Bipolar Junction Transistor MCQ Quiz - Objective Question with Answer for Bipolar Junction Transistor - Download Free PDF

Bipolar RAM and Its Use of BJTs

Bipolar RAM relies on the switching capabilities of BJTs to store and retrieve data. Each memory cell in a bipolar RAM is typically composed of a combination of BJTs configured in such a way that they can represent a binary state (0 or 1). When a specific memory cell is accessed, the BJTs in that cell switch states, either allowing or blocking the flow of current, which corresponds to reading or writing data.

Advantages:

• High Speed: The primary advantage of Bipolar RAM is its extremely fast switching speed, which makes it suitable for time-critical applications.
• Reliability: Bipolar RAM is robust and reliable in performance, especially in environments requiring consistent, high-speed data access.

Disadvantages:

• Power Consumption: BJTs consume more power compared to other types of transistors like MOSFETs, making bipolar RAM less energy-efficient.
• Cost: The manufacturing cost of bipolar RAM is higher due to the complexity of BJTs and their associated circuitry.

Applications: Due to its high speed, bipolar RAM is commonly used in specialized computing systems, such as high-performance processors, military-grade devices, and certain types of industrial controllers.

Correct Option Analysis:

The correct option is:

Option 4: BJTs

This option is correct because Bipolar RAM is explicitly designed around the use of Bipolar Junction Transistors (BJTs). These transistors are known for their high-speed switching capabilities, which is a defining characteristic of bipolar RAM. The use of BJTs is what gives Bipolar RAM its speed advantage over other types of memory technologies.

The correct answer is: 3) both (1) and (2)

Explanation:

• Junction Parasitics (like capacitance and resistance at the PN junctions) slow down the switching speed of the device, especially at high frequencies.

• Electron Mobility affects how quickly charge carriers can move through the semiconductor. Lower mobility results in slower device response, thus limiting high-frequency operation.

Both factors contribute to the frequency limitation of bipolar semiconductor devices.

Explanation:

Key Features of Depletion Mode MOSFETs:

• High-Speed Operation: Depletion mode MOSFETs have low capacitance and high electron mobility, which allows for faster switching. The smaller parasitic capacitance associated with the gate and drain terminals enables rapid charging and discharging, which is crucial for high-speed applications.
• Normally "On" Operation: Unlike enhancement mode MOSFETs, depletion mode MOSFETs are naturally conductive. This characteristic eliminates the need for additional circuit components to maintain conduction in certain applications, further improving speed and efficiency.
• Low Power Consumption: Due to their high input impedance, depletion mode MOSFETs draw negligible current from the gate terminal, which reduces power losses and enhances overall efficiency.
• Wide Frequency Response: The high-speed switching capability of depletion mode MOSFETs makes them suitable for applications in high-frequency circuits, such as RF amplifiers and communication systems.

Concept of Common-Base (CB) Configuration:

In a common-base BJT amplifier:

• Input signal is applied to the emitter

• Output is taken from the collector

• Base is common to both input and output (grounded for AC signals)

 Additional Information

1) Suitable for high-frequency applications

• Correct: The CB configuration has excellent high-frequency response due to:

  • Low input impedance

  • No Miller effect (capacitance multiplication)

  • Better bandwidth compared to CE configuration

2) Voltage gain is very high

• Correct: While current gain (α) is slightly less than 1, the CB amplifier provides high voltage gain (comparable to CE).

3) Operates as an amplifier in saturation region

• Incorrect (Answer):

  • BJT acts as an amplifier only in the active region (where IC = βIB holds).

  • In saturation, the transistor acts as a closed switch (not for amplification).

4) Works as an off switch when both junctions are reverse-biased

• Correct: This describes the cutoff region (transistor acts as an open switch).

Conclusion:

The incorrect statement is Option 3, because a CB amplifier must operate in the active region for proper amplification, not saturation.

Final Answer: 3) The CB configuration operates as an amplifier when the transistor is in the saturation region. 

V-I characteristics of a PN junction diode

Forward Bias:

When the p-type side of the diode is connected to a higher potential than the n-type side, the diode is forward-biased. This reduces the width of the depletion region, making it easier for electrons to flow from the n-type side to the p-type side and holes to flow from the p-type side to the n-type side, resulting in a large forward current. 

Reverse Bias:

When the n-type side of the diode is connected to a higher potential than the p-type side, the diode is reverse-biased. This widens the depletion region, making it difficult for the majority carriers to cross the junction. The reverse saturation current, which flows due to the thermally generated minority carriers, is very small compared to the forward current. 

Explanation

• The forward current of a diode is always significantly greater than its reverse saturation current. When a diode is forward-bias, it allows a substantial amount of current to flow, while in reverse bias, only a very small amount of reverse saturation current flows.
• This is due to the depletion region being much narrower in forward bias, allowing majority carriers to cross the junction easily. At the same time, it's wider in reverse bias, hindering the flow of majority carriers and allowing only minority carriers to contribute to the current. 

Current amplification factor: It is defined as the ratio of the output current to the input current. In the common-base configuration, the output current is emitter current IC, whereas the input current is base current IE.

Thus, the ratio of change in collector current to the change in the emitter current is known as the current amplification factor. It is expressed by the α.

\(\alpha = \frac{{{\rm{\Delta }}{I_C}}}{{{\rm{\Delta }}{I_E}}}\)

Where, I_E = I_C + I_B

Calculation:

Given,

I_E = 2 mA

I_B = 20 μA = 0.02 mA

From above concept,

I_C = 2 mA - 0.02 mA = 1.98 mA

Current amplification factor is given as,

\(\alpha=\frac{I_C}{I_E}=\frac{1.98}{2}=0.99\)

• The term impedance matching is simply defined as the process of making one impedance look like another
• Impedance matching is a process in which the impedance of an electrical load is made equal to the source impedance to maximize the power transfer or minimize signal reflection from the load
• The Power amplifiers generally use transformer coupling because the transformer permits impedance matching
• The disadvantage of impedance matching is that it gives distorted output

Transistors:

• Transistors are semiconductor devices, which are commonly used in amplifiers or electrically controlled switches
• Transistors are the basic building block that regulates the operation of computers, mobile phones, and all the other modern electronic circuits
• The most important transistor function is as a current amplifier and as a switch
• Transistors as current amplifiers are often used in amplifiers while transistors as switches are often found in automatic light circuits
• The functions of a transistor are variable resistor and switching device

Concept:

ICEO is the reverse leakage current in the common-emitter configuration of BJT when the base is open.

ICBO is the reverse leakage current in the common-base configuration of BJT when the emitter is open.

Also, ICEO > ICBO

And they are related by the relation:

ICEO = (1 + β) ICBO

Total collector current is given by:

IC = βIB + ICBO(β + 1)

Application:

The total collector current for the given configuration will be:

IC = βIB + ICBO(β + 1)   ---(1)

Given: I_B = 0 A, β = 50, and ICBO = 2.5 μA.

Substituting these values in Equation (1), we get:

IC = (50)(0) + (2.5 μ) (50 + 1)

I_C = 2.5 × 10^-6 × 51 A

IC = 0.1275 mA

The operating point is also called the quiescent point and for a transistor, it is given as \(\left( {{{\rm{I}}_{\rm{C}}},{\rm{\;}}{{\rm{V}}_{{\rm{CE}}}}} \right)\)

• The ac load line of a transistor circuit is steeper than its dc load line but the two intersect at Q point
• When AC and DC Load lines are represented in a graph, it can be understood that they are not identical
• Both lines intersect at the Q-point or quiescent point; The endpoints of the AC load line are saturation and cut off points; This is understood from the figure below

Concept:

The power capacity of a transistor is given by:

P_diss. = V_CE × I_C

V_CE = Collector emitter voltage

I_C = Collector current

Application:

Given P_diss(max) = 50 mW

V_CE = 10 V

P_diss(max) = V_CE(max)× I_C(max)

\(I_C(max)=\frac{50~mW}{10~V}=5~mA\)

To prevent the transistor from destroying or from excessive heating, the safe collector current would be 2.5 mA, which is less than the maximum allowed collector current of 5 mA.

I_CO = It is the reverse saturation current across reverse biased collector junction.

I_CEO = Base cut-off current

I_CEO = (1 +β) I_CO

\(\beta = \frac{\alpha }{{1 - \alpha }} = \frac{{0.995}}{{1 - 0.995}} = 199\)

I_CEO = (200) I_CO

I_CEO = (200) 0.5 μA

A transistor is a 3 layered 2 junction device as shown below.

I_E = I_B + I_C

I_C = βI_B

• The Emitter-follower circuit is also known as a common collector configuration.
• It is called the emitter follower configuration because the emitter voltage follows the base voltage.
• It is mostly used as a voltage buffer.

Important:

CB configuration: Low Input impedance and High output impedance

CC configuration: (Emitter follower): High Input impedance and Low output impedance