Unit 3 - Practice Quiz

BJT stands for Bipolar Junction Transistor. It is called 'bipolar' because its operation involves both electrons and holes as charge carriers.

A BJT has three terminals, which are named the Emitter, Base, and Collector.

In the active region, the BJT acts as an amplifier. This requires the emitter-base junction to be forward-biased to allow current flow, and the collector-base junction to be reverse-biased to collect the charge carriers.

According to Kirchhoff's Current Law applied to the transistor, the emitter current () is the sum of the base current () and the collector current ().

In the Common Emitter (CE) configuration, the input signal is applied between the base and emitter, and the output is taken from between the collector and emitter. Thus, the emitter terminal is common.

A BJT acts as a linear amplifier when it is biased to operate in the active region. The cut-off and saturation regions are used for switching applications.

As a switch, a transistor is either fully OFF (cut-off region, acting as an open switch) or fully ON (saturation region, acting as a closed switch).

Alpha () is the DC current gain for the common-base configuration. It is the ratio of the collector current () to the emitter current (). Its value is always slightly less than 1.

A PNP transistor is formed by sandwiching a thin layer of N-type semiconductor (the base) between two thicker layers of P-type semiconductor (the emitter and collector).

Beta () is the DC current gain for the common-emitter configuration. It is the ratio of the collector current () to the base current (). Its value is typically high, often between 50 and 400.

The Common Emitter (CE) configuration provides the highest current gain, represented by beta (), which is typically much larger than 1. The CB configuration has a current gain slightly less than 1, and the CC configuration has a high current gain similar to CE.

The Common Collector (CC) configuration is called an emitter follower because the output voltage taken at the emitter closely follows the input voltage applied at the base. It has a voltage gain of approximately 1.

The Common Collector (CC) or emitter follower configuration is widely used as a buffer because it has a high input impedance (doesn't load the previous stage) and a low output impedance (can drive low-impedance loads).

The emitter is heavily doped to inject a large number of charge carriers into the base. The base is lightly doped and very thin, while the collector is moderately doped.

The 'ON' state of a transistor switch corresponds to the saturation region, where the transistor acts like a closed switch with a small voltage drop across it, allowing maximum current to flow.

In the Common Base (CB) configuration, the base terminal is common. The input signal is applied to the emitter, and the output signal is taken from the collector.

An amplifier's main function is to take a weak input signal (voltage or current) and produce a stronger output signal that is a magnified replica of the input.

Rise time () is a standard measure of a transistor's switching speed, specifically defining how quickly it transitions from the cut-off state to the saturation state once it begins to turn on.

The Common Base (CB) configuration provides a high voltage gain but its current gain () is always slightly less than one. It is often used in high-frequency applications.

Delay time () is the time interval from the application of the input 'ON' pulse to the point where the collector current reaches 10% of its final value. It represents the initial delay before the transistor begins to turn on.

In the saturation region, both the emitter-base junction and the collector-base junction are forward-biased. This allows maximum current to flow from collector to emitter, making the transistor act like a closed switch.

The correct option follows directly from the given concept and definitions.

The relationship between and is given by the formula . Substituting , we get .

Placing the Q-point in the center of the active region provides the largest possible room for both the positive and negative swings of the AC output signal before it gets clipped by hitting the saturation or cutoff regions, thus avoiding distortion.

The collector saturation current is . The minimum base current required for saturation is .

The Common Collector (CC) configuration, also known as an emitter follower, is characterized by a high input impedance and a low output impedance. This makes it ideal for use as a buffer or for impedance matching between stages.

When a transistor is in saturation, the base is flooded with more charge carriers than needed. Storage time is the delay required for these excess minority carriers in the base to recombine or be swept out before the transistor can begin to turn off.

A thin and lightly doped base minimizes the chance for charge carriers (electrons in NPN, holes in PNP) injected from the emitter to recombine with the majority carriers in the base. This allows almost all injected carriers to reach the collector, resulting in a high current gain ().

is the collector-emitter leakage current with the base open. This small leakage current acts as a base current and gets amplified by the transistor, resulting in a much larger collector current given by .

In a CE amplifier, when the input voltage at the base increases, it increases the base current () and consequently the collector current (). The increased causes a larger voltage drop across the collector resistor (), which in turn decreases the output voltage (). This inverse relationship results in a 180-degree phase shift.

The Common Emitter (CE) configuration is the most widely used because it provides both significant voltage gain and significant current gain. The product of these two results in the highest power gain among the three configurations.

Input characteristics show the relationship between the input current and input voltage. For a CB configuration, the input is at the emitter-base junction. Therefore, the plot is of input current () versus input voltage (), while keeping the output voltage () constant.

The Early effect describes how the reverse bias voltage across the collector-base junction affects the transistor's behavior. An increase in this reverse bias widens the depletion region, reducing the effective width of the base. This leads to a slight increase in collector current, causing the output characteristic curves to have a non-zero slope.

Ideally, in the OFF state (cutoff), the transistor acts as an open circuit, so no current flows and the output voltage is equal to the supply voltage . In the ON state (saturation), it acts as a perfect closed switch, so there is no voltage drop across it, making equal to 0 V.

The turn-on time () is the total time it takes for the transistor to switch from the OFF state to the ON state. It is composed of the delay time (), which is the time before the collector current starts to rise, and the rise time (), which is the time it takes for the current to rise from 10% to 90% of its final value. So, .

Heavy doping of the emitter ensures that when the emitter-base junction is forward-biased, the current is dominated by charge carriers injected from the emitter into the base, rather than carriers injected from the base into the emitter. This high injection efficiency is crucial for achieving a high current gain.

The CB configuration has a current gain () that is always slightly less than 1. However, it can provide a very high voltage gain because its high output impedance allows a large output voltage swing for a small input voltage change. It is often used in high-frequency applications.

Removing the bypass capacitor introduces the emitter resistor () into the AC circuit. This creates negative feedback, also known as emitter degeneration. This feedback stabilizes the amplifier but drastically reduces the AC voltage gain. The gain becomes approximately .

While both emitter and collector are N-type, their doping levels and physical sizes are very different. The emitter is heavily doped for high injection efficiency, and the collector is large to dissipate heat. Swapping them makes the new 'emitter' (the original collector) a poor injector of carriers, resulting in a very low reverse .

The reverse saturation current () is highly dependent on temperature, approximately doubling for every 10°C rise. Since the total collector current in a CE configuration is , an increase in leads to a significant increase in the overall collector current, which can cause thermal runaway if not properly stabilized.

Emitter injection efficiency for a PNP BJT is . Increasing base doping increases the back-injected electron current () from the base to the emitter, thus decreasing . The current gain depends directly on (since ). A decrease in leads to a decrease in and a significant decrease in . Additionally, higher base doping increases recombination, which also reduces gain.

The Miller effect occurs when an impedance (like ) is connected between the input and output of an inverting amplifier. In a CB configuration, the input is the emitter and the output is the collector. The base is the common terminal (AC ground). Therefore, the capacitance is connected between the output (collector) and AC ground, not providing a feedback path from output to input. This eliminates the Miller multiplication effect, contributing to the CB amplifier's excellent high-frequency response.

Storage time () is caused by the excess minority carriers stored in the base during saturation. A Schottky diode clamp (Baker clamp) prevents the BJT from hard-saturating by diverting excess base current to the collector, thus preventing large excess charge storage. A 'speed-up' capacitor provides a large negative current pulse at the beginning of turn-off, which helps to quickly remove any stored charge. This combination is highly effective at minimizing .

The first stage needs high input impedance, for which the Common-Collector (CC) stage is ideal. The second stage requires high voltage gain, which is the primary strength of the Common-Emitter (CE) stage. The third (output) stage must provide a low output impedance to effectively drive the low-impedance load, a task for which the Common-Collector (CC) stage is perfectly suited. Therefore, the CC-CE-CC configuration meets all the specified requirements.

The Early effect is modeled by the output resistance . First, calculate the output resistance at the initial operating point: k. The change in collector current due to a change in is given by Ohm's law for this resistance: mA.

Punch-through (or reach-through) is a breakdown mechanism distinct from avalanche or thermal runaway. It is caused by the widening of the collector-base depletion region under high reverse bias. If the base is sufficiently narrow and the voltage is high enough, the depletion region can span the entire base. When this happens, the emitter and collector are effectively shorted, and the base loses control over the current, leading to a large, uncontrolled current flow.

1. Calculate the collector saturation current: mA.
2. Find the minimum base current for saturation: A.
3. Apply the overdrive factor (ODF): A.

Base transit time is proportional to the square of the base width (), so a narrow base is critical for high speed. Base resistance is inversely proportional to base width and base doping concentration. One could try to compensate for a narrow base by heavily increasing the base doping to lower . However, if the base doping becomes comparable to the emitter doping, the emitter injection efficiency plummets, destroying the transistor's gain. This forces a compromise between a narrow, lightly-doped base (fast but high resistance) and a wider, more heavily-doped base (slower but lower resistance).

The current gain is a product of emitter injection efficiency () and base transport factor. In forward active mode, the heavily doped emitter injects carriers very efficiently into the less-doped base, making close to 1. In reverse active mode, the collector acts as the emitter. Since the collector is lightly doped compared to the base, its ability to inject carriers into the base is very poor, resulting in a low reverse injection efficiency . This poor injection efficiency is the primary reason why is much smaller than .

The gain-bandwidth product for a BJT states that the transition frequency is approximately equal to the product of the low-frequency current gain and the beta cutoff frequency . The relationship is . Therefore, we can find by rearranging the formula: MHz.

The output resistance is the change in collector voltage divided by the resulting change in collector current (), with the input source set to zero. Emitter degeneration provides negative feedback. If a change in (e.g., due to the Early effect) causes to change, this also changes , which alters the voltage drop across . This change is fed back to the base-emitter loop, opposing the initial change in . This opposition makes much less sensitive to variations in , which by definition means the output resistance is higher.

The switching times are directly related to the movement of charge. The rise time () is the time it takes for the collector current to rise from 10% to 90% of its final value, which corresponds to the time taken by the base current to supply the charge needed to support the active-region current. The storage time () is the delay before the transistor begins to turn off, which is the time required for the reverse base current to remove the excess saturation charge from the base.

The Miller input capacitance is given by , where is the voltage gain of the first stage. In a cascode amplifier, the first CE stage (Q1) is loaded by the second CB stage (Q2). The input impedance of a CB stage is its emitter resistance, , which is very small (e.g., a few ohms). Therefore, the voltage gain of the CE stage, . With a voltage gain close to -1, the Miller multiplication factor is only about 2, compared to potentially >100 in a normal CE stage. This drastic reduction in the effective input capacitance is what gives the cascode its superior high-frequency performance.

Instantaneous power is . During the transition from OFF to ON, falls from to 0 (approx) and rises from 0 to . If we assume linear ramps, and , where T is the transition time. Then . This is a parabolic function of , which has its maximum at the midpoint, . At this point, and , and the peak power is .

The collector current in a BJT is determined by the diffusion of minority carriers across the base, and its magnitude is inversely proportional to the total number of majority carriers in the base, which is the Gummel Number. So, . The base current is primarily due to recombination, which is directly related to the amount of charge in the base, and thus also increases with . Since , and decreases while increases with , the current gain is strongly and inversely dependent on the Gummel number.

In an HBT, the base (SiGe) has a smaller bandgap than the emitter (Si). This creates an energy barrier in the valence band that blocks the back-injection of holes from the base into the emitter. This unwanted current is a major component of the base current in normal BJTs and is what limits the maximum useful base doping. Since this current is suppressed in an HBT, the base can be doped very heavily. This drastically reduces the base resistance () without compromising the high emitter injection efficiency, leading to simultaneously high gain and high maximum frequency of oscillation ().

While changes in and contribute, the dominant effect is the temperature dependence of . The collector current is exponentially dependent on (). For a fixed base voltage , as temperature rises, required for a certain current drops. This means the forward bias voltage () across the junction increases, causing to rise exponentially. This increases power dissipation (), which raises the temperature further, creating a strong positive feedback loop that can destroy the device.

To find the output resistance, we apply a test voltage at the output (emitter) and find the current , with the input voltage source set to zero. The test voltage at the emitter creates a base current that flows back through the input loop. With the input source shorted, this flows through the source resistance , creating a base voltage . The base-emitter voltage is then . The entire behavior of the transistor is coupled through . Because influences , which is itself dependent on , the relationship between and the resulting output current (which is a function of ) is fundamentally tied to the value of .

The Kirk effect occurs when the injected minority carrier density in the collector becomes comparable to the collector's doping density. This mobile charge effectively cancels out the fixed charge from the ionized dopant atoms, collapsing the electric field in the collector-base depletion region. This causes the effective (neutral) base to expand into the physical collector region. This increase in the effective base width () dramatically increases the base transit time (), which directly causes the transition frequency () to decrease.

The correct option follows directly from the given concept and definitions.