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參數資料
| 型號： | AN103 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | The FET Constant-Current Source/Limiter |
| 中文描述： | 場效應管的恒流源/限制器 |
| 文件頁數： | 2/5頁 |
| 文件大?。?/td> | 56K |
| 代理商： | AN103 |
AN103
2
Siliconix
10-Mar-97
D
S
D
S
–
+
V
DS2
–
+
–
+
(a)
(b)
Figure 3.
Cascade FET V
GS1
= 0
Q
2
Q
1
V
GS2
g
fs2
g
oss2
I
2
g
oss1
V
DS1
= –V
GS2
= I
O
/g
oss1
I
O
V
O
Now, I
D
is regulated by Q
1
and V
DS1
= –V
GS2
. The dc val-
ue of I
D
is controlled by R
S
and Q
1
. However, Q
1
and Q
2
both affect current stability. The circuit output conduc-
tance is derived as follows:
If g
oss1
= g
oss2
(7)
(8)
g
o
g
oss
2
gfs
goss
when RS
0 as in Figure 2
(9)
g
o
g
oss2
g
fs
1
R
S
g
fs
In either case (R
S
= 0 or R
S
conductance is considerably lower than the g
oss
of a
single FET.
0), the circuit output
In designing any cascaded FET current source, both FETs
must be operated with adequate drain-gate voltage, V
DG
.
That is,
V
DG
V
GS(off)
, preferably V
DG
2V
GS(off)
(10)
If V
DG
< 2 V
GS(off)
, the g
oss
will be significantly
increased, and circuit g
o
will deteriorate. For example: A
JFET may have a typical g
oss
= 4 S at V
DS
= 20 V and
V
GS
= 0. At V
DS
–V
GS(off)
= 2 V, g
oss
100 S.
The best FETs for current sources are those having long
gates and consequently very low g
oss
. The Siliconix
2N4340, J202, and SST202 exhibit typical g
oss
= 2 S at
V
DS
= 20 V. These devices in the circuit of Figure 4 will
provide a current source adjustable from 5 A to 0.8 mA
with internal impedance greater than 2 M 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 R
S
= 1 M
R
S
= 1 M
R
S
200 (Optional)
Instead of the adjustable resistor, the JFETs can be put in
I
DSS
range groupings with an appropriate R
S
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 A to 0.8 mA with internal resistance
greater than 10 M
D
S
–
+
30 V
D
S
Q
2
Q
1
100
(Optional)
R
S
Q
1
= 2N4340, J202, SST202
Q
2
= 2N4341, J304, SST304
R
S
= 1 M
Figure 5.
Cascade FET Current Source
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