MCQ Set 1 - Transistor Fundamentals

Emitter bias is a type of circuit configuration where the emitter current is kept constant. In this configuration, a resistor is connected in series with the emitter terminal of a transistor, which helps to stabilize the current flowing through the emitter. This biasing technique is commonly used in transistor amplifiers to ensure a stable and predictable operation of the transistor. By keeping the emitter current fixed, the circuit can provide consistent amplification of the input signal.

When using a DMM to test a transistor, an approximate reading of 0.7V will be found with two polarity connections. This is because a transistor has two junctions - the base-emitter junction and the base-collector junction. The base-emitter junction typically has a forward voltage drop of around 0.7V, which can be measured using the DMM. The base-collector junction does not have a significant voltage drop, so it does not affect the reading. Therefore, the DMM will show a reading of approximately 0.7V when testing a transistor with two polarity connections.

The upper Q point on the load line represents the maximum current gain. This is because the load line represents the relationship between the collector current and collector-emitter voltage in a transistor circuit. The upper Q point indicates the highest current gain that can be achieved in the circuit, meaning that the transistor is operating at its maximum amplification capability.

The current gain of a transistor is defined as the ratio of the collector current to the base current. This means that the base current is used to control or modulate the collector current in a transistor. By adjusting the base current, the collector current can be amplified or controlled, making the base current an important factor in determining the overall performance of the transistor.

When the base supply voltage increases, it causes an increase in the collector current. This, in turn, shifts the Q point of the transistor upwards along the load line. As the Q point moves up, the operating point of the transistor shifts towards saturation, resulting in a higher output voltage and a lower output current. Therefore, the correct answer is "Up".

When measuring the voltage on an npn transistor's base, a DMM (Digital Multimeter) will show a 0.7V reading when the positive lead of the DMM is connected to the base of the transistor. This is because the base-emitter junction of an npn transistor typically has a forward voltage drop of around 0.7V. Therefore, connecting the positive lead of the DMM to the base will result in a 0.7V reading.

If the emitter resistor is open, it means that there is no resistor connected to the emitter terminal of the transistor. In this case, the emitter current will be very small or zero, and the collector current will also be very small or zero. As a result, the voltage at the collector terminal will be close to the supply voltage, which is typically high. Therefore, the correct answer is high.

As the Q point moves along the load line, VCE (voltage between collector and emitter) decreases when the collector current increases. This is because the load line represents the relationship between VCE and IC (collector current) for a given circuit. As the collector current increases, the voltage drop across the load resistance also increases, causing VCE to decrease. Conversely, when the collector current decreases, the voltage drop across the load resistance decreases, leading to an increase in VCE. Therefore, the correct answer is "Increases".

If the collector resistor opens in a base-biased circuit, it means that there is no resistance in the collector circuit. This will result in a maximum current flowing through the collector, causing the load line to become horizontal. This is because the voltage across the collector resistor is directly proportional to the current flowing through it, and without any resistance, the voltage will remain constant regardless of the current. Therefore, the load line will be a horizontal line on the voltage-current graph.

If a transistor operates at the middle of the load line, it means that the Q point is already balanced and stable. When the base resistance decreases, it allows more current to flow through the base-emitter junction. This increase in current causes the Q point to move upwards along the load line, towards the saturation region. Therefore, the correct answer is "Up".

The collector current is determined by the product of the base current and the current gain (hfe) of the transistor. In this case, the collector current is given as 1.5mA and the current gain is 50. To find the base current, we can rearrange the formula: collector current = base current * current gain. Plugging in the given values, we can solve for the base current: 1.5mA = base current * 50. Rearranging the equation, we find that the base current is 30uA.

If a transistor operates at the middle of the load line, it means that it is operating at a point where the collector current is halfway between its maximum and minimum values. If the current gain of the transistor decreases, it means that the transistor becomes less efficient in amplifying the input signal. As a result, the collector current will decrease, causing the Q point (the operating point of the transistor) to move downwards along the load line.

The base current is multiplied by the current gain to determine the collector current in a transistor. In this case, the base current is 50uA and the current gain is 100. Therefore, the collector current can be calculated by multiplying the base current by the current gain: 50uA * 100 = 5000uA, which is equivalent to 5mA.

When there is no base current in a transistor switch, the transistor is in the cutoff region. In this state, the transistor is turned off and does not conduct current. As a result, the output voltage from the transistor is high, indicating an "off" state.

When the emitter resistance decreases, it means that the emitter current increases. This increase in emitter current causes the Q point, which represents the operating point of a transistor, to move up on the load line. As a result, the collector current decreases because the transistor is operating at a higher voltage and closer to saturation. Therefore, the correct answer is that the Q point moves up.

If the base resistor is very small, it means that the base current will be large. In the saturation region of a transistor, both the base-emitter junction and the base-collector junction are forward-biased. This means that a large base current will allow a large collector current to flow, resulting in the transistor being in saturation. Therefore, if the base resistor is very small, the transistor will operate in the saturation region.

If the base supply voltage is disconnected, it means that there is no voltage being applied to the base of the transistor. In this case, the transistor will be in cutoff mode and will not conduct any current. When the transistor is in cutoff mode, the collector-emitter voltage will be equal to the collector supply voltage. Therefore, the correct answer is "Collector supply voltage."

A phototransistor has a higher sensitivity compared to a photodiode. This means that it is more responsive to light and can detect even small amounts of light more accurately. The increased sensitivity of a phototransistor allows for better performance in low-light conditions and enhances its ability to detect and convert light into electrical signals. This advantage makes phototransistors suitable for applications where high sensitivity and accuracy are required, such as in optical communication systems, light sensors, and imaging devices.

In emitter-based circuits, the emitter current is the first step to analyze. The emitter current is the total current flowing into the emitter terminal of the transistor. By finding the emitter current, we can determine the current flowing through the base and collector terminals as well. This information is crucial in understanding the behavior and characteristics of the transistor circuit. Therefore, analyzing the emitter current is the initial step in studying emitter-based circuits.

When the emitter resistance increases, it reduces the amount of current flowing through the emitter, which in turn reduces the voltage drop across the emitter resistor. As a result, the voltage at the collector terminal increases. This is because the collector voltage is determined by the difference between the supply voltage and the voltage drop across the emitter resistor. Therefore, when the emitter resistance increases, the collector voltage increases.

As the base resistor increases, the base current decreases, which in turn reduces the collector current in a common emitter configuration. This leads to an increase in the collector voltage Vc​ due to less voltage being dropped across the collector resistor Rc​.

The collector-emitter saturation voltage refers to the voltage drop across the collector-emitter junction when the transistor is in saturation mode. In this case, the question states that the bulk resistance of the collector diode is being ignored. This means that there is no resistance in the collector diode, resulting in a negligible voltage drop across it. Therefore, the collector-emitter saturation voltage is 0V.

The graph of current gain versus collector current indicates that the current gain varies slightly. This means that the current gain does not remain constant but instead shows a small amount of variation as the collector current changes. This could be due to factors such as temperature variations or manufacturing tolerances. However, the variation is not significant enough to classify it as varying significantly. Additionally, the statement that the current gain equals the collector current divided by the base current is not supported by the given information.

If the collector resistor is open, it means that there is no path for current to flow through the collector. As a result, the collector voltage will be low because there is no voltage drop across the resistor.

If the base resistor has zero resistance, it means that there is no resistance between the base terminal of the transistor and the input signal. This can lead to a high current flowing into the base terminal, which can potentially damage or destroy the transistor. Therefore, the correct answer is "Destroyed".

In an emitter-biased circuit, the collector current is determined by the emitter current and the current gain. When the current gain increases from 50 to 300, it means that the transistor is more efficient in amplifying the current. However, the emitter current remains almost the same because the circuit is emitter-biased, which means the biasing conditions are set to keep the emitter current constant. As a result, the collector current will also remain almost the same.

If the base resistor is open, it means that there is no current flowing through the base of the transistor. As a result, the transistor will not be biased and will not operate in the active region. Without biasing, the Q point (or operating point) of the transistor will be at the lower end of the load line. This is because the lower end of the load line corresponds to zero collector current and maximum collector voltage, indicating that the transistor is not conducting and is in an off state.

In an emitter-biased circuit, the base current is determined by the emitter current and the current gain of the transistor. If the current gain is unknown, it becomes impossible to accurately calculate the base current. The base current is crucial for determining the overall performance and characteristics of the transistor circuit. Therefore, without knowing the current gain, it is not possible to calculate the base current accurately.

When testing an npn transistor using an ohmmeter, the collector-emitter resistance will be low when the transistor is defective. This is because a low resistance indicates a short circuit between the collector and emitter terminals, which is an abnormal behavior for a properly functioning transistor.

The current gain of a device can be affected by various factors, including temperature. Depending on the specific characteristics of the device and its design, the current gain can either decrease, remain the same, or increase as the temperature increases. Therefore, it is possible for the current gain to be any of the above options.

The current gain, also known as the beta (β) of a transistor, represents the ratio of the collector current to the base current. It is a measure of how effectively the transistor amplifies the input signal. When the collector current increases, the current gain can either decrease, stay the same, or increase. This depends on various factors such as the type of transistor, the biasing conditions, and the operating region of the transistor. Therefore, any of the given options can be true depending on the specific circumstances.