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Discrete audio amplifier basics - Part 2: JFETs, MOSFETs and other circuit configurations

JFETs (cont.)
3/24/2010 06:00 PM EDT
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kendallcp
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re: Discrete audio amplifier basics - Part 2: JFETs, MOSFETs and other circuit configurations
kendallcp   7/20/2010 12:34:31 PM
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It's lovely to see such extended treatment of JFET amplifiers. How many other people out there get the same rather wistful feeling that I do, though? The big semiconductor manufacturers have culled their JFET ranges to extinction, leaving the market mostly to specialist vendors such as Linear Systems. Depletion-mode FETs have the wonderful characteristic of being conducting when no gate potential is applied; this behaviour is really hard to replicate otherhow. There are a few depletion MOS devices out there; single-source and in quite large packages. I feel rather sad that the JFET is slowly fading from our design consciousness. What do you think?

WKetel
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re: Discrete audio amplifier basics - Part 2: JFETs, MOSFETs and other circuit configurations
WKetel   4/1/2010 12:18:46 AM
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This explanation of the symbols is useful. Now I need to convince others that it has value.

bcarso
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re: Discrete audio amplifier basics - Part 2: JFETs, MOSFETs and other circuit configurations
bcarso   3/28/2010 8:09:16 PM
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You need to establish a base bias suffficent to operate the lower device in/near the pinchoff region, i.e., where its output impedance is reasonably high. You do not want the base voltage to (partially) track with the circuit output voltage---this tends to defeat the purpose of the cascade, which is to minimize the voltage swing on the drain of the input device, thus reducing drain-gate capacitance multiplication ("Miller effect"). In some cases it is advantageous to derive the base voltage from a sample of the actual circuit input voltage, reducing the effective FET input capacitance further---however, this will result in negative input impedance at high frequencies and potential instabilities.

wreeve
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re: Discrete audio amplifier basics - Part 2: JFETs, MOSFETs and other circuit configurations
wreeve   3/28/2010 4:39:18 AM
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I would like to explore the cascode configuration of fig. 9.15 but with Q2 replaced with a BJT. My question concerns biasing of Q2 when it is a BJT in a common base configuration. Does it matter if the base biasing resistor is connected directly to Vcc or the bottom of R2 where the Output is tapped? How do both arrangements affect the overall cascode circuit gain?

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