Specifications and information are subject to change without notice.
WJ Communications, Inc
• Phone 1-800-WJ1-4401 • FAX: 408-577-6621 • e-mail: sales@wj.com • Web site: www.wj.com
June 2003
FP1189
½-Watt HFET
Product Information
The Communications Edge
TM
Application Note: Constant-Current Active-Biasing
Special attention should be taken to properly bias the FP1189.
Power supply sequencing is required to prevent the device from
operating at 100% Idss for a prolonged period of time and possibly
causing damage to the device. It is recommended that for the safest
operation, the negative supply be “first on and last off.” With a
negative gate voltage present, the drain voltage can then be applied
to the device. The gate voltage can then be adjusted to have the
device be used at the proper quiescent bias condition.
An optional active-bias current mirror is recommended for use with
the application circuits shown in this datasheet. Generally in a
laboratory environment, the gate voltage is adjusted until the drain
draws the recommended operating current.
The gate voltage
required can vary slightly from device to device because of device
pinchoff variation, while also varying slightly over temperature.
The active-bias circuit, shown on the right, uses dual PNP transistors
to provide a constant drain current into the FP1189, while also
eliminating the effects of pinchoff variation. This configuration is
best suited for applications where the intended output power level of
the amplifier is backed off at least 6 dB away from its compression
point. With the implementation of the circuit, lower P1dB values
may be measured for a Class-AB amplifier, where the device will
attempt to source more drain current while the circuit tries to provide
a constant drain current. The circuit should be connected directly in
line with where the voltage supplies would be normally connected
with the amplifier circuit, as shown the diagram. Any required
matching circuitry remains the same, although it is not shown in the
diagram.
This recommended active-bias constant-current circuit
adds 7 components to the parts count for implementation, but should
cost only an extra $0.144 to realize ($0.10 for U1, $0.0029 for R1,
R3, R4, R5, $0.024 for R2, and $0.0085 for C1).
Temperature compensation is achieved by tracking the voltage
variation with the temperature of the emitter-to-base junction of the
two PNP transistors. As a 1st order approximation, this is achieved
by using matched transistors with approximately the same Ibe
current. Thus the transistor emitter voltage adjusts the HFET gate
voltage so that the device draws a constant current, regardless of the
temperature. A Rohm dual transistor - UMT1N - is recommended
for cost, minimal board space requirements, and to minimize the
variation between the two transistors. Minimizing the variability
between the base-to-emitter junctions allow more accuracy in setting
the current draw. More details can be found in a separate application
note “Active-bias Constant-current Source Recommended for
HFETs” found on the WJ website.
Parameter
FP1189
Pos Supply, Vdd
+8 V
Neg Supply, Vgg
-5 V
Vds
+7.75 V
Ids
125 mA
R1
62
Ω
R2
2.0
Ω
R3
1.8 k
Ω
R4
1 k
Ω
R5
1 k
Ω
6
1
5
2
4
3
R1
R2
R3
R4
1 k
Ω
-Vgg
+Vdd
U1
Rohm UMT1N
RF IN
RF OUT
DUT
M.N.
M.N.
HFET Application Circuit
R5
C1
.01
µF
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