On Nov 6, 5:20 pm, nx7u wrote:
....
3. No the gates are not interchangable.

That's generally true of dual-gate mosfets, of course. Gate 2 is
similar to the control element of the upper transistor in a cascode.
Thus, in typical operation, gate 2 has very little effect on drain
current.

4. The datasheet does not give any Vg2s/Id curves, and Vg1s cannot be
positive (max gate current is exceeded nearly immediately) so you're
stuck with 7mA for this part.

?? That seems very strange. I would expect that, based on the
diagram shown in the data sheet for the part, the current into the
gate terminal would be very small up to the point where either the
protection diodes turned on, or the gate oxide broke down. If the
latter happens, you won't have any FET behavior ever again with that
part. ;-) The data sheet says 10mA max gate current (the allowable
gate current before the part is damaged), and that presumably is
through the protection diodes. For the BF991, that would be at a
minimum of 6 volts, and in any event should not be less than a
standard forward diode drop. If Francesco has a circuit where gate 2
is biased through a resistive divider which would limit the gate
current to less than 10mA, I would think there is no problem with
having reversed gate 1 and gate 2, though of course it won't work as
an amplifier that way.

Though it's possible to damage a transistor in a way that it will not
work well as an RF amplifier but will still show normal DC behavior,
I'd say it's very unlikely that's the case for biasing Gate 1 a bit
too positive. So if the DC behavior is normal (nominal zero-gate-1-
voltage drain current and decreasing drain current with negative gate
1 voltage; cutoff complete by -2.5V and likely somewhat before that;
"flat" drain current with changes in drain voltage above the gate 2
voltage...), I'd expect it to still be OK, or at least worth trying in
an RF amplifier.


The BF991 does look to be the same die as the BF981, just in the SMT
package.

In which case, I would expect no more than 50nA of gate-lead current
up to 5V g-s voltage, at 25C...


Cheers,
TOm

Tom et. al.,
You're right...I misread the datasheet, it shows Id skyrocketing if
Vg1s0. But of course the protection diodes should do the trick. The
datasheet also specifies Vg1s and Vg2s breakdown at 6V. So although
for practical purposes the gates are not interchangeable, it shouldn't
have killed the part.



On Nov 7, 10:18 pm, K7ITM wrote:
On Nov 6, 5:20 pm, nx7u wrote:
...

3. No the gates are not interchangable.

That's generally true of dual-gate mosfets, of course. Gate 2 is
similar to the control element of the upper transistor in a cascode.
Thus, in typical operation, gate 2 has very little effect on drain
current.

4. The datasheet does not give any Vg2s/Id curves, and Vg1s cannot be
positive (max gate current is exceeded nearly immediately) so you're
stuck with 7mA for this part.


TOm