I've seen several JFETs which have interchangeable drain and source terminals, which makes sense since at the die level their structure is symmetrical.
I've read that sometimes the channel is not the same thickness all the way along, and gate-drain capacitance differs from gate-source capacitance. Sometimes the datasheet mentions this, sometimes not.
How might I go about determining if a given JFET is indeed symmetrical? Or does it not really matter in practice when dealing with low-frequency applications like audio?
I've read that sometimes the channel is not the same thickness all the way along, and gate-drain capacitance differs from gate-source capacitance. Sometimes the datasheet mentions this, sometimes not.
How might I go about determining if a given JFET is indeed symmetrical? Or does it not really matter in practice when dealing with low-frequency applications like audio?
The different capacitance comes simply from the fact that there are
different DC voltages at source and drain in actual use.
Source and gate will usually be close together, so you have a diode
without much back bias, so capacitance is quite large.
The gate-drain voltage is usually a few volts, so the diode has more
back bias and thus less C. The voltage drop across the channel is
gradual, so you would have to inspect every few percent of the channel
separately to be really precise.
Every diode is a varicap, after all. If you switch source and drain,
so does the channel profile. It is not a constant. The voltages
used for testing are usually in the data sheet.
cheers, Gerhard
different DC voltages at source and drain in actual use.
Source and gate will usually be close together, so you have a diode
without much back bias, so capacitance is quite large.
The gate-drain voltage is usually a few volts, so the diode has more
back bias and thus less C. The voltage drop across the channel is
gradual, so you would have to inspect every few percent of the channel
separately to be really precise.
Every diode is a varicap, after all. If you switch source and drain,
so does the channel profile. It is not a constant. The voltages
used for testing are usually in the data sheet.
cheers, Gerhard
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I think I read somewhere that it is simpler to assume they are all symmetric. You should ask "how can I find those few which are not symmetric?".
In any case, it only matters for those circuits which assume a symmetric device.
In any case, it only matters for those circuits which assume a symmetric device.
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