FM Power Amplifier 88-108 MHz 600W BLF574

BLF574, 50V, push-pull transistor using NXP’s High Voltage 6th generation of LDMOS technology was introduced recently and makes this FM amplifier possible with a single transistor. This eliminates the need for combiners and associated losses. The BLF574 is the latest high performance technology from NXP. These new 50 volt LDMOS transistors are ideal for that 1KW amplifier project. 600W BLF574 48V FM Pallet info.

The two push-pull sections of the device are completely independent from each other inside the package. There is an internally integrated ESD diode protecting the gates of the device. The device is unmatched and is designed to be used for applications anywhere below 600 MHz where very high power and efficiency are required. Common applications would be FM and VHF broadcast, or in laser or ISM applications.

Great care has been taken during the design of the high voltage process to ensure that the device achieves high ruggedness. This is a critical parameter for successful broadcast operations. The device can achieve greater than a 10:1 VSWR for all phase angles at full operating power.

Another design goal was to minimize the size of the applications circuit. This is important in that it will allow amplifier designers to maximize the power in a given amplifier size. The design highlighted in this application note achieves 600W in the 88-108 MHz band. The FM amplifier circuits is only as wide as the transistor itself, so transistors can be arranged in the final amplifier as close as physically possible and still enable adequate room for the circuit implementation.

This application note describes the design and the performance of the BLF574 for Class-B CW and FM type applications in the 88-108 MHz frequency band.

FM Amplifier Bias Circuit
A temperature compensated bias circuit is used. The bias circuit is supplied by an 8V voltage regulator (Q1). Q2 is the temperature sensor and must be mounted in good thermal contact with the device under test, Q3. R1 sets the quiescent current. The gate voltage correction is about –4.8 to – 5.0 mV/°C. R2 is used to reduce the Vgs range. Q2 generates the basic –2.2mV/°C at its base and this is multiplied up by the R14/R15 ratio for a temperature slope of about -15mV/°C. The multiplication ability of the transistor is the reason it is used rather than a diode. A portion of the – 15mV/°C is summed into the potentiometer R1. R4 sets the amount of temperature compensation.

The ideal value proved to be 2kΩ. The value of R11 and R13 are not important to the temperature compensation. They are used only for base band stability and to improve IMD asymmetry at lower power levels.

Taken from NXP BLF574 Amplifier Application Note