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JFET Amplifiers
15 questions By Tony R. Kuphaldt
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Question 10 of 15
Define what a common-gate transistor amplifier circuit is. What distinguishes this amplifier configuration from the other single-FET amplifier configurations, namely common-drain and common-source? What configuration of BJT amplifier circuit does the common-gate FET circuit most resemble in form and behavior?
Also, describe the typical voltage gains of this amplifier configuration, and whether it is inverting or non-inverting.
Reveal answerThe common-gate amplifier configuration is defined by having the input and output signals referenced to the source and drain terminals (respectively), with the gate terminal of the transistor typically having a low AC impedance to ground and thus being “common” to one pole of both the input and output voltages.
The common-gate amplifier configuration most resembles the common-base BJT amplifier configuration in both form and behavior.
Common-gate amplifiers are characterized by moderate voltage gains, and a non-inverting phase relationship between input and output.
Notes:The answers to the question may be easily found in any fundamental electronics text, but it is important to ensure students know why these characteristics are such. I always like to tell my students, “Memory will fail you, so you need to build an understanding of why things are, not just what things are.”
One exercise you might have your students do is come up to the board in front of the room and draw an example of this circuit, then everyone may refer to the drawn image when discussing the circuit’s characteristics.
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Question 11 of 15
Define what a common-drain transistor amplifier circuit is. What distinguishes this amplifier configuration from the other single-FET amplifier configurations, namely common-source and common-gate? What configuration of BJT amplifier circuit does the common-drain FET circuit most resemble in form and behavior?
Also, describe the typical voltage gains of this amplifier configuration, and whether it is inverting or non-inverting.
Reveal answerThe common-drain amplifier configuration is defined by having the input and output signals referenced to the gate and source terminals (respectively), with the drain terminal of the transistor typically having a low AC impedance to ground and thus being “common” to one pole of both the input and output voltages.
The common-drain amplifier configuration most resembles the common-collector BJT amplifier configuration in both form and behavior.
Common-drain amplifiers are characterized by low voltage gains (less than unity), and a non-inverting phase relationship between input and output.
Notes:The answers to the question may be easily found in any fundamental electronics text, but it is important to ensure students know why these characteristics are such. I always like to tell my students, “Memory will fail you, so you need to build an understanding of why things are, not just what things are.”
One exercise you might have your students do is come up to the board in front of the room and draw an example of this circuit, then everyone may refer to the drawn image when discussing the circuit’s characteristics.
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Question 12 of 15
Determine whether this amplifier circuit is inverting or non-inverting (i.e. the phase shift between input and output waveforms):
Be sure to explain, step by step, how you were able to determine the phase relationship between input and output in this circuit. Also identify the type of amplifier each transistor represents (common-???).
Reveal answerNon-inverting. The JFET is connected as a common-source, while the BJT is connected as a common-emitter.
Notes:There are several other questions you could ask about this amplifier circuit. For example:
- How is the Q-point bias established for the JFET?
- How is the Q-point bias established for the BJT?
- What purpose does the potentiometer serve?
- Is there another possible location for the potentiometer that would perform the same function?
Note: the schematic diagram for this circuit was derived from one found on page 36 of John Markus’
Guidebook of Electronic Circuits, first edition. Apparently, the design originated from a Motorola publication on using field effect transistors (“Tips on using FET’s,” HMA-33, 1971).
