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JFET cascode current source

Hi,
I am building a cascode current source which is derived from a self
biased JFET current source. I do not get how these are biased. If J1
is in constant current region how will J2 is biased. Please help me
understand this.
 
W

Winfield Hill

Jan 1, 1970
0
Hi,
I am building a cascode current source which is derived from a self
biased JFET current source. I do not get how these are biased. If J1
is in constant current region how will J2 is biased. Please help me
understand this.
.
|
|-+
| J1
o------>|-+
| |
| |-+
| | J2
.------>|-+
| |
| .-.
| | |R1
| | |
| '-'
| |
'---------'

This requires that J1 have a higher Vgs at the
operating Id than J2 has. J2 operates with its
Vds equal to the Vgs difference. Because this
Vds voltage changes very little with Vd on J1,
the circuit exhibits a lower Id change with Vd.
 
W

Winfield

Jan 1, 1970
0
JosephKK said:
Winfield Hill posted to sci.electronics.design:


Win, please do not do these twits homework for them.

Sorry, but I see that his answer will be marked
wrong, because I left out a critical piece.
 
Thanks a ton Win,
By the way, i think i have passed the age of getting homeworks long
ago:).
I found an old scope and am tearing it apart (being a biologist i like
the term "tearing apart") to look at some of the circuits.
 
S

Spehro Pefhany

Jan 1, 1970
0
Thanks a ton Win,
By the way, i think i have passed the age of getting homeworks long
ago:).
I found an old scope and am tearing it apart (being a biologist i like
the term "tearing apart") to look at some of the circuits.

Boston University medical campus, eh?

Best regards,
Spehro Pefhany
 
W

Winfield Hill

Jan 1, 1970
0
Thanks a ton Win,
By the way, i think i have passed the age of getting
homeworks long ago:).
I found an old scope and am tearing it apart (being a
biologist i like the term "tearing apart") to look at
some of the circuits.

The missing piece of analysis is, of course, that
Vgs1 must be greater than Vgs2 + Id R1. So you've
found this circuit in an old Tek scope? I wonder
what JFETs they used. IIRC, if they're special Tek
numbers we may still be able to find equivalents,
but it's likely they would be selected parts, for
the desired Vgs voltage range.
 
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