Q.2.13 why the base region is thin?
Answer: For normal operation of a transistor, the emitter base junction is forward biased and collector base junction is reversed. The forward biasing of the emitter base junction permits a large number of electrons is an npn transistor to be injected into base. These electrons after entering the base region produced a concentration being greater at the emitter base junction than at the collector base junction. These injected electrons move by diffusion through the base towards the collector junction. While passing through the base region the electrons are lost due to recombination with holes in base region. The base region is therefore, made very thin so that number of electrons lost by recombination is very small and collector current is almost equal to the emitter current.
Q 2.14 what are advantages of a transistor over a triode tube?
Answer: A transistor has following advantages over a triode value:
- Its size is very small and hence circuits using transistors are very compact.
- It operates at a very low voltage and hence consumes a very low power.
- It requires no heating of filament.
- It can withstand mechanical vibrations.
- It is very cheap.
A transistor, however, is very sensitive to temperature, gives low power output and has a high noise level due to a large value of revers current.
Q.2.15 why hybrid parameters are called so?
Answer: The hybrid parameters are has dimensions of ohm, are simple numbers and has dimensions of conductance and measured in mhos. This is why these parameters are known as hybrid parameters i.e. parameters having mixed dimensions.
Q.2.16 On what factors hybrid parameters depend?
Answer: When small ac signals are involved, a transistor behaves like a linear device because its output ac signal varies directly as the input signal. Hence for small ac signals, each transistor has its own characteristic set of h parameters. The h parameters depend on various factors such as,
- Transistor type
- Transistor configuration
- Operating point
- Temperature
- Frequency
The h parameters can be found experimentally or graphically. The h parameters are determined from input characteristics and are found from output characteristics of common emitter transistor.
Q.2.17 what are bias conditions of base emitter and base collector junctions for a transistor to operate as an amplifier?
Answer: For operation of an amplifier, base-emitter junction should be forward biased and base-collector junction is reverse biased.
Q.2.18 At what point on load line does cut off begin? At what point does saturation begin?
Answer: Cut-off occurs at the interaction of load line and $I_B=0$ curve. Saturation begins at intersection of vertical portion of collector curve and load line.
Review Questions
R.2.1 what separates the three regions in a bipolar junction transistor?
Answer: The three regions of a transistor are separated by two pn-junctions.
R.2.2 what are bias conditions of base emitter and base collector junctions for a transistor to operate as an amplifier?
Answer: For operation of an amplifier, base-emitter junction should be forward biased and base-collector junction is reverse biased.
R.2.3 Define . Also give relation between .
Answer: is the ratio of collector to base current. $\alpha_{DC}$ is collector to emitter current ratio. Moreover $\beta_{DC}=h_{FE}.$
R.2.4 Explain variation of with temperature.
Answer: $\mathbit{\beta}_{\mathbit{DC}}$ increases with temperature.
R.2.5 when a transistor is used as a switch? When is collector current maximum?
Answer: A transistor switch operates in cut off and saturation. Collector current is maximum in saturation.
R.2.6 At what point on load line does cut off begin? At what does saturation begin?
Answer: Cut off occurs at the intersection of load line and $I_B=0$ curve. Saturation begins at intersection of vertical portion of collector curve and load line.
R.2.7 Can we interchange emitter and collector of a transistor?
Answer: No, because emitter is heavily doped and collector is moderately doped. Therefore emitter and collector cannot be interchanged.
R.2.8 which of the three currents IB, Ic and IE is largest?
Answer: The emitter current IE is largest from remaining two currents.
R.2.9 Can two separate pn-junction diodes placed back to back be used to form pnp transistor?
Answer: For a transistor base must be very thin and it must be lightly doped. These two conditions will not be satisfied when we make a transistor by joining two pn-junctions. The thickness of base will be large and doping will be heavy. So pnp transistor cannot be formed by joining two pn-junctions.
Q.3.1 what is meant by feedback amplifier?
Answer: A feedback amplifier is that in which a fraction of the amplifier output is feed back to the input circuit. This partial dependence of amplifier input on its output helps to control to output. A feedback amplifier consists of two parts: an amplifier and a feedback circuit.
Q.3.2 what are various forms of negative feedback?
Answer: There are four basic arrangements for using negative feedback.
- Voltage- series feedback
- Voltage-shunt feedback
- Current-series feedback
- Current-shunt feedback
Q.3.3 what is an oscillator?
Answer: An electric device which converts dc energy into ac energy at very high frequency is called oscillator. It is a circuit which generates an ac output signal without requiring any externally applied input signal.
Q.3.4 what are different types of an oscillator?
Answer: Electronic oscillators may be broadly divided into two groups:
- Sinusoidal oscillators.
- Non-sinusoidal (or relaxation) oscillators.
Sinusoidal oscillators may be further subdivided into:
- Tuned circuits or LC feedback oscillators such as feedback oscillators such as Hartely, Colpitts and Clap,etc.
- RC phase shift oscillators such as Wien-bridge oscillator.
- Negative resistance oscillators such as tunnel diode oscillator.
- Crystal oscillators such as Pierce oscillator.
Q.3.5 Give a comparison of an amplifier and oscillator.
Amplifier: An amplifier produces an output signal whose waveform is similar to input signal. It takes energy from dc power source and converts it into ac energy at signal frequency. If there is no input signal, there is no energy conversion and hence there is no output signal.
Amplifier: An oscillator does not require an external signal either to start or maintain energy conversion process. It keeps producing an output signal so long as the dc power source is connected. The frequency of output signal can be varied at will.
Q.3.6 what are advantages of negative feedback?
Answer: Various advantages of negative feedback are:
- Highly stabilized gain.
- High fidelity i.e., more linear operation
- Increased bandwidth i.e., frequency response.
- Reduced noise.
- Less amplitude distortion.
- Input and output impedances can be modified as desired
- Less harmonic distortion.
- Less phase distortion.
- Less frequency distortion.
Q.3.7 what is meant by logic gate?
Answer: A logic gate is an electronic circuit which makes logic decisions. It has one output and one or more inputs. Logic gates are basic building blocks from which most of digital systems are built up.
Q.3.8 what are various logic families?
Answer: Logic gates are available in form of various IC families, some of which are transistor –transistor logic (TTL), emitter-coupled logic (ECL), metal oxide-semiconductor (MOS) and complementary metal oxide semiconductor (CMOS) etc.
Q.3.9 Explain why?
- NOR gate is a universal gate
- NAND gate is a universal gate. OR why NAND & NOR gates are widely used?
Answer: NOR gate as a universal gate: It is important to note that a NOR gate can be realize the basic logic functions: OR, AND and NOT. That is why it is often called a universal gate. Let us see how NOR gate can be used as OR gate, AND gate and NOT gate. The output from NOR gate is . By using another inverter in the output, the final output is inverted and is given by C =A + B, which is logic function of an OR gate.
The function of NOR gate as AND gate. Here two inverters have been used one for each input. Thus the inputs have been inverted before they are applied to the NOR gate. The output is $\overline{\overline{A}+B}$.
By De-Morgan theorem,
$$\overline{\overline{A}+\overline{B}}=A.B$$
So output of this gate is $C=A.B$ which is function of AND gate.
The function of NOR gate as NOT gate. The two inputs have been tied together and output is $\overline{A+A,}$ which by De-Morgan theorem is equal to A, which is logic function of NOT gate.
NAND gate as a universal gate: It is interesting to note that a NAND gate can be used to realize the basic logic functions: OR, and NOT. That is why it is often referred to as a universal gate. Let us see how NAND gate can be used as OR gate, AND gate and NOT gate.
The function of NAND gate as AND gate. The output from first gate is . By using $\overline{A.B}$ another NAND gate in the output, the final output is inverted and is given by C = A.B gate.
Show how OR gate can be made out of three NAND gates. The output is $\overline{A+\overline{B}}$ which is turn by De-Morgan theorem is equal to A+B, same as logic function of OR gate.
Shows that how a NOT gate can be made out of a NAND gate by connecting its two inputs together.
Q.3.10 Differentiate between bi-stable multi vibrator and as-table multi-vibrator.
Answer: Bi-stable Multi-vibrator:
- It has two absolutely stable states.
- It has no energy storage element.
- It is driven type.
As-table Multi-vibrator:
- It has no stable state.
- It has two energy storing elements i.e., two capacitors.
- It is non-driven type.
Q.3.11 Give some uses of Bi-stable Multi-vibrator.
Answer:
- It is used in timing circuits as a frequency divider.
- It is used in coming circuits.
- Bi-stable multi-vibrator circuit is used in computer memory circuits.
Q.3.12 Under what condition does feedback reduce distortion?
Answer: Negative feedback reduces distortion.
Q.3.13 what is digital circuit?
Answer: Digital circuit is an electronic circuit with discrete levels of input and output voltage.
Q.3.14 what is effect of negative feedback on input impedance and output impedance?
Answer: When negative feedback is inserted in series with the amplifier, its input impedance increases. The use of current feedback increase output impedance of amplifier.
Q.3.15 In a-stable multi-vibrator circuit Find frequency of circuit oscillation.
Answer:
$$R_1=R_2=R=10\times{10}^3\mathrm{\Omega},\ C_1=C_2=C=0.01\times{10}^{-6}F$$
We know that
$$f=\frac{0.7}{RC}=\frac{0.7}{10\times{10}^3\times0.01\times{10}^{-6}}=7.00\times{10}^3Hz=7.00\ kHz$$
Review Questions
R.3.1 what is digital circuit?
Answer: Digital circuit is an electronic circuit with discrete levels of inputs and output voltage.
R.3.2 what are uses of LED?
Answer: LED’s are used as:
- Indicator lamps
- Signal lamps
- Displays in computer and calculators
R.3.3 Give any three applications of multi-vibrators.
Answer: Multi-vibrator can be used as:
- Frequency dividers
- Saw tooth generators
- Memory elements in computers
R.3.4 Under what condition does feedback reduces distortion?
Answer: Negative feedback reduces distortion.
R.3.5 what is truth table?
Answer: A truth table is defined as a table which gives output state for all possible input combinations.
R.3.6 Define positive logic gate and negative gate.
Answer: In computing systems, the umber symbols 0 and 1 represent the two possible states of a circuit. It makes no difference if these two states are referred to as ON and OFF, CLOSED and OPEN, HIGH and LOW, PLUS and MINUS, TRUE and FALSE depending on the circumstances.
In positive logic, 1 represents, an ON circuit, a CLOSED switch, a HIGH voltage, a PLUS sign, a TRUE statement consequently, a 0 represents,
An OFF circuit, an OPEN switch, a LOW voltage, a MINUS sign, a FALSE statement. In negative logic, just opposite conditions prevail.
R.3.7 If we input 0 to NOT gate, what is output?
Answer: Output is 1.
