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? on biasing JFETs

D

Dave

Jan 1, 1970
0
Are JFETs biased differently from standard bipolar transistors? Them must
be, or the schematic I have is crap. The gate/drain junction is not biased
with reverse polarity, like I am used to with, say, 2N3904s. It looks like
it is forward based just like the source/gate junction. Any words of wisdom
are welcome. Obvious newbie here...

Thanks,

Dave
 
E

Eeyore

Jan 1, 1970
0
Dave said:
Are JFETs biased differently from standard bipolar transistors?

Totally. Because they work on an entirely different principle.
Them must be, or the schematic I have is crap. The gate/drain junction is not
biased
with reverse polarity, like I am used to with, say, 2N3904s. It looks like
it is forward based just like the source/gate junction. Any words of wisdom
are welcome. Obvious newbie here...

Actually, bipolar transistors are *forward* biased. You need some base current
for them to operate as linear amplifiers (as opposed to switches)

An n-channel jfet will however have 'negative bias' (wrt the source) on it's
gate. When Vgs = 0 the fet will pass the maximum current possible for that
device (Idss).

Graham
 
D

Dave

Jan 1, 1970
0
Eeyore said:
Totally. Because they work on an entirely different principle.


Actually, bipolar transistors are *forward* biased. You need some base
current
for them to operate as linear amplifiers (as opposed to switches)

An n-channel jfet will however have 'negative bias' (wrt the source) on
it's
gate. When Vgs = 0 the fet will pass the maximum current possible for that
device (Idss).

Graham

AAAhhhhh. Okay, I had no idea that Vgs=0 would pass *maximum* current for
that device. I obviously have a lot to learn... Thank you. Now I have
some idea as to what I am looking for. Much appreciated.

Dave
 
G

Guy Macon

Jan 1, 1970
0
Dave said:
Are JFETs biased differently from standard bipolar transistors?
Them must be, or the schematic I have is crap.

Don't assume that the two are mutually exclusive. It could be that
JFETs are biased differently from bipolars (they are) *and* the
schematic you have is crap.

I hope this helps. :)
 
T

Tim Wescott

Jan 1, 1970
0
Dave said:
Are JFETs biased differently from standard bipolar transistors? Them must
be, or the schematic I have is crap. The gate/drain junction is not biased
with reverse polarity, like I am used to with, say, 2N3904s. It looks like
it is forward based just like the source/gate junction. Any words of wisdom
are welcome. Obvious newbie here...
If the gate is a higher voltage than the drain (assuming the schematic
isn't crap) check to see if the JFET is P-channel (analogous to a PNP
bipolar).

Generally a JFET will conduct serious current when the gate-source
voltage is zero, so you will often see N-channel JFETs biased with a DC
path to the negative rail and the source connected to the same rail
through a small resistor.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
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