Simulating a transistor to investigate the collector

Question

Lab Assignment

Objective:

To Study NPN transistor IN curves by:

• Simulating a transistor to investigate the collector current versus the collector-to-emitter voltage.
• Implementing a circuit and taking measurements of the I_C vs. V_CE curves.

Materials:

1. Laboratory setup, including breadboard
2. 1 NPN transistor (e.g., 2N2222)
3. Several wires and resistors of varying sizes

Part 1: Simulation:

Consider the circuit shown below.

Enter the circuit into your simulator's schematic editor, applying DC voltage supplies to the base and to the collector of the transistor.

I_C vs.V_BE

• While setting V_CE to a constant value of 5V, sweep the base voltage from 0V to 0.8V in increments of 0.1 V. Plot a curve of I_C vs. V_BE and place the plot in your lab writeup.

• At what value of V_BE does current begin to conduct?
• What are the values of I_B and IC when V_BE: 0.7V?
• Based on these numbers, what is your estimate of β?

I_C vs. V_CE

• For three values of V_BE (0.6 V, 0.7 V. and 0.8 V), sweep the collector voltage from 0V to 2V in increments of 0.1 V. Plot the curves for I_C vs. V_CE, using a graphing program indicating the value of V_BE; next to each curve.

• What observations can be made about the curves?

Part 2: Measurements:

Assemble the circuit from the figure above, using a power supply to generate the DC voltages.

I_C vs. V_BE

• While setting V_CE to a constant of 5 V. sweep the base voltage from0 to 0.8V in increments of 0.1V, and measure the collector current using the power supply. (Note that not all power supplies allow you to accurately measure current. It this is the case for your lab setup, you place a small resistor in series with the collector and measure the voltage drop across the resistor.)

• Plot a curve of I_C vs V_BE
• At what value of V_BE does the current turn on?
• Using a small resistor placed in series with the base and collector terminals, measure I_B and I_C for V_BE = 0.7V
• Based on these numbers, what is your 3?

I_C VS. V_CE

• For three values of V_BE (0.6 V, 0.7 V, and 0.8 V), sweep the V_CE from 0 V to 1 V in increments of 0.1 V, and measure the collector current using the power supply.

• Plot the curves for I_C vs. V_CE using a graphing program and clearly indicating the value of next to each curve.

Part 3: Post-Measurement Exercise:

Simulation vs. Measurement

• What are the main differences between your simulated and measured curves? Can you explain the differences?

Early voltage, V_A

• Based on your simulated I_C vs. V_CE curves for an active transistor, extract the Early voltage V_A.
• Does VA change significantly for each value of V_BE?
• What is the average value of V_A?

Part 4: Design and Simulation

Consider the circuit shown below.

Design the circuit such that I_C = 1mA, V_B = 0, and V_C = +5V. Use supplies of V+ = V- = 15V. Although there will be variations tram transistor to transistor, you may assume a value 100 for β in your calculations. Also assume that VBE = 0.7V.

In your lab writeup, perform the following.

Hand Calculations

• What are l_B and 1_E? Based on these numbers, what is V_E?

• You now have enough information to calculate R_C and RC. Are the calculated values available in your kit? Can you achieve these values by combining several resistors? Comment.

• Derive the Thevenin equivalent of R₁ and R₂. What values of R₁ and do you need to use to achieve V_B = 0 V? Remember that I_B ≠ 0. Is the problem completely specified? If not, what needs to be specified?

Simulations

• Simulate your circuit using values of R_E, R_C, R₁, and R₂ based on you r calculations.

• Report the values of V_E, V_C, V_B, I_C, and I_B. How closely do they match your calculations? (Remember: The simulator has its own, more complex model of the real transistor, so there should be some small variations.)

Prototyping and Measurement

• Assemble the circuit onto a breadboard.
• Using a digital multimeter, measure V_E,V_C, and V_B.
• Using a digital multimeter, measure all resistors to three significant digits.

Post-Measurement Exercise

• What are the measured values of V_BE and V_CE? How do they compare to your pre-lab calculations? Explain any discrepancies.

• Based on the measured values of V_C and V_E and your measured resistor values, what are the measured values of I_B, I_C and I_E based on your lab measurements?

Part 5: NPN in Saturation Mode:

Hand Calculations

• Based on the specifications, calculate V_E and V_B.

• You now have enough information to calculate R_C and R_E. Are the calculated values available in your kit? Can you achieve this value by combining several resistors? Comment.

• What is β_forced?

• What values of R₁ and R₂ do you need to use? Is the problem in completely specified?

Simulation

• Simulate your circuit using values of R_E, R_C, R₁ and R₂ based on your calculations.

• Report the values of V_E, V_C, V_E, I_E, I_C, and I_B. How closely do they match your calculations?

Part 6:Post-Measurement Exercise:

Prototyping and Measurement

• Assemble the circuit onto a breadboard.
• Using your volt multimeter, measure V_E, V_C and V_B. Report them in your lab writeup.
• Using a digital multimeter, measure all resistors to three significant digits.

Post-Measurement Exercise

• What are the measured values of V_BE and V_CE? How do they compare to your pre-lab calculations? Explain any discrepancies.

• Based on the measured voltages and resistor values, what are the measured values of I_B, I_C and I_E, based on your lab measurements? What is βforced?