TL082 input current?

[Chris Carlen]

Greetings:

The TL08{1,2,4} datasheet lists no input current spec, and only has this
to say:

"The magnitude of the input voltage must never exceed the magnitude of
the supply voltage or 15V, whichever is less."

I would have expected to find a max input current spec of 10-20mA or
something.

Is there some reason why the JFET input amp cannot tolerate ANY input
current?

I was hoping to use a resistor, to a bipolar clamp to ground, preceding
another resistor to the opamp input for protection. But if they are to
be taken seriously, then this wouldn't protect in the absence of power,
because current would flow. Thus, I may have to go back to the clamp to
rails approach, which forces me out of the cheap TL082 to something else
that can tolerate slightly more input voltage than 15V (in other words,
specified input current max).

Don't ya' just love datasheets!

Thanks for comments.


Good day!




--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarle@sandia.gov -- NOTE: Remove "BOGUS" from email address to reply.

 

[John Doe]

Chris Carlen <crcarle@BOGUS.sandia.gov> wrote:

....

Don't ya' just love datasheets!

Yes, but I'm not an engineer.
Good luck.

 

[Anthony Fremont]

Chris Carlen wrote:

Greetings:

The TL08{1,2,4} datasheet lists no input current spec, and only has
this to say:

"The magnitude of the input voltage must never exceed the magnitude of
the supply voltage or 15V, whichever is less."

I would have expected to find a max input current spec of 10-20mA or
something.

Is there some reason why the JFET input amp cannot tolerate ANY input
current?

I was hoping to use a resistor, to a bipolar clamp to ground,
preceding another resistor to the opamp input for protection. But if
they are to be taken seriously, then this wouldn't protect in the
absence of power, because current would flow. Thus, I may have to go
back to the clamp to rails approach, which forces me out of the cheap
TL082 to something else that can tolerate slightly more input voltage
than 15V (in other words, specified input current max).

Don't ya' just love datasheets!

Thanks for comments.

An ideal op-amp has infinite input impedance, therfore the current
flowing into an input would be 0 regardless of the voltage present.
Obviously this is the real world and the input impedance is "much"
lower, you should expect several uA of input current to flow. Just
don't present a voltage to an input that is outside the supply rails.

 

[Winfield Hill]

Joerg wrote...

JFETs can be rather fickle. I remember when I was young and used BF245
and others in an antenna pre-amp. With protection diodes and all. Almost
after every thunderstorm they had either gone to lalaland or exhibited
very degraded performance. To avoid having to climb a slippery roof all
the time I finally replaced it with, ahem, a tube amp. No more trips
onto the wet roof, ever. No forming of ice clumps on the amp either ;-)

With a Ciss of only 2 - 4pF at -1V (where Id ~= 1mA), the BF245 appears
to be a rather fragile device. Hmm, I've noticed they're pitching much
higher-capacitance JFETs now for receivers.


--
Thanks,
- Win

 

[gwhite]

Winfield Hill wrote:

With a Ciss of only 2 - 4pF at -1V (where Id ~= 1mA), the BF245 appears
to be a rather fragile device.

Why do those data points suggest fragility?

 

[Ken Smith]

In article <42019E11.314E5111@deadend.com>, gwhite <gwhite@deadend.com> wrote:

Winfield Hill wrote:


With a Ciss of only 2 - 4pF at -1V (where Id ~= 1mA), the BF245 appears
to be a rather fragile device.

Why do those data points suggest fragility?

Low capacitance at low voltages means a physically small device. Small
devices are easier to blow up.

--
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