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JFET Amplifiers
15 questions By Tony R. Kuphaldt
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Question 7 of 15
This is a schematic of an RF amplifier using a JFET as the active element:
What configuration of JFET amplifier is this (common drain, common gate, or common source)? Also, explain the purpose of the two iron-core inductors in this circuit. Hint: inductors L1 and L2 are often referred to as RF chokes.
Reveal answerThis is a common-gate amplifier. The iron-core inductors block (“choke”) the high-frequency AC signals from getting to the DC power supply.
Notes:Be sure to ask your students why it would not be good for the RF signals to find their way to the DC power supply. There is more than one possible answer to this question!
This schematic was derived from an evaluation amplifier schematic shown in an
ON Semiconductor J308/J309/J310 transistor datasheet.
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Question 8 of 15
Calculate the approximate input impedance of this JFET amplifier circuit:
Explain why it is easier to calculate the Zin of a JFET circuit like this than it is to calculate the Zin of a similar bipolar transistor amplifier circuit. Also, explain how calculation of this amplifier’s output impedance compares with that of a similar BJT amplifier circuit - same approach or different approach?
Reveal answerZin = 89.2 kΩ
Notes:Ask your students to explain why input impedance is an important factor in amplifier design. Why should we care how much input impedance an amplifier has?
Also, ask your students to explain why such high-value bias resistors (150 kΩ and 220 kΩ) would probably not be practical in a BJT amplifier circuit.
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Question 9 of 15
Define what a common-source transistor amplifier circuit is. What distinguishes this amplifier configuration from the other single-FET amplifier configurations, namely common-drain and common-gate? What configuration of BJT amplifier circuit does the common-source 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-source amplifier configuration is defined by having the input and output signals referenced to the gate and drain terminals (respectively), with the source 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-source amplifier configuration most resembles the common-emitter BJT amplifier configuration in both form and behavior.
Common-source amplifiers are characterized by moderate voltage gains, and an 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.
