AN103
2
Siliconix
10-Mar-97
D
S
D
S
–
+
–
+
–
+
(a)
(b)
Figure 3.
Cascade FET VGS1 = 0
Q2
Q1
VGS2gfs2
goss2
I2
VDS2
goss1
VDS1 = –VGS2
= IO/goss1
IO
VO
Now, ID is regulated by Q1 and VDS1 = –VGS2. The dc val-
ue of ID is controlled by RS and Q1. However, Q1 and Q2
both affect current stability. The circuit output conduc-
tance is derived as follows:
If goss1 = goss2
(7)
(8)
go
+
goss
2
)
g
fs
goss
when RS
0 0 as in Figure 2
(9)
go
[
goss2
g
fs 1 ) RSgfs
In either case (RS = 0 or RS
0), the circuit output
conductance is considerably lower than the goss of a
single FET.
In designing any cascaded FET current source, both FETs
must be operated with adequate drain-gate voltage, VDG.
That is,
VDG
u VGS(off), preferably VDG u 2VGS(off)
(10)
If VDG < 2 VGS(off), the goss will be significantly
increased, and circuit go will deteriorate. For example: A
JFET may have a typical goss = 4
mS at VDS = 20 V and
VGS = 0. At VDS
–VGS(off) = 2 V, goss 100 mS.
The best FETs for current sources are those having long
gates and consequently very low goss. The Siliconix
2N4340, J202, and SST202 exhibit typical goss = 2
mS at
VDS = 20 V. These devices in the circuit of Figure 4 will
provide a current source adjustable from 5
mA to 0.8 mA
with internal impedance greater than 2 M
W at 0.2 mA.
Other Siliconix part types such as the 2N4392, J112, and
SST112 can provide 10 mA or higher current.
Q1
D
S
–
+
30 V
Figure 4.
Adjustable Current Source RS = 1 MW
RS = 1 MW
RS
200
W (Optional)
Instead of the adjustable resistor, the JFETs can be put in
IDSS range groupings with an appropriate RS resistor
selected for each group. This method is common in high
volume applications.
The cascade circuit of Figure 5 provides a current
adjustable from 2
mA to 0.8 mA with internal resistance
greater than 10 M
W.
D
S
–
+
30 V
D
S
Q2
Q1
100
W
(Optional)
RS
Q1 = 2N4340, J202, SST202
Q2 = 2N4341, J304, SST304
RS = 1 MW
Figure 5.
Cascade FET Current Source
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
