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Details, datasheet, quote on part number:NCO8801
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Datasheet text preview:
APPLICATION NOTE
The BLF246 as an H.F.-S.S.B. amplifier
NCO8801
Philips Semiconductors
The BLF246 as an H.F.-S.S.B. amplifier
CONTENTS 1 2 3 4 5 SUMMARY INTRODUCTION NARROW BAND TEST AT 28 MHz WIDEBAND OPERATION ACKNOWLEDGEMENT
Application Note NCO8801
1998 Mar 23
2
Philips Semiconductors
The BLF246 as an H.F.-S.S.B. amplifier
1 SUMMARY
Application Note NCO8801
This report gives information on the BLF246 as a linear amplifier at 28 MHz for S.S.B. signals. Typically the device produces an output power of 80 W P.E.P. at an I.M. distortion of -34 dB. At a supply voltage of 28 V the power gain is 20 dB and the 2-tone efficiency 40%. For the design of wideband amplifiers in the range of 1.5 - 30 MHz additional information is presented showing that the power gain drops to appr. 19 dB with a variation of ±0.4 dB. 2 INTRODUCTION
The BLF246 is an R.F. power MOS-transistor in SOT121 package specified at a frequency of 108 MHz and a supply voltage of 28 V for an output power of 80 W C.W. During the development period this device has also been tested in a class-AB amplifier at 28 MHz to investigate its behaviour as a linear amplifier for possible application in the H.F. band (1.5 - 30 MHz) with S.S.B. modulation. In that case the 3rd and 5th order intermodulation products are of special interest. 3 NARROW BAND TEST AT 28 MHz
The R.F. circuit used for this purpose is depicted in Fig.1. The optimum drain quiescent current for linear operation of this device in class-AB in 0.6 A. This is achieved with a Vgs which is appr. 0.48 V higher than the specified Vgs(th). The output circuit has been aligned with a dummy load consisting of the parallel connection of a 3.5 resistor and a 400 pF capacitor. This results in a load seen by the transistor of appr. 3.3 with a negligible reactive part. The capacitors C9 and C10 are used to reduce the second harmonic voltage at the drain. In a wideband amplifier this is not necessary because the same effect is obtained by other means like a centre-tapped drain choke in a push-pull amplifier. The input circuit is aligned for minimum reflection at 50% of the maximum output power. The gate-source damping resistors R1 and R2 are needed for 2 reasons: 1. Stability i.e. to prevent oscillation when the output circuit is detuned 2. Low intermodulation distortion. In general it can be said that I.M. distortion is improved by lowering the value of these resistors. Of course they will influence the power gain of the amplifier. The average performance of the BLF246 in this amplifier is shown in Figs 2, 3, 4 and 5. At an output power of 80 W P.E.P. the average performance is: Gp = 20 dB Eff. = 40% (2-tone) d3 = -34 dB d5 = -42 dB. The distortion products have been measured with respect to one tone. It must be mentioned that the information given here is typical i.e. it is not guaranteed by measurements on individual transistors. 4 WIDEBAND OPERATION
For operation in the H.F. range (1.5 - 30 MHz) it will be necessary to reduce the value of Rgs from 18 as used in the narrow band amplifier down to 12 to obtain a more constant power gain over the band and to achieve a smaller variation in the input impedance to allow an easier matching to a driver stage or a 50 source. 1998 Mar 23 3
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