BJT Switches: Master the Fundamentals for Digital Circuit Applications

Question 1

Which of the following statements about BJT switches is NOT true?

Accepted Answer

BJTs can only be used as amplifiers, not as switches.

Answer

In saturation mode, the BJT acts like a closed switch.

Answer

The switching speed of a BJT is affected by the lifetime of its minority carriers.

Answer

BJT switches can be used to build logic gates.

Question 2

In a BJT switch operating in saturation mode, what is the approximate value of collector-emitter voltage (Vce)?

Accepted Answer

Near zero

Answer

Equal to Vcc

Answer

Equal to Vbe

Answer

High, but not equal to Vcc

Question 3

In a common-emitter BJT switch, what is the relationship between base current (Ib) and collector current (Ic)?

Accepted Answer

Ic = βIb

Answer

Ib = Ic

Answer

Ic = αIb

Answer

Ib = αIc

Question 4

Which of the following applications is most suited for BJT switches?

Accepted Answer

Digital logic circuits

Answer

High-power amplifiers

Answer

Analog filters

Answer

Audio signal processing

Question 5

In cutoff mode, the BJT acts as a:

Accepted Answer

Switch

Answer

Amplifier

Answer

Rectifier

Answer

Oscillator

Question 6

When a BJT is primarily used as a digital switch, in which modes is it predominantly operating?

Accepted Answer

Saturation and cutoff

Answer

Linear and saturation

Answer

Cutoff and linear

Answer

Saturation and linear

Question 7

In a BJT switch, the base-emitter junction is:

Accepted Answer

Forward-biased

Answer

Reverse-biased

Answer

Unbiased

Answer

Diode-connected

Question 8

Design a simple digital circuit using a BJT switch to turn on an LED. Explain how the circuit demonstrates the switching behavior of a BJT.

Accepted Answer

A BJT switch connected to an LED, controlled by a pushbutton

Answer

A BJT switch with only a resistor in the circuit

Answer

A BJT switch connected to a voltage source without a resistor

Answer

A BJT switch used as an amplifier in the circuit

Question 9

Which of the following factors does not influence the switching speed of a BJT?

Accepted Answer

Supply voltage

Answer

Base width

Answer

Collector capacitance

Answer

Emitter resistance

Question 10

In a saturated BJT switch, how does the output voltage compare to the saturation voltage?

Accepted Answer

Approximately equal

Answer

Greater than

Answer

Unaffected

Question 11

For a BJT to function effectively as a switch, which condition must be met?

Accepted Answer

Base-emitter junction forward biased, collector-base junction reverse biased

Answer

Base-emitter junction reverse biased, collector-base junction forward biased

Answer

Both base-emitter and collector-base junctions forward biased

Answer

Both base-emitter and collector-base junctions reverse biased

Question 12

When a BJT switch is turned on, which region is it operating in?

Accepted Answer

Saturation

Answer

Reverse Active

Answer

Cutoff

Answer

Linear

Question 13

What is the key function of a base resistor in a BJT switch circuit?

Accepted Answer

To limit base current and prevent transistor damage

Answer

To provide bias to the base for establishing quiescent conditions

Answer

To increase base voltage and enhance switching speed

Question 14

In a common-emitter BJT switch circuit, how does the input signal relate to the output signal?

Accepted Answer

Inverted

Answer

Amplified

Answer

Non-inverted

Answer

Phase-shifted

Question 15

Which practical application commonly employs BJT switches?

Accepted Answer

Digital logic gates

Answer

Power supplies

Answer

Audio amplifiers

Question 16

What is the primary purpose of a pull-up resistor in a BJT switch circuit?

Accepted Answer

To provide a default high output when the switch is off

Answer

To reduce transistor saturation

Answer

To increase current gain

Question 17

What is the main purpose of a Schottky diode in a BJT switch circuit?

Accepted Answer

To reduce reverse recovery time

Answer

To protect the transistor from overvoltage

Answer

To increase switching speed

Answer

To provide negative feedback

Question 18

In a BJT switch that is saturated, what is the approximate value of the collector-emitter voltage?

Accepted Answer

0.2V

Answer

1V

Answer

Vcc

Answer

0.7V

Question 19

When a BJT is used as a switch, it operates in which two distinct modes?

Accepted Answer

Saturation and Cutoff

Answer

Active and Cutoff

Answer

Active and Saturation

Answer

Linear and Saturation

Question 20

In saturation mode, a BJT switch exhibits:

Accepted Answer

Low collector-emitter voltage (Vce) and high collector current (Ic)

Answer

High Vce and low Ic

Answer

Low Vce and low Ic

Question 21

In cutoff mode, a BJT switch has:

Accepted Answer

High Vce and negligible Ic

Answer

Low Vce and negligible Ic

Answer

Low Vce and high Ic

Question 22

Which of the following configurations is commonly used to drive a BJT switch into saturation?

Accepted Answer

Common-emitter configuration with a resistor in the base

Answer

Common-collector configuration with a resistor in the emitter

Answer

Common-base configuration with a resistor in the collector

Question 23

What is the primary role of the base resistor in a BJT switch?

Accepted Answer

To limit base current and prevent damage to the transistor

Answer

To provide a voltage drop to drive the transistor into saturation

Answer

To increase base current and enhance switching speed

Question 24

Which of the following applications commonly utilizes BJT switches?

Accepted Answer

Digital logic gates

Answer

Power converters

Answer

Analog amplifiers

Question 25

What is the main purpose of a pull-up resistor in a BJT switch circuit?

Accepted Answer

To provide a defined output voltage when the switch is open

Answer

To enhance switching speed

Answer

To limit base current

Question 26

In a BJT switch, which state has a low output impedance?

Accepted Answer

Saturation

Answer

Cutoff

Answer

Reverse

Answer

Active

Question 27

What is the primary role of a base resistor in a BJT switch?

Accepted Answer

Limit base current and prevent excessive saturation

Answer

Provide a path for collector current

Answer

Increase base current

Question 28

Which factors influence the switching time of a BJT switch?

Accepted Answer

Base resistance, transistor capacitance, and collector current

Answer

Only base resistance

Answer

Emitter resistance and collector voltage

Question 29

Which application is most suitable for a BJT switch?

Accepted Answer

Driving a relay

Answer

Amplifying small signals

Answer

High-speed data transmission

Answer

RF power amplification

Question 30

Explain the difference between hard saturation and soft saturation in a BJT switch.

Accepted Answer

Hard saturation occurs when the base current is high enough to drive the BJT well into saturation, while soft saturation occurs when the base current is just sufficient to saturate the BJT.

Answer

Hard saturation occurs when the collector current is high, while soft saturation occurs when the collector current is low.

Question 31

What is the purpose of a snubber circuit across a BJT switch?

Accepted Answer

Suppress voltage transients and protect the BJT from damage

Answer

Increase the switching speed of the BJT

Answer

Provide a path for base current during turn-off

Question 32

Which parameter primarily limits the maximum switching frequency of a BJT switch?

Accepted Answer

Transition frequency (fT)

Answer

Breakdown voltage (BVCEO)

Answer

Base resistance (rb)

Answer

Saturation voltage (VCE(sat))

Question 33

How does temperature affect the switching characteristics of a BJT switch?

Accepted Answer

Increased temperature reduces saturation voltage and increases switching time.

Answer

Increased temperature increases saturation voltage and decreases switching time.

Answer

Temperature has no effect on the switching characteristics of a BJT switch.

Question 34

In a BJT switch, the transistor is biased in either of two extreme states. Name these states.

Accepted Answer

Saturation and cutoff

Answer

Linear and nonlinear

Answer

Forward active and reverse active

Answer

Ohmic and non-ohmic

Question 35

Which of the following is NOT a characteristic of a BJT switch in saturation mode?

Accepted Answer

High collector-emitter voltage

Answer

Short circuit between collector and emitter

Answer

Low base drive current

Answer

High collector current

Question 36

What is the primary purpose of a base resistor in a BJT switch?

Accepted Answer

Limit base current and prevent saturation

Answer

Reduce base current and promote cutoff

Answer

Increase base current and enhance saturation

Answer

Provide a path for collector current to flow

Question 37

In a BJT switch, which transistor parameter primarily influences the switching time?

Accepted Answer

Transition capacitance

Answer

Base resistance

Answer

Reverse saturation current

Answer

Forward current gain

Question 38

In a Darlington pair switch, the primary reason for using two transistors is to achieve:

Accepted Answer

Higher current gain

Answer

Reduced power dissipation

Answer

Lower on-resistance

Answer

Faster switching speed

Question 39

Which of the following applications commonly employs BJT switches?

Accepted Answer

Digital logic circuits

Answer

Power supplies

Answer

Sensors

Answer

Analog amplifiers

Question 40

In a totem-pole switch, the purpose of the totem is to:

Accepted Answer

Provide both high and low logic level outputs

Answer

Reduce switching time

Answer

Increase current gain

Answer

Isolate input from output

Question 41

In a BJT switch circuit, what factor primarily determines the collector current when the BJT is operating in saturation mode?

Accepted Answer

Base current

Answer

Collector-emitter voltage

Answer

Supply voltage

Answer

Load resistance

Question 42

In a digital circuit, which operating modes of a BJT are typically employed?

Accepted Answer

Saturation and cutoff

Answer

Linear and active

Answer

Cutoff only

Answer

Saturation only

Question 43

In a BJT switch, what is the main purpose of a pull-up or pull-down resistor?

Accepted Answer

Providing a well-defined output voltage when the BJT is off

Answer

Enhancing the current gain of the BJT

Answer

Preventing potential overheating of the BJT

Question 44

Which factors can affect the switching speed of a BJT?

Accepted Answer

Transistor type, base resistance, and load capacitance

Answer

Supply voltage and collector current only

Answer

Base-emitter voltage and collector-emitter voltage only

Question 45

In a BJT switch circuit, what happens when the base current increases?

Accepted Answer

Decreased saturation voltage and increased collector current

Answer

Decreased saturation voltage and decreased collector current

Answer

Increased saturation voltage and decreased collector current

Question 46

A BJT switch in digital circuits serves to:

Accepted Answer

Permit or block current flow between the input and output terminals

Answer

Generate sinusoidal waveforms

Answer

Amplify voltage signals

Question 47

When a BJT switch operates in saturation mode, it behaves like:

Accepted Answer

A closed switch with minimal resistance

Answer

An open switch with very high resistance

Answer

A linear amplifier

Answer

A non-linear oscillator

Question 48

To drive a BJT switch into cutoff mode (off-state), the:

Accepted Answer

Base-Emitter junction must have reverse bias

Answer

Emitter-Base junction must have forward bias

Answer

Base-Collector junction must have forward bias

Answer

Collector-Emitter junction must have reverse bias

Question 49

The switching speed of a BJT switch is influenced by:

Accepted Answer

Transistor characteristics and circuit parameters

Answer

Load impedance only

Answer

Supply voltage only

Answer

Ambient temperature only

Question 50

A key advantage of utilizing BJT switches in digital circuits is their:

Accepted Answer

Ability to handle high currents

Answer

Low noise

Answer

Low power consumption

Answer

High bandwidth

Question 51

In a BJT emitter-follower switch:

Accepted Answer

The output voltage follows the input voltage with a gain less than 1

Answer

The output voltage is inverted and amplified

Answer

The output voltage is delayed with respect to the input voltage

Question 52

When designing a BJT switch circuit, it is essential to consider:

Accepted Answer

Power dissipation and thermal management

Answer

Only the switching time

Answer

Only the voltage gain

Question 53

In the design of a BJT switch, the choice of transistor is influenced by:

Accepted Answer

Voltage and current ratings, switching speed, and cost

Answer

Only the voltage ratings

Answer

Only the current ratings