Performance-Based Assessments for Semiconductor Circuit Competencies

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.). Use a sine-wave function generator to supply an audio-frequency input signal, and make sure its amplitude isn’t set so high that the amplifier clips.

I have had good success using the following values:

V_DD = 9 volts

V_in = 1 volt (peak), f = 2 kHz

R_G = 100 kΩ

R_S = 10 kΩ

C₁ = 0.47 μF

Please note that the quiescent output voltage is impossible to precisely predict, as it depends on the particular characteristics of the JFET used (I_D versus V_GS). The fact that this circuit uses self-biasing instead of voltage divider biasing makes the situation worse. Predicting quiescent gate voltage, however should be extremely easy (0 volts) if one understands how JFETs function.

An interesting parameter to explore in this circuit is the effect of the source resistor value on voltage gain. The theoretical voltage gain of a simple common-drain amplifier circuit is unity (1), but this may be approximated only with relatively large load resistor (R_S) values. Try substituting a 1 kΩ or less resistor for R_S, and notice what happens to the gain. Then, have your students explain why this happens!

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.). Use a sine-wave function generator to supply an audio-frequency input signal, and make sure its amplitude isn’t set so high that the amplifier clips.

I have had good success using the following values:

V_DD = 12 volts

V_in = 0.5 volt (peak-to-peak), f = 2 kHz

R_G = 100 kΩ

r_S = 2.2 kΩ

R_S = 10 kΩ

R_D = 10 kΩ

C₁ = 0.47 μF

C_bypass = 10 μF

Please note that the quiescent output voltage is impossible to precisely predict, as it depends on the particular characteristics of the JFET used (I_D versus V_GS). The fact that this circuit uses self-biasing instead of voltage divider biasing makes the situation worse. Predicting quiescent gate voltage, however should be extremely easy (0 volts) if one understands how JFETs function.

All quiescent circuit values depend on V_DD, so if things aren’t biased the way you would like, simply adjust the power supply voltage to suit.

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.