Performance-Based Assessments for Semiconductor Circuit Competencies

I have had great success with the following values:

V = 7 to 24 volts

C₁ and C₂ = 0.001 μF

C₃, C₄, and C₅ = 0.47 μF

L₁ = 100 μH (ferrite core RF choke)

R₁ = 22 kΩ

R₂ = 1.5 MΩ

R₃ = 6.8 kΩ

R₄ = 100 kΩ

Q₁ = part number 2N3403

Q₂ = part number MPF 102

With these component values, the carrier waveform was quite clean and the frequency was almost exactly 700 kHz:

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

Modulation isn’t that great, due to the crude nature of the circuit, but it is certainly good enough to hear over an appropriately tuned AM radio. Setting V_signal and f_signal is a matter of experimentation, to achieve the desired degree of modulation and tone pitch.

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.).

This circuit demonstrates the use of passive integrators to convert a square wave into a pseudo-sine wave output. The multivibrator portion 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. Component values I’ve used with success are 1 kΩ for R₁ and R₄, 100 kΩ for R₂ and R₃, and 0.001 μF for C₁ and C₂.

Resistors R₅ and R₆, along with capacitors C₃ and C₄, form a dual passive integrator network to re-shape the square-wave output of the multivibrator into a pseudo-sine wave. These components’ values must be chosen according to the multivibrator frequency, so that the integration is realistic without the attenuation being excessive. Integrator component values that have worked well for the multivibrator components previously specified are 10 kΩ for R₅ and R₆, and 0.1 μF for C₃ and C₄.

Transistor Q₃ is just an emitter follower, placed there to give the amplifier section a high input impedance. Q₃‘s emitter resistor value is not critical. I have used a 1 kΩ resistor for R₇ with good success.

The last transistor (Q₄) is for voltage amplification. A “trimmer” style potentiometer (10 kΩ recommended for R_pot) provides easy adjustment of biasing for different supply voltages. Using the potentiometer, I have operated this circuit on supply voltages ranging from -6 volts to -27 volts. Use a bypass capacitor (C₇) large enough that its reactance at the operating frequency is negligible (less than 1 ohm is good), such as 33 μF. Resistor values I’ve used with success are 10 kΩ for R₈ and 4.7 kΩ for R₉. Coupling capacitor values are not terribly important, so long as they present minimal reactance at the operating frequency. I have used 0.47 μF for both C₅ and C₆ with good success.

You may find that the relatively high operating frequency of this circuit complicates matters with regard to parasitic capacitances. The fast rise and fall times of the strong square wave tend to couple easily to the sine-wave portions of the circuit, especially when the sine wave signal is so severely attenuated by the double integrators. One solution to this dilemma is to lower the operating frequency of the circuit, allowing a lower cutoff frequency for the double integrator (two-pole lowpass filter) section which in turn will improve the signal-to-noise ratio throughout. If you wish to try this, you may use these suggested component values:

R₁ = 1 kΩ

R₂ = 100 kΩ

R₃ = 100 kΩ

R₄ = 1 kΩ

R₅ = 100 kΩ

R₆ = 100 kΩ

R₇ = 1 kΩ

R₈ = 10 kΩ

R₉ = 4.7 kΩ

R_pot = 10 kΩ

C₁ = 0.047 μF

C₂ = 0.047 μF

C₃ = 0.1 μF

C₄ = 0.047 μF

C₅ = 1 μF

C₆ = 1 μF

C₇ = 33 μF

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. Have students calculate the necessary current-limiting resistor for their LEDs based on measured values of V_forward for the LED (using a multimeter with a “diode-check” function). Let students research the typical forward current for their LED from an appropriate datasheet. Any LED should suffice for this activity.

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.