Transistor Testing

Use the Type 575 semiconductor curve tracer in the electronic lab to test 2N3904 transistors in common emitter mode. Observe and take pictures of both the input and output characteristic plots.

Important note: To make sure you do not burn your transistor, (a) study the datasheet of the transistor to determine the leads for E, B and C, (b) do not apply $V_{CC}$ voltage higher than 30 V.

Initial settings before you start:

• Display control (top-right)
  • Adjust both the vertical and horizontal zero positions for the light spot to be at the lower-left corner of the grids;
  • Vertical axis (for $I_B$ or $I_C$): 0.5 or 1 mA/Div;
  • Horizontal axis ($V_{BE}$ or “base voltage”): 0.1 or 0.5 V/Div;
  • Horizontal axis ($V_{CE}$ or “collector voltage”): 2 V/Div.
• Collector voltage (lower-left)
  • Peak voltage range: x1 (be careful to use x10 as maximum voltage is 200 V!)
  • Polarity: + for npn transistors
  • Peak Volt ($V_{CC}$): 10 to 20 Volts
  • Dissipation limiting resistor ($R_C$): 1 k$\Omega$
• Base voltage (lower-right)
  • Repetitive/Single scan: single for input characteristic plot, repetitive for output characteristic plot
  • Steps/family: 12
  • Polarity: + for npn transistors
  • Steps/Sec: 240
  • Series resistor ($R_B$) (no relevance here)
  • Step selection: $0.005 mA = 5 \mu A$

What to observe:

• The input characteristics (confirm $V_{BE}\approx 0.7 V$ when $I_C=\beta I_B$ is in the range of 1 to 10 mA.)
• The output characteristics, observe specifically:
  • how the load line changes when $V_{CC}$ is changed;
  • how the load line changes when $R_C$ is changed;
  • find $\beta=\Delta I_C/\Delta I_B$ for different transistors. ($I_C$ is represented by the vertical axis (mA/Div) and $I_B$ is represented by the steps (mA/step).)

What to submit:

• Take screenshots of both the input and output characteristic plots observed on the semiconductor curve tracer with the following clear labeled (with meaningful increment):
  • The voltage (horizontal axis) $V_{BE}$ or $V_{CE}$
  • The current (vertical axis) $I_B$ or $I_C$
  • For the output plot, each one of the 12 $I_B$ values in the family
• Determine the value $\beta=I_C/I_B$ (also referred to as the “forward transfer current ratio” and denoted by $h_f$ or $h_{FE}$ on the 2N3904 datasheet (also see class notes) from the output plot.
• Determine $V_{BE}$ from the input plot when $I_B$ is within the range of 0.01 to 0.1 mA, i.e., $I_C$ is within the range of $0.01\;\beta$ to $0.1\;\beta$.
• Describe how the load line changes in the output plot when $R_C$ and $V_{CC}$ are increased or decreased.