AN10714_1
© NXP B.V. 2010. All rights reserved.
Application note
Rev. 01 — 26 January 2010
9 of 21
NXP Semiconductors
AN10714
Using the BLF574 in the 88 MHz to 108 MHz FM band
2.4 PCB form factor
Care has been taken to minimize board space for the design.
can be generated in a space only as wide as the transistor itself.
3.
Amplifier design
3.1 Mounting considerations
To ensure good thermal contact, a heatsink compound (such as Dow Corning 340) should
be used when mounting the BLF574 in the SOT539A package to the heatsink. Improved
thermal contact is obtainable when the devices are soldered on to the heatsink. This
lowers the junction temperature at high operating power and results in slightly better
performance.
When greasing the part down, care must be taken to ensure that the amount of grease is
kept to an absolute minimum. The NXP Semiconductors’ website can be consulted for
application notes on the recommended mounting procedure for this type of device or from
your local NXP salesperson.
3.2 Bias circuit
A temperature compensated bias circuit is used and comprises the following:
An 8 V voltage regulator (Q1) supplies the bias circuit. The temperature sensor (Q2) must
be mounted in good thermal contact with the device under test (Q3). The quiescent
current is set using a potentiometer (R1). The gate voltage correction is approximately
−
4.8 mV/
°
C to
−
5.0 mV/
°
C. The V
GS
range is also reduced using a resistor (R2).
Fig 5.
Photograph of the BLF574 circuit board
001aal307
