The BJT: Current-Controlled Switch & Amplifier

Learning Objectives

Key Terms & Concepts

🖥 Tinkercad Setup Notes for Transistor Circuits

You know the routine from the Diode and Rectifier labs — this lab adds the transistor parts and a pushbutton:

• Transistors: search "NPN" and "PNP" in the parts panel — the parts are named NPN Transistor (BJT) and PNP Transistor (BJT). Hover over each of the three pins to see Tinkercad's collector / base / emitter labels before wiring anything. Do not assume pin order — verifying pinout is the habit that saves real hardware.
• Pushbutton: the 4-pin button connects its two sides when pressed; pins on the same side are already joined. Wire diagonal pins to be safe.
• LED polarity: the bent lead / flat side is the cathode. Anode toward positive. A red LED drops about 1.8–2 V at 10 mA.
• Two ammeters at once: Part 2 measures I_B and I_C simultaneously — one multimeter in A mode in the base loop, another in the collector loop. Expect I_B in microamps and I_C in milliamps; the unit prefix difference IS the story of this lab.
• β varies: Tinkercad's generic BJT model has a fixed β in the neighborhood of 100–200, but treat it as unknown — your job in Part 2 is to measure it. Real parts of the same number vary 2:1 or more.

🔧 Virtual Parts List

📐 The Circuits You Will Build

Three schematics, three jobs: the NPN LED switch (Part 1), the β-measurement circuit with both ammeters (Parts 2–3), and the PNP high-side switch (Part 4). Note the emitter arrows — they are your polarity compass all lab long.

Expected Results at a Glance

🛠 Step-by-Step Procedure

Part 1 — NPN Transistor as a Switch (≈25 min)

1. Create the workspace. New circuit named "LastName – BJT Lab." Place a small breadboard and a Power Supply set to 5.00 V, wired to the bottom + and − rails.
2. Identify the pins. Place an NPN Transistor (BJT) straddling three adjacent columns. Hover over each pin and write down which column holds the collector, base, and emitter. Every wiring mistake in this lab starts with skipping this step.
3. Ground the emitter. Black wire from the emitter column to the − rail.
4. Build the collector load. From the + rail: red wire → LED anode; LED cathode → 220 Ω resistor → the collector column. (Rail → LED → 220 Ω → collector, matching Figure 1.)
5. Build the base drive. From the + rail: wire → pushbutton (use diagonal pins) → 10 kΩ base resistor → the base column.
6. Test the switch. Start the simulation. LED off. Click and hold the pushbutton — LED on. A base current under half a milliamp is switching a collector current nearly thirty times larger. Record both states in Data Table 1.
7. Measure the proof. Add a voltmeter across collector-emitter. Record V_CE with the button up (≈5 V, cutoff/open) and pressed (≈0.2 V, saturated/closed). Then move the voltmeter to base-emitter and confirm V_BE ≈ 0.7 V while pressed — the Diode lab's junction, alive inside the transistor.

Part 2 — Measure the Current Gain β (≈25 min)

1. Rebuild for measurement. Remove the LED, button, and 220 Ω. Install the Figure 2 circuit: 1 kΩ from + rail toward the collector through multimeter M2 (mode A), and 470 kΩ from + rail toward the base through multimeter M1 (mode A). Voltmeter M3 across C-E.
2. Predict first. I_B = (5 − 0.7)/470 k ≈ 9 µA. Write your predicted I_B in Data Table 2 before simulating.
3. Run and record — 470 kΩ. Log I_B, I_C, and V_CE. Compute β = I_C/I_B. V_CE should sit somewhere in the middle volts — active region, so this β is legitimate.
4. Repeat — 100 kΩ. Swap the base resistor, re-run, record, compute I_C/I_B again. Compare against the 470 kΩ row: still close to the same β? Check V_CE — still comfortably above 0.3 V?
5. Repeat — 10 kΩ. Swap, run, record. I_B jumped to ≈430 µA but I_C is stuck near 4.8 mA and V_CE collapsed to ≈0.2 V. Compute I_C/I_B for this row too — it is far below your measured β. That number is not the transistor's gain; it is the collector resistor talking.
6. State your β. Average the two active-region rows and record your measured β at the bottom of Data Table 2, with one sentence on why the 10 kΩ row was excluded.

Part 3 — Map the Three Regions (≈15 min)

1. Cutoff. Disconnect the base resistor from the + rail entirely (base open). Run: I_C ≈ 0, V_CE ≈ 5 V. Record as the "cutoff" row of Data Table 2.
2. Sweep with the supply. Reinstall the 470 kΩ base resistor. Set the supply to 1.0 V, 2.0 V, 3.5 V, 5.0 V, re-running each time and logging I_B, I_C, V_CE. Watch the transistor wake up as the base drive crosses ~0.7 V and march toward saturation.
3. Classify. Label every row of Data Table 2 with its region — cutoff, active, or saturation — using the V_CE referee: ≈supply = cutoff, ≈0.2 V = saturation, in between = active. Cross-check with the interactive explorer below.

Part 4 — PNP High-Side Switch (≈20 min)

1. Rebuild with the PNP. Restore the supply to 5.00 V. Place a PNP Transistor (BJT), hover-verify its pinout, and wire Figure 3: emitter to the + rail, LED anode to the collector, LED cathode → 220 Ω → − rail.
2. Base network. Base → 10 kΩ → pushbutton → − rail. The button now connects the base toward ground, not toward the supply.
3. Add the pull-up (recommended). A 100 kΩ resistor from base to the + rail. With the button up this holds V_EB = 0 and guarantees a clean OFF; without it the base floats when released, and a floating base is bad practice in real hardware.
4. Test the inverted logic. Run. Button up: LED off. Button pressed (base pulled LOW): LED on. Record both states, including V_CE — note its sign relative to the NPN circuit — in Data Table 3.
5. Articulate the mirror. One sentence for the report: state the turn-on condition of each transistor type in terms of where the base must sit relative to the emitter (±0.7 V).
6. Wrap up. Complete the Analysis Questions and the Knowledge Check, then Print/Save your report with the data tables filled in.

📋 Data Tables

Type readings directly into the tables — they persist when you print this page.

Data Table 1 — NPN LED Switch (Part 1)

Data Table 2 — β Measurement & Region Map (Parts 2–3, R_C = 1 kΩ)

Data Table 3 — PNP High-Side Switch (Part 4)

📈 Interactive BJT Curve & Load-Line Explorer

This companion simulator models the Figure 2 measurement circuit (V_CC = 5 V). The navy curves are the transistor's characteristic family — collector current versus V_CE for different base currents. The amber dashed line is the load line imposed by R_C, and the red dot is the operating point where the two must agree. Drive the base and watch the Q-point slide from cutoff, up the load line through the active region, and pile into saturation.

✍ Analysis Questions

Answer in complete sentences in your lab report.

1. In Part 1, compute the ratio I_C/I_B for the pressed-button state using your Data Table 1 values and the expected currents. Why is this ratio smaller than the β you measured in Part 2?
2. Using your Data Table 2 active-region rows, show the β calculation for each and explain why the two values agree (or account for any difference).
3. For the R_B = 10 kΩ row, I_C/I_B was far below β. What physically limits I_C in this condition? Support your answer with the measured V_CE.
4. The base-emitter junction measured ≈0.7 V whenever the transistor conducted. Connect this observation to the Basic Diode lab: what is the base-emitter junction, structurally?
5. State the turn-on condition for the NPN and the PNP in one sentence each, referencing the base-emitter voltage polarity. Why is the PNP called an "active-LOW" device?
6. Your thermostat's 5 V logic output must switch a 12 V fan whose chassis-grounded wiring cannot be interrupted on the ground side. Which transistor type and switch topology from this lab fits, and why?
7. A classmate reports β = 11 measured with R_B = 10 kΩ and concludes the transistor is defective. Using the load-line explorer or your data, explain their error and describe the measurement conditions they should use instead.

Key Facts Reference Box

Interactive Knowledge Check