Output Characteristics of an NPN Transistor

Experiment: Output Characteristics of an NPN Transistor (Common-Emitter)

1. Aim

To plot the output characteristics ($I_C$ vs $V_{CE}$ for fixed $I_B$) and the transfer characteristics ($I_C$ vs $I_B$) of an NPN transistor in the common-emitter configuration, and to determine the current gain $h_{FE}$ and output impedance from the curves.

2. Apparatus / Components Required

• SEELab3 unit (both PV1 and PV2 required)
• NPN transistor: 2N2222 ($h_{FE} \approx 200$)
• Collector resistor $R_C = 1\text{ k}\Omega$
• Base resistor $R_B = 100\text{ k}\Omega$
• Connecting wires
• PC with Python 3 (libraries: expeyes, numpy, scipy, matplotlib)

3. Theory & Principle

In the common-emitter (CE) configuration, the emitter is the common terminal between input (base) and output (collector) circuits. The transistor has three operating regions:

• Cut-off: $V_{BE} < 0.6\text{ V}$ — both junctions reverse biased; $I_C \approx 0$.
• Active: $V_{BE} \approx 0.7\text{ V}$, $V_{CE} > V_{CE,sat}$ — base-emitter forward biased, base-collector reverse biased. Collector current is controlled by base current: $I_C = h_{FE} \cdot I_B$.
• Saturation: $V_{CE} < V_{CE,sat} \approx 0.2\text{ V}$ — both junctions forward biased; $I_C$ no longer controlled by $I_B$.

The DC current gain:

For the 2N2222, $h_{FE} \approx 200$, meaning a base current of $10\text{ }\mu\text{A}$ drives a collector current of $\approx 2\text{ mA}$.

Indirect current measurement

Both $I_B$ and $I_C$ are measured without ammeters, using the same technique as the diode experiments:

where A2 monitors the base voltage and A1 monitors the collector voltage.

Output impedance

In the active region the $I_C$ vs $V_{CE}$ curves are nearly — but not perfectly — horizontal. The slight upward slope gives the output impedance:

A steep slope means low $r_o$ (less ideal current source); a flat curve means high $r_o$ (more ideal).

4. Circuit Diagram / Setup

1. Connect PV2 → $R_B$ ($100\text{ k}\Omega$) → Base of 2N2222. Connect A2 at the Base.
2. Connect PV1 → $R_C$ ($1\text{ k}\Omega$) → Collector. Connect A1 at the Collector.
3. Connect Emitter to GND.

5. Procedure

Part A — Output characteristics (app-based)

1. Open the SEELab3 app and select the “NPN Output Characteristics” experiment.
2. Set PV2 to a fixed value (e.g., $1.5\text{ V}$) to establish a base current $I_B \approx (1.5 - 0.7)/100\text{k} = 8\text{ }\mu\text{A}$.
3. The software sweeps PV1 from $0$ to $3.3\text{ V}$ in steps, recording $V_{A1}$ (collector voltage) at each step and computing $I_C = (V_{PV1} - V_{A1})/R_C$.
4. The $I_C$ vs $V_{CE}$ curve is plotted. Identify the saturation region (steep rise, $V_{CE} < 0.3\text{ V}$) and the active region (flat plateau).
5. Repeat for at least three different PV2 values to generate a family of curves.

Part B — Transfer characteristics and $h_{FE}$

1. Fix $V_{PV1}$ at a value that keeps the transistor in the active region (e.g., $2\text{ V}$).
2. Sweep PV2 in steps, recording $I_B$ and the corresponding $I_C$ at each step.
3. Plot $I_C$ vs $I_B$ — the slope of this line is $h_{FE}$.

Part C — Python automation

The following Python programs automate the sweep and analysis. Download them from the links below:

• npn-ce-output.py — sweeps PV1 for each of several PV2 (base current) values and saves $V_{CE}$, $I_C$, $I_B$ to a CSV file.
• Sample data (npn-ce-output.csv) — reference dataset for testing the analysis script.
• npn-ce-output-analyse.py — reads the CSV, plots the family of $I_C$ vs $V_{CE}$ curves, and prints $h_{FE}$ and $r_o$ for each base current.
• npn-ce-ibic.py — plots the transfer characteristic ($I_B$ vs $I_C$).
• npn-ce-ibic-lsfit.py — fits the $I_B$ vs $I_C$ data by least squares to extract $h_{FE}$.

Mobile App

Python Output

6. Observation Table

6a. Output Characteristics — $I_C$ vs $V_{CE}$

For each $I_B$ row, record $I_C$ (mA) at the listed $V_{CE}$ values.

6b. Transfer Characteristics and $h_{FE}$

6c. Output Impedance (active region, from Python slope)

7. Results and Discussion

• The output characteristics showed a clear saturation region ($V_{CE} < \approx 0.3\text{ V}$) where $I_C$ rises steeply, and an active region ($V_{CE} > 0.5\text{ V}$) where $I_C$ is nearly constant and proportional to $I_B$.
• The measured current gain was $h_{FE} =$ ____, against the datasheet value of $\approx 200$ for the 2N2222.
• The output impedance in the active region was $r_o \approx$ ____ k$\Omega$, indicating that the transistor acts as a good (though not ideal) current source in CE configuration.
• Increasing $I_B$ shifted the active-region plateau upward by $h_{FE} \times \Delta I_B$, confirming the linear relationship between base and collector currents.

8. Precautions

1. Never leave the base floating: A floating base picks up stray voltages and drives the transistor into an unpredictable state. Always connect $R_B$ from PV2 to the base, even when PV2 = 0 V.
2. PV1 limit is 3.3 V: The collector sweep is limited to $V_{CE,max} \approx 3.3\text{ V}$. This is sufficient to clearly observe both the saturation knee and the active plateau for the base currents used.
3. Power dissipation: At $I_C = 2\text{ mA}$ and $V_{CE} = 3\text{ V}$, the transistor dissipates $P = 6\text{ mW}$ — well within the 2N2222’s $625\text{ mW}$ limit. Do not use base currents that push $I_C$ beyond $3\text{ mA}$ with this setup.
4. Transistor pinout: The 2N2222 in TO-18 metal can and TO-92 plastic package have different pin orders. Verify the pinout (E, B, C) from the datasheet before inserting.

9. Troubleshooting