Performance-Based Assessments for Semiconductor Circuit Competencies

Use a variable-voltage, regulated power supply to supply any amount of DC voltage below 30 volts. Specify standard resistor values, all between 1 kΩ and 100 kΩ (1k5, 2k2, 2k7, 3k3, 4k7, 5k1, 6k8, 10k, 22k, 33k, 39k 47k, 68k, etc.).

This circuit produces nice, sharp-edged square wave signals at the transistor collector terminals when resistors R₁ and R₄ are substantially smaller than resistors R₂ and R₃. This way, R₂ and R₃ dominate the capacitors’ charging times, making calculation of duty cycle much more accurate. Component values I’ve used with success are 1 kΩ for R₁ and R₄, 100 kΩ for R₂ and R₃, and 0.1 μF for C₁ and C₂.

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.

Use a variable-voltage, regulated power supply to supply any amount of DC voltage below 30 volts. Specify standard resistor values, all between 1 kΩ and 100 kΩ (1k5, 2k2, 2k7, 3k3, 4k7, 5k1, 6k8, 10k, 22k, 33k, 39k 47k, 68k, etc.).

R_pot serves the purpose of providing variable AC gain in the first amplifier stage to meet the Barkhausen criterion.

I have had good success with the following values:

V_CC = 12 volts

C₁ and C₂ = 0.001 μF

C₃ = 47 μF

C₄ = 0.47 μF

R₁ and R₂ = 4.7 kΩ

R₃ = 4.7 kΩ

R₄ = 39 kΩ

R₅ = 22 kΩ

R₆ = 27 kΩ

R₇ = 3.3 kΩ

R_pot = 10 kΩ, linear

Q₁ and Q₂ = part number 2N2222

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.

I have had great success with the following values:

V_CC = 7 to 24 volts

C₁ and C₂ = 0.22 μF

C₃ = 0.47 μF

L₁ = 100 μH (ferrite core RF choke)

R₁ = 22 kΩ

R₂ = 1.5 MΩ

Q₁ = part number 2N3403

With these component values, the output waveform was quite clean and the frequency was very close to predicted:

$$f_{out} = \frac{1}{2{\pi}\sqrt{\frac{LC_1C_2}{C_1+C_2}}}$$

You might want to quiz your students on the purpose of resistor R₂, since it usually only has to be present at power-up to initiate oscillation!

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.