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FP31QF-PCB1900 Datasheet(PDF) 11 Page - WJ Communication. Inc. |
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FP31QF-PCB1900 Datasheet(HTML) 11 Page - WJ Communication. Inc. |
11 / 13 page 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 November 2004 FP31QF 2-Watt HFET Product Information The Communications Edge TM Application Note: Constant-Current Active-Biasing Special attention should be taken to properly bias the FP31QF. 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 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 FP31QF, 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 FP31QF Pos Supply, Vdd +9 V Neg Supply, Vgg -5 V Vds +8.75. V Ids 450 mA R1 62 Ω R2* 0.56 Ω R3 2 k Ω R4 1 k Ω R5 1 k Ω *R2 should be of size 1206 to dissipate 0.113 Watts. This should be of 1% tolerance. Two 1.1 Ω resistors in parallel of size 0805 can also be used. 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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