Oral Exam 1 (2/2)

What is a BJT IV curve?

A plot of collector current IC versus collector-emitter voltage VCE for different fixed base currents IB.

What is another name for the BJT IV curve measurement?

A curve trace.

What is on the x-axis of the BJT IV curve?

VCE, the collector-emitter voltage.

What is on the y-axis of the BJT IV curve?

IC, the collector current.

What does each separate curve on the BJT IV plot represent?

One fixed value of base current IB.

How is the BJT IV curve obtained in simulation or lab?

Sweep VCE while stepping IB through different values, then record IC.

For the single 2N3904 BJT curve trace, what base currents were used in simulation?

Typically 0 µA to 80 µA in 10 µA steps.

What does the BJT IV curve show about current gain?

A small base current in microamps controls a much larger collector current in milliamps.

What is beta in a BJT?

Beta, or hFE, is the current gain: beta = IC / IB.

What does it mean when the IV curves are mostly flat?

IC is only slightly affected by VCE, so the BJT acts approximately like a current-controlled current source.

In the active region, what is the approximate relationship between IC and IB?

IC ≈ beta IB.

What region is used when the BJT acts as an amplifier?

The active region.

What bias condition is needed for active mode?

The base-emitter junction is forward biased and the base-collector junction is reverse biased.

For an NPN BJT, what is the usual forward-biased VBE approximation?

About 0.7 V.

What happens when IB = 0?

The transistor is in cutoff, so IC is approximately zero.

What is cutoff?

The BJT is off; base-emitter is not forward biased and IC is approximately zero.

What is saturation?

The BJT is fully on; both base-emitter and base-collector junctions are forward biased, and VCE is very small.

What happens to the IV curve at very low VCE?

The curve bends into saturation, where the transistor no longer behaves like an ideal current amplifier.

How is the IV curve used in this lab?

It is used to characterize the actual BJT, compare it to simulation, find beta, and choose or verify the operating point.

Why compare the measured IV curve to the simulated IV curve?

To see whether the real transistor behaves like the model and to explain discrepancies.

What lab transistor is used for the BJT curve trace?

A 2N3904 NPN BJT.

What is the key sentence for describing the IV curve?

The BJT IV curve is a family of IC versus VCE curves for different IB values, showing cutoff, active operation, and saturation.

What does the IV curve tell you about a BJT?

It tells how the transistor itself behaves for different base currents and collector-emitter voltages.

What is a load line?

A straight line on the IC versus VCE plot representing all operating points allowed by the external circuit.

What determines the load line?

The external circuit values, mainly VCC and collector/emitter resistors.

For a simple collector resistor circuit, what is the load line equation?

VCE + IC RC = VCC.

How can the simple load line equation be rearranged?

IC = (VCC - VCE) / RC.

What is the cutoff intercept of the load line?

At IC = 0, VCE = VCC.

What is the saturation-side intercept of the load line?

At VCE ≈ 0, IC ≈ VCC / RC.

What does the slope of the load line depend on?

The collector resistance; for the simple case, slope = -1/RC.

What is the Q-point?

The DC operating point of the transistor.

How do you find the Q-point graphically?

Find the intersection between the load line and the BJT IV curve for the chosen base current.

What does the Q-point give you?

The operating values of VCE, IC, and IB.

Why is the Q-point important?

It determines whether the BJT is in cutoff, active region, or saturation.

Why do we bias a BJT?

To place the Q-point in the desired operating region, usually the active region for amplification.

For an amplifier, where should the Q-point usually be?

In the active region, away from cutoff and saturation, so the signal can swing without clipping.

What happens if the Q-point is too close to cutoff?

The output can clip because the transistor turns off during part of the signal.

What happens if the Q-point is too close to saturation?

The output can clip because the transistor cannot lower VCE much further.

What is the relationship between the IV curve and the load line?

The IV curve describes the transistor, and the load line describes the external circuit constraint.

What is the best short phrase for IV curve vs load line?

IV curve = transistor behavior; load line = circuit constraint; Q-point = intersection.

Does the load line change if the BJT is replaced but the resistors and supply stay the same?

No. The load line is set by the external circuit, not by the specific transistor.

What changes when the BJT is replaced?

The BJT IV curves can change because beta, VBE, leakage, and Early effect can differ.

If a new BJT has a different beta, what happens to the Q-point?

The Q-point may move because the transistor curve or base current relationship changes.

Why does beta variation not directly change the load line?

Beta is a transistor property, while the load line comes from KVL around the external collector circuit.

What is a common wrong answer about replacing the BJT?

Saying the load line changes because beta changes.

What is the correct answer about replacing the BJT?

The load line stays the same if VCC and resistors are unchanged, but the operating point may change.

When would the load line change?

If VCC, RC, RE, or the external load changes.

In the notes example, what is the load line equation?

375 IC + VCE = 4.5, with IC in amps.

In the notes example, what was the approximate Q-point for IB = 30 µA?

VCE ≈ 2.7 V and IC ≈ 4.8 mA.

What does the load line physically represent?

The tradeoff between collector current and collector-emitter voltage caused by the voltage drop across the circuit resistors.

Why does increasing IC reduce VCE on the load line?

Because more collector current causes a larger voltage drop across RC, leaving less voltage across the transistor.

What does the IV curve show in the active region?

For each IB, IC is approximately constant over a wide range of VCE.

What does the load line show in the active region?

Which one of the possible transistor operating points is allowed by the external circuit.

What is the oral-exam one-sentence answer for the IV curve?

The IV curve is a family of IC versus VCE curves for different IB values, used to identify cutoff, active region, saturation, beta, and the operating point.

What is the oral-exam one-sentence answer for the load line?

The load line is the external circuit constraint relating IC and VCE, and its intersection with the BJT IV curve gives the DC operating point.

What is the most important distinction to remember?

The IV curve belongs to the transistor, while the load line belongs to the circuit.