Transistors MCQ Quiz - Objective Question with Answer for Transistors - Download Free PDF

The correct option is 3

Concept:

Power Amplifier:

• Power amplifiers deal with high voltage amplification (in Volts range instead of the normal mV range). generating a lot of heat.
• Since metals are good thermal conductors, the transistors used in them are mounted on the metallic plate as they help radiate the heat dissipated from transistors.
• Power amplifiers are also having low output impedance to match the load.
• The common collector or emitter follower circuit is normally used as the power amplifier because it has a low o/p impedance. 
• Due to the use of heat sinks and large-size power transistors, the power amplifiers become bulky.
• A transformer may also be used for impedance matching on the o/p side.​

Concept:

Bipolar junction transistor

• The Bipolar Junction Transistor (BJT) is a three-terminal device i.e. Base, Emitter, and Collector.
• There are two main types of bipolar junction transistors the NPN and the PNP transistor.
• The emitter is a heavily doped region of the BJT transistor, providing the majority of carriers into the base region.
• The base region is a thin, lightly doped region sandwiched between the emitter and collector.
• The majority of carriers from the emitter pass through the base region and its flow can be externally controlled.

Working configuration of BJT

Concept:

The schematic of the PUT is shown below:

The schematic of the thyristor is shown as:

It has a four-layered construction just like the thyristors and has three terminals named anode(A), cathode(K), and gate(G) again like the thyristors.

It is called a programmable UJT just because its characteristics and parameters have many similarities to that of the unijunction transistor.

The characteristics of the UJT and the PUT are shown as:

Conclusion:

The PUT is similar to that of the thyristor.

Concept:

Two or more amplifier circuits can be cascaded to increase the gain of an ac signal.

The overall gain Av can be calculated by simply multiplying each gain together. This is as shown:

Mathematically, this is defined as:

\({{\rm{A}}_{\rm{v}}} = {{\rm{A}}_{{{\rm{v}}_1}}}.{{\rm{A}}_{{{\rm{v}}_2}}}.{{\rm{A}}_{{{\rm{v}}_3}}} \ldots .\)  

In dB, the overall gain is defined as:

\({A_v}\left( {dB} \right) = 20\;log{A_v}\)

This can be written as:

\({A_v}\left( {dB} \right) = 20\;log{A_v} = 20\;{\rm{log}}({A_{{v_1}}}.{A_{{v_2}}}.{A_{{v_3}}} \ldots .\;\)

\({A_v}\left( {dB} \right) = 20log{A_{v1}} + 20\;log{A_{v2}} + \;20\;log{A_{v3}}\; + \ldots \)

Hence,

\({A_v}\left( {dB} \right) = {A_{v1\left( {dB} \right)}} + {A_{v2\left( {dB} \right)}} + \;{A_{v3\left( {dB} \right)}}\; + \; \ldots \)

∴ If the gain of each amplifier stage is expressed in decibels (dB), the total gain will be the sum of the gains of individual stages.

Calculation:

The voltage gain of 1st amplifier = 10

The voltage gain of 2nd amplifier = 20

The voltage gain of 3rd  amplifier = 30

Voltage gain of the combined amplifier = 10 x 20 x30 =6000

The correct answer option 4):(Triggering circuits )

Concept:

• A unijunction transistor is a three-lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch. The UJT is not used as a linear amplifier.
• The most common application of a unijunction transistor is as a triggering device for SCR's and Triacs but other UJT applications include sawtoothed generators, simple oscillators, phase control, and timing circuits.
• UJT does not have the ability to amplify but it has the ability to control a large ac power with a small signal. It exhibits a negative resistance characteristic and so it can be employed as an oscillator. The structure of a UJT is quite similar to that of an N-channel JFET. In a unijunction transistor, the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 kilo Ohm when the emitter is open

A transistor can be operated in one of the three modes: 

1) Active Region

2) Saturation Region

3) Cut-off Region

The biasing for different mode of operation is as shown in the table:

∴ For a transistor operating in the saturation region, both the junctions need to be forward bias.

• Common collector configuration, also known as emitter follower provides high input impedance and low output impedance. So they are used for the purpose of impedance matching.
• In common collector configuration, the collector terminal is common to both input and output terminals.

The DC current gain is therefore given by the ratio of emitter current to the base current, i.e.

\(\gamma=\frac{I_E}{I_B}\)

IE = Emitter Current

IB = Base Current

Also, IE = IB + IC

DC current gain will be:

\(\gamma=\frac{I_C+I_B}{I_B}=\frac{I_C}{I_B}+1\)   ---(1)

The DC current gain for a common emitter configuration is defined as:

\(\beta=\frac{I_C}{I_B}\)

Equation (1) now becomes:

\(\gamma=\beta +1\)

Important Differences between different transistor configuration is as shown:

Concept:

The frequency response of the Direct-coupled amplifier is as shown:

• Lower frequency does not exist because the DC-amplifier circuit does not use coupling or bypass capacitors. So it’s gain does not affect at low frequency.
• Lower frequency gain is the same as mid-frequency gain

Important Points

• In other amplifiers like RC-coupled amplifier, Transformer coupled amplifier and Impedance-coupled amplifier coupling, and bypass capacitor and transformer is used.
• So it affects the gain of the amplifier at low frequencies as well as at high frequencies.
• So in all the above amplifier, lower and higher cut-off frequency exist in frequency response.

Explanation:

• Frequency Distortion occurs in a transistor amplifier when the level of amplification varies with frequency
• Non-uniformity in the rate of change between the base voltage and the collector current means that for a given DC bias at the base, a sinusoidal signal would have its negative peaks rounded off and the positive peaks sharpened up which causes harmonic distortion
• Frequency distortion due to harmonics occurs because of reactive elements such as capacitance or inductance

Bipolar Junction Transistor (BJT)

BJT is a current-controlled device.

BJT is a three-terminal device i.e. base, collector, and emitter.

The conduction in BJT is due to the minority as well as the majority charge carrier.

Current ratios in BJT

1.) Common base current gain

It is the ratio of collector current to emitter current.

It is denoted by α

\(α={I_C\over I_E}\)

2.) Common emitter current gain

It is the ratio of collector current to base current.

It is denoted by β

\(β={I_C\over I_B}\)

3.) Common collector current gain

It is the ratio of emitter current to base current.

It is denoted by γ 

\(γ={I_E\over I_B}\)

Also, γ = α + β 

The symbolic diagram of the Darlington pair is shown below:

• A Darlington pair is a two-transistor circuit with the emitter of one transistor is connected to the base of other transistors, while both collector terminals are connected to the common terminal
• It has high current gain β; (equal to the product of current gain of individual transistors) and is useful in applications where current amplification or switching is required.
• It also has high input impedance and low output impedance.
• The current gain of a Darlington pair in common emitter configuration is approximate β²

SCR:

DIAC:

The structure of a power transistor is as shown:

• We can clearly observe that a power transistor is a vertically oriented four-layer structure of alternating p-type and n-type.
• The vertical structure is preferred because it maximizes the cross-sectional area and through which the current in the device is flowing. This also minimizes on-state resistance and thus power dissipation in the transistor.
• The doping of the emitter layer and collector layer is quite large typically 1019 cm-3. A special layer called the collector drift region (n-) has a light doping level of 1014 cm-3.
• The thickness of the drift region determines the breakdown voltage of the transistor. More the thickness of the collector drift region more will be the breakdown voltage (The transistor will block large voltage during OFF state)
• Small base thickness is preferred in order to have good amplification capabilities. However, if the base thickness is small the breakdown voltage capability of the transistor is compromised.

​

A conventional transistor consists of thin p-layer sandwiched between two n layers and vice versa to form a three-terminal device with terminals named as Emitter, Base, and Collector as shown:

The common-emitter current gain (β) is the ratio of the transistor's collector current to the transistor's base current, i.e.

\(β = \frac{{{I_C}}}{{{I_B}}}\)

And the common base DC current gain (α) is a ratio of the transistor's collector current to the transistor's emitter current, i.e.

\(α = \frac{{{I_C}}}{{{I_E}}}\)

The transistor currents are related by the relation:

I_E = I_B + I_C

α can now be written as:

\(α = \frac{{{I_C}}}{{{I_B+I_C}}}\)

Dividing both the numerator and denominator by IB, we get:

\(α = \frac{{{I_C/I_B}}}{{{1+I_C/I_B}}}\)

Since \(β = \frac{{{I_C}}}{{{I_B}}}\)

\(α = \frac{β }{{β + 1}}\) 'or'

\(β = \frac{α }{{1-α}}\)

Concept:

For a transistor, the base current, the emitter current, and the collector current are related as:

I_E = I_B + I_C

where I_C = β_dc I_B

β_dc = Current gain of the transistor which is defined as:

\(β=\frac{I_C}{I_B}\)

β is the fraction of emitter current passing to the collector side.

Calculation:

Let the emitter current be I_E.

Since 1% of the emitted current carriers are lost in the base recombination, the collector current will be 99% of the emitter current, i.e.

I_C = 0.99 I_E

And the base current will be 1%, i.e. 0.01 I_E

∴ β will be:

\(β=\frac{I_C}{I_B}=\frac{0.99I_E}{0.01I_E}\)

β = 99

When the junction temperature of a transistor increases, it increases the collector current causing a further increase in temperature and this process is called Thermal runway.

So, option (2) is correct.

Notes:

Thermal resistance between collector-base junction develops due to thermal run-way.

\(\theta = \frac{{{T_j} - {T_A}}}{{{P_D}}}\left( {K/watt} \right)\) 

Where,

θ = Thermal resistance

T_j = collector Junction Temperature

T_A = Ambient temperature

P_D = Power dissipation along collector junction.

Thermal resistance increases due to increase in Junction temperature.

Thermal run-away can be avoided if:

\(\frac{{\partial {P_c}}}{{\partial {T_j}}} \le \frac{{\partial {P_D}}}{{\partial {T_j}}} = \frac{1}{\theta }\) 

Where,

\(\frac{{\partial {P_c}}}{{\partial {T_j}}}\)  is rate at which heat is released.

\(\frac{{\partial {P_D}}}{{\partial {T_j}}}\) is rate at which heat is dissipated.