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Details, datasheet, quote on part number:NCO8602
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APPLICATION NOTE
A wideband power amplifier (25 - 110 MHz) with the MOS transistor BLF245
NCO8602
Philips Semiconductors
A wideband power amplifier (25 - 110 MHz) with the MOS transistor BLF245
CONTENTS 1 2 3 3.1 3.2 3.3 4 4.1 4.2 4.3 4.4 4.5 5 6 7 SUMMARY INTRODUCTION DESIGN OF THE AMPLIFIER General remarks Output circuit Input circuit MEASURED PERFORMANCE Constant input power Constant output power Constant frequency Stability Mismatch CONCLUSIONS REFERENCES APPENDIX
Application Note NCO8602
1998 Mar 23
2
Philips Semiconductors
A wideband power amplifier (25 - 110 MHz) with the MOS transistor BLF245
1 SUMMARY
Application Note NCO8602
For military communication purposes a wideband class-AB power amplifier has been designed around the BLF 245 with the frequency range 25 to 110 MHz. The DC-setting is VD = 28 V and IDQ = 200 mA. In the input and output matching networks asymmetrical 1 : 4 transformers on 4C6 ferrite core material have been applied. Table 1 The main properties are: UNIT gain at PO bandwidth VD IDQ efficiency input VSWR 2 INTRODUCTION 17.7 ± 0.5 25 - 110 28 200 55 - 67 1.6 dB MHz V mA %
The BLF245 is an RF power MOS transistor for the VHF frequency range in a SOT123 encapsulation. For application in military communication equipment a wideband power amplifier has been developed with a frequency range from 25 to 110 MHz. The transistor operates in class-AB at VDS = 28 V and a quiescent current IDQ = 200 mA. The useful output power is in the range of 25 - 30 W. 3 3.1 DESIGN OF THE AMPLIFIER General remarks
The amplifier has been developed with 1 : 4 impedance transformers in the input as well as in the output circuit. These transformers of the transmission line type with a ferrite core transform the 50 system impedance at the input and output to about 12.5 . An LC compensation circuit has been applied to transform this 12.5 to the optimum load impedance of the transistor. At the input a circuit matches the 12.5 to the gate impedance of the transistor and also takes care of a flat gain over the whole bandwidth. 3.2 Output circuit
For an optimum alignment of the output circuit the transistor has been replaced by a dummy. This dummy consists of a resistor of 12 parallel with a capacitor of 82 pF. The real part of the dummy has been determined by the available drain voltage and the required output power.
2 VD 28 R L = ---------- R L = ---------- = 13.1 2P O 2.30 2
This is near to the value of 12.5 mentioned in Section 3.1. The capacitor is about 15% higher than the output capacitance of the transistor. The RF choke at the drain side must have a sufficient high reactance at the lower end of the frequency range. Choosing this reactance appr. a factor 5 higher than the transistor loadresistance we get an inductance of 455 nH for L4. The output capacitance of the transistor can be compensated according to the Appendix. The result is: L6 = 18.6 nH and C11 = 82 pF. To transform the achieved 12.5 to the 50 system impedance an asymmetrical 1 : 4 transformer has 1998 Mar 23 3
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