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AN103
The FET Constant-Current Source/Limiter
Introduction
The combination of low associated operating voltage and high output impedance makes the FET attractive as a constant-current source. An adjustable-current source (Figure 1) may be built with a FET, a variable resistor, and a small battery. For optimum thermal stability, the FET should be biased near the zero temperature coefficient point.
D S RL RS - +
A change in supply voltage or a change in load impedance, will change ID by only a small factor because of the low output conductance goss.
DID = (DVDS)(goss)
(3)
The value of goss is an important consideration in the accuracy of a constant-current source where the supply voltage may vary. As goss may range from less than 1 mS to more than 50 mS according to the FET type, the dynamic impedance can be greater than 1 MW to less than 20 kW. This corresponds to a current stability range of 1 mA to 50 mA per volt. The value of goss also depends on the operating point. Output conductance goss decrease approximately linearly with ID. The relationship is
ID g oss + IDSS g oss
(4) (5) (6)
Figure 1. Field-Effect Transistor Current Source
NO TAG
where goss = g when VGS = 0
oss
Whenever the FET is operated in the current saturated region, its output conductance is very low. This occurs whenever the drain-source voltage VDS is at least 50% greater than the cut-off voltage VGS(off). The FET may be biased to operate as a constant-current source at any current below its saturation current IDSS.
So as VGS VGS(off), goss Zero. For best regulation, ID must be considerably less than IDSS.
Cascading for Low goss Basic Source Biasing
For a given device where IDSS and VGS(off) are known, the approximate VGS required for a given ID is
V GS + V GS(off) ID IDSS
1k
It is possible to achieve much lower goss per unit ID by cascading two FETs, as shown in Figure 2.
S Q1 S RL D Q2
D
1-
(1)
where k can vary from 1.8 to 2.0, depending on device geometry. If K = 2.0, the series resistor RS required between source and gate is
RS + V GS ID or RS + V GS(off) ID ID IDSS
RS - VDD +
1-
(2)
Figure 2. Cascade FET Current Source
Updates to this app note may be obtained via facsimile by calling Siliconix FaxBack, 1-408-970-5600. Please request FaxBack document #70596.
Siliconix 10-Mar-97
1
AN103
D Q2 S D Q1 S
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 MW at 0.2 mA. Other Siliconix part types such as the 2N4392, J112, and SST112 can provide 10 mA or higher current.
+ + VO - D Q1 S 200 W (Optional) -
(a)
IO VGS2gfs2 goss2 + VDS2 - I2
30 V
+ goss1 VDS1 = -VGS2 - = IO/goss1
(b)
RS
RS = 1 MW
Figure 3. Cascade FET VGS1 = 0 Figure 4. Adjustable Current Source RS = 1 MW
Now, ID is regulated by Q1 and VDS1 = -VGS2. The dc value of ID is controlled by RS and Q1. However, Q1 and Q2 both affect current stability. The circuit output conductance is derived as follows: If goss1 = goss2
go + g oss 2)
g fs g oss
(7) (8)
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 MW.
+ -
when RS 0 0 as in Figure 2
g oss 2 go [ g fs 1 ) R Sg fs
(9)
D Q2 S D Q1 S 100 W (Optional)
30 V
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)
Q1 = 2N4340, J202, SST202 Q2 = 2N4341, J304, SST304 RS = 1 MW
RS
If VDG < 2 VGS(off), the goss will be significantly increased, and circuit go will deteriorate. For example: A 2
Figure 5. Cascade FET Current Source
Siliconix 10-Mar-97
AN103
IF VF CR160 CR180 CR200 CR220 CR240 CR270 TO-18 2-Lead CR300 Package CR330 CR360 CR390 CR430 CR470
Part Type
J500 J501 SST/J502 SST/J503 SST/J504 SST/J505 SST/J506 SST/J507 SST/J508 SST/J509 J = TO-226AA 2-Lead Package SST = TO-236 (SOT-23) Package SST/J510 SST/J511 0.1 0.2 0.5 1 IF - Regulator Current (mA) 2 5 10
Figure 6. Standard Series Current Regulator Range
Standard Two-Leaded Devices
Siliconix offers a special series of two-leaded JFETs with a resistor fabricated on the device, thus creating a "10% current range. Devices are available in ranges from 1.6 mA (CR160) to 4.7 mA (CR470). For designs requiring a "20% current range, Siliconix offers devices rated from 0.24 mA typical (J500) through 4.7 mA typical (J511) in a two-leaded TO-226A (TO-92) package. The SST502 series is available in surface mount TO-236 (SOT-23). Each of these two-leaded devices can be used to replace several typical components. Figure 6 shows the current ranges of these two device series. Further information is contained in the individual data sheets appearing elsewhere in this data book or from Siliconix FaxBack. The CR160 series features guaranteed peak operating voltage minimum of 100 V with a typical of 180 V. The J500 series features 50 V minimum with a typical of 100 V. The lower current devices in both series provide excellent current regulation down to as little as 1 V. Siliconix 10-Mar-97
Bias Resistor Selection
All industry JFET part types exhibit a significant variation in IDSS and VGS(off) on min/max specifications and device-to-device variations. Using the simple source biasing current source as illustrated in Figure 1, the designer can graphically calculate the RS which best fits the desired drain current ID. Figure 7 plotting ID versus VGS over the military temperature range shows the resulting ID for different values of RS. The RS lines are constructed by drawing the slope of the RS desired value starting at the origin, eg. RS = 2 k slope. Find a convenient point on the X - Y axis to mark a
V GS of 2 kW such as VGS = -1.5 V and ID = 0.75 mA. ID
Then, draw a straight line from this point to the origin. The intersection of this RS line and the device ID versus VGS will be the operating ID. In this example, the resulting ID = 0.35 mA at TJ = 25_C. The intercepts of the TJ = -55_C and 125_C show the minimal variation with temperature. Also note that JFETs have a ID current where there is no change with temperature variation. To achieve this 0TC, the -VGS voltage (ID x RS) is approximately:
VGS(0TC) ] VGS(off) - 0.65 V
(11) 3
AN103
2.00 1.75 I D - Drain Current (mA) 1.50 0.5 k 1.25 1.00 0.75 0.50 0.25 0 0 -0.4 -0.8 -1.2 -1.6 VGS - Gate-Source Voltage (V) 125_C 5k 10 k 20 k -2 2k 25_C TJ = -55_C RS = 0.2 k VDS = 4 to 20 V 2N4339 max SST/J202 (low end) 1k
Figure 7. JFET Typical Transfer Characteristic
1000 VDD = 5 to 30 V TJ = 25_C except as noted I D - Drain Current ( mA)
TJ = -55_C 25_C 2N/PN4118A SST4118 Max 125_C 2N/PN4119A SST4119 Max
100
2N/PN4117A SST4117 Max 2N/PN4118A SST4118 Min 2N/PN4119A SST4119 Min 2N/PN4117A SST4117 Min
10 0.1 0.5 1 5 RS - Source Resistance (kW) 10 50 100
Figure 8. Source Biased Drain-Current vs. Source Resistance
2
TJ = -55_C 25_C 2N4339 Max
SST/J202 Max
1 I D - Drain Current (mA)
125_C SST/J201 Max 2N4338 Max SST/J202 Min 2N4339 Min SST/J201 2N4338 Min
VDD = 4 to 20 V TJ = 25_C except as noted
0.1
VDD
RS
0.01 0.1 0.5 1 RS - Source Resistance (kW) 5 10 20
Figure 9. JFET Source Biased Drain-Current vs. Source Resistance
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Siliconix 10-Mar-97
AN103
Choosing the Correct JFET for Source Biasing
Each of the Siliconix device data sheets include typical transfer curves that can be used as illustrated in Figure 7. Several popular devices are ideal for source biased current sources covering a few mAs to 20 mA. To aid the designer, the devices in Table 1 have been plotted to show the drain current, ID, versus the source resistance, RS, in Figures 8, 9, and 10. Most plots include the likely worst case ID variations for a particular RS. For tighter current control, the JFET production lot can be divided into ranges with an appropriate resistor selection for each range.
Table 1: Source Biasing Device Recommendations Practical Current Range ID (mA)
0.01 - 0.02 0.01 - 0.04 0.02 - 0.1 0.01 - 0.1 0.02 - 0.3 0.1 - 2 0.2 - 10
Through-Hole Plastic Device
PN4117A PN4118A PN4119A J201 J202 J113 J112
Surface Mount Device
SST4117 SST4118 SST4119 SST201 SST202 SST113 SST112
Metal Can Device
2N4117A 2N4118A 2N4119A 2N4338 2N4339 2N4393 2N4392
20 10 I D - Drain Current (m A)
-55_C TJ = 25_C
VDD = 5 to 30 V TJ = 25_C except as noted
Mid 2N4392, SST/J112
125_C Mid Min 2N4393 SST/J113 VDD 2N4393, SST/J113 Max Min 2N4392, SST/J112
1
RS
0.1 0.1 0.5 1 RS - Source Resistance (kW) 5 10
Figure 10. JFET Source Biased Drain-Current vs. Source Resistance
Siliconix 10-Mar-97
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